Strings

Lecture 13

Mehran Sahami Handout #26 CS 106A October 22, 2007

Mehran Sahami Handout #26
CS 106A October 22, 2007
Strings and Ciphers
Based on a handout by Eric Roberts.
Cryptography, derived from the Greek word meaning hidden, is the science of
creating and decoding secret messages whose meaning cannot be understood by others
who might intercept the message. In the language of cryptography, the message you are
trying to send is called the plaintext; the message that you actually send is called the
ciphertext. Unless your adversaries know the secret of the encoding system, which is
usually embodied in some privileged piece of information called a key, intercepting the
ciphertext should not enable them to discover the original plaintext version of the
message. On the other hand, the intended recipient, who is in possession of the key, can
easily translate the ciphertext back into its plaintext counterpart.
Caesar ciphers
One of the earliest documented uses of ciphers is by Julius Caesar. In his De Vita
Caesarum, the Roman historian Suetonius describes Caesar’s encryption system like this:
If he had anything confidential to say, he wrote it in cipher, that is, by so changing
the order of the letters of the alphabet, that not a word could be made out. If
anyone wishes to decipher these, and get at their meaning, he must substitute the
fourth letter of the alphabet, namely D, for A, and so with the others.
Even today, the technique of encoding a message by shifting letters a certain distance in
the alphabet is called a Caesar cipher. According to the passage from Suetonius, each
letter is shifted three letters ahead in the alphabet. For example, if Caesar had had time to
translate the final words Shakespeare gives him, ET TU BRUTE would have come out as
HW WX EUXWH, because E gets moved three letters ahead to H, T gets moved three to W,
and so on. Letters that get advanced past the end of the alphabet wrap around back to the
beginning, so that X would become A, Y would become B, and Z would become C.
Caesar ciphers have been used in modern times as well. The “secret decoder rings” that
used to be given away as premiums in cereal boxes were typically based on the Caesar
cipher principle. In early electronic bulletin board systems, users often disguised the
content of postings by employing a mode called ROT13, in which all letters were cycled
forward 13 positions in the alphabet. And the fact that the name of the HAL computer in
Arthur C. Clarke’s 2001 is a one-step Caesar cipher of IBM has caused a certain amount of
speculation over the years.
Let's consider writing a simple program that encodes or decodes a message using a Caesar
cipher. The program needs to read a numeric key and a plaintext message from the user
and then display the ciphertext message that results when each of the original letters is
shifted the number of letter positions given by the key. A sample run of the program
might look like the example on the following page.
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For the Caesar cipher, decryption does not require a separate program as long as the
implementation is able to accept a negative key, as follows:
Letter-substitution ciphers
Although they are certainly simple, Caesar ciphers are also extremely easy to break.
There are, after all, only 25 nontrivial Caesar ciphers for English text. If you want to
break a Caesar cipher, all you have to do is try each of the 25 possibilities and see which
one translates the ciphertext message into something readable. A somewhat better
scheme is to allow each letter in the plaintext message to be represented by an arbitrary
letter instead of one a fixed distance from the original. In this case, the key for the
encoding operation is a translation table that shows what each of the 26 plaintext letters
becomes in the ciphertext. Such a coding scheme is called a letter-substitution cipher.
The key in such a cipher can be represented as a 26-character string, which shows the
mapping for each character, as shown in the following example:
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Q W E R T Y U I O P A S D F G H J K L Z X C V B N M
Letter-substitution ciphers have been used for many, many years. In the 15th century, an
Arabic encyclopedia included a section on cryptography describing various methods for
creating ciphers as well as techniques for breaking them. More importantly, this same
manuscript includes the first instance of a cipher in which several different coded
symbols can stand for the same plaintext character. Codes in which each plaintext letter
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maps into a single ciphertext equivalent are called monoalphabetic ciphers. More
complex codes in which the representation for a character changes over the course of the
encryption process are called polyalphabetic ciphers.
The second problem to consider is to write a program that implements this more general
letter-substitution cipher. The program should ask the user to enter a 26-letter key and
the plaintext message. It should then display the ciphertext and, to ensure that the
encryption is working, what you get if you decrypt the ciphertext using the same key:
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Caesar ciphers solution
/*
* File: CaesarCipher.java
* -----------------------
* This program translates a line of text into its Caesar cipher
* form.
*/
import acm.program.*;
public class CaesarCipher extends ConsoleProgram {
public void run() {
println("This program uses a Caesar cipher for encryption.");
int key = readInt("Enter encryption key: ");
String plaintext = readLine("Plaintext: ");
String ciphertext = encryptCaesar(plaintext, key);
println("Ciphertext: " + ciphertext);
}
/*
* Encrypts a string by adding the value of key to each character.
* The first line makes sure that key is always positive by
* converting negative keys to the equivalent positive shift.
*/
private String encryptCaesar(String str, int key) {
if (key < 0) {
key = 26 - (-key % 26);
}
String result = "";
for (int i = 0; i < str.length(); i++) {
char ch = encryptCharacter(str.charAt(i), key);
result += ch;
}
return result;
}
/*
* Encrypts a single character using the key given. This
* method assumes the key is non-negative. Non-letter
* characters are returned unchanged.
*/
private char encryptCharacter(char ch, int key) {
if (Character.isLetter(ch)) {
ch = (char) ('A'
+ (Character.toUpperCase(ch) - 'A' + key) % 26);
}
return ch;
}
}
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Letter-substitution ciphers solution
/*
* File: LetterSubstitutionCipher.java
* -----------------------------------
* This program translates a line of text using a letter-substitution
* cipher.
*/
import acm.program.*;
public class LetterSubstitutionCipher extends ConsoleProgram {
public void run() {
println("This program implements a letter-substitution cipher.");
String key = readKey();
String plaintext = readLine("Plaintext: ");
String ciphertext = encrypt(plaintext, key);
String decryption = decrypt(ciphertext, key);
println("Ciphertext: " + ciphertext);
println("Decryption: " + decryption);
}
/**
* Reads an encryption key from the user and checks it for
* legality (see isLegalKey below). If the user enters an
* illegal key, the user is asked to reenter a valid one.
*/
private String readKey() {
String key = readLine("Enter 26-letter key: ");
while (!isLegalKey(key)) {
println("That key is not legal.");
key = readLine("Reenter key: ");
}
return key;
}
/**
* Checks to see whether a key is legal, which means it meets
* the following conditions:
* (1) It is 26 characters long,
* (2) It contains only uppercase letters, and
* (3) It has no duplicated letters.
*/
private boolean isLegalKey(String key) {
if (key.length() != 26) return false;
for (int i = 0; i < 26; i++) {
char ch = key.charAt(i);
if (!Character.isUpperCase(ch)) return false;
for (int j = i + 1; j < 26; j++) {
if (ch == key.charAt(j)) return false;
}
}
return true;
}
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/**
* Encrypts a string according to the key.
*/
private String encrypt(String str, String key) {
String result = "";
for (int i = 0; i < str.length(); i++) {
char ch = encryptCharacter(str.charAt(i), key);
result += ch;
}
return result;
}
/**
* Encrypts a single character according to the key.
*/
private char encryptCharacter(char ch, String key) {
if (Character.isLetter(ch)) {
ch = key.charAt(Character.toUpperCase(ch) - 'A');
}
return ch;
}
/**
* Decrypts a string according to the key.
*/
private String decrypt(String str, String key) {
String result = "";
for (int i = 0; i < str.length(); i++) {
char ch = decryptCharacter(str.charAt(i), key);
result += ch;
}
return result;
}
/**
* Decrypts a single character according to the key.
*/
private char decryptCharacter(char ch, String key) {
int index = key.indexOf(ch);
if (index != -1) {
ch = (char) ('A' + index);
}
return ch;
}
}

Mehran Sahami Handout #25 CS 106A October 22, 2007

Mehran Sahami Handout #25 CS 106A October 22, 2007 Strings Examples Based on examples by Eric Roberts and Patrick Young. Checking for palindromes public boolean isPalindrome(String str) { for(int i = 0; i < str.length() / 2; i++) { if (str.charAt(i) != str.charAt(str.length() - (i + 1))) { return false; } } return true; } Reversing strings and a simpler version of checking for palindromes
public String reverseString(String str) { String result = ""; for(int i = 0 ; i < str.length(); i++) { result = str.charAt(i) + result; } return result; } public boolean simpleIsPalindrome(String str) {
return (str.equals(reverseString(str))); } Counting uppercase characters import acm.program.*; public class CountUppercase extends ConsoleProgram { private int countUppercase(String str) { int upperCount = 0; for(int i=0; i<str.length(); i++) { char ch = str.charAt(i); if (Character.isUpperCase(ch)) { upperCount++; } } return upperCount; } public void run() { String str = readLine("Enter String: "); println(countUppercase(str) + " upper case letters"); } }
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Replace first occurrence public String replaceFirstOccurrence(String str, String orig, String repl) { int index = str.indexOf(orig); if (index != -1) { str = str.substring(0, index) + repl + str.substring(index + orig.length()); } return str; }

Mehran Sahami Handout #24 CS 106A October 22, 2007

Mehran Sahami Handout #24
CS 106A October 22, 2007
Section Handout #4: String Processing
Portions of this handout by Eric Roberts and Patrick Young
1. Adding commas to numeric strings (Chapter 8, Exercise 13, page 290)
When large numbers are written out on paper, it is traditional—at least in the United States—to use commas to separate the digits into groups of three. For example, the number one million is usually written in the following form:
1,000,000
To make it easier for programmers to display numbers in this fashion, implement a method
private String addCommasToNumericString(String digits)
that takes a string of decimal digits representing a number and returns the string formed by inserting commas at every third position, starting on the right. For example, if you were to execute the main program
public void run() {
while (true) {
String digits = readLine("Enter a numeric string: ");
if (digits.length() == 0) break;
println(addCommasToNumericString(digits));
}
}
your implementation of the addCommasToNumericString method should be able to produce the following sample run:
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2. Deleting characters from a string
Write a method
public String removeAllOccurrences(String str, char ch)
that removes all occurrences of the character ch from the string str. For example, your method should return the values shown:
removeAllOccurrences("This is a test", 't') returns "This is a es"
removeAllOccurrences("Summer is here!", 'e') returns "Summr is hr"
removeAllOccurrences("---0---", '-') returns "0"
3. Heap/stack diagrams
Using the style of heap/stack diagram introduced in Chapter 7, show the state of both the heap and the stack at the point in the computation indicated by the arrow in the following code, where the Rationale class is the one defined in Chapter 6.
public void run() {
Rational r = new Rational(1, 2);
r = raiseToPower(r, 3);
println("r ^ 3 = " + r);
}
private Rational raiseToPower(Rational x, int n)
Rational result = new Rational(1);
for (int i = 0; i < n; i++) {
result = result.multiply(x);
}
return result;
}
Diagram at this point
Indicate which values in the heap are garbage at this point in the calculation.
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4. Tracing method execution
For the program below, show what output is produced by the program when it runs.
/*
* File: Mystery.java
* ------------------
* This program doesn't do anything useful and exists only to test
* your understanding of method calls and parameter passing.
*/
import acm.program.*;
public class Mystery extends ConsoleProgram {
public void run() {
ghost(13);
}
private void ghost(int x) {
int y = 0;
for (int i = 1; i < x; i *= 2) {
y = witch(y, skeleton(x, i));
}
println("ghost: x = " + x + ", y = " + y);
}
private int witch(int x, int y) {
x = 10 * x + y;
println("witch: x = " + x + ", y = " + y);
return x;
}
private int skeleton(int x, int y) {
return x / y % 2;
}
}

Mehran Sahami Handout #24A CS 106A October 24, 2007 Solution to Section #4 Portions of this handout by Eric Roberts and Patrick Young

Mehran Sahami Handout #24A CS 106A October 24, 2007 Solution to Section #4 Portions of this handout by Eric Roberts and Patrick Young
1. Adding commas to numeric strings
private String addCommasToNumericString(String digits) { String result = ""; int len = digits.length(); int nDigits = 0; for (int i = len - 1; i >= 0; i--) { result = digits.charAt(i) + result; nDigits++; if (((nDigits % 3) == 0) && (i > 0)) { result = "," + result; } } return result; }
2. Deleting characters from a string
private String removeAllOccurrences(String str, char ch) { String result = ""; for (int i = 0; i < str.length(); i++) { if (str.charAt(i) != ch) { result += str.charAt(i); } } return result; }
A slightly different approach that involves a while loop instead of a for loop:
private String removeAllOccurrences(String str, char ch) { while (true) { int pos = str.indexOf(ch); if (pos >= 0) { str = str.substring(0, pos) + str.substring(pos + 1); } else break; } return str; }
Could you generalize these functions to remove substrings? For instance, removeAllSubstrings("Mississippi", 'si') returns "Missppi". Would these two functions ever return different values for the same input?
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3. Heap/Stack diagrams 100012munned102011munned104012munnedFFC01000rheapFFB83FFB41080n106014munnedstackFFB03itluser108018munnedFFB81000xgarbage 4. Tracing method execution

Programming Methodology-Lecture13 Instructor (Mehran Sahami)

Programming Methodology-Lecture13
Instructor (Mehran Sahami): So welcome back to Week 5 of CS 106A. We're getting
so close to the middle of the class, so a couple of quick announcements before we start.
First of which, there are three handouts. Hopefully, you should pick them up. There’s this
week’s section from some string examples, and kind of some more string examples that
we're gonna go over today.
And now, since we're getting to the middle of the quarter, it’s that time for the mid-term
to be coming up. So, in fact, the mid-term is next week. It’s a week from Tuesday. I
know. The quarters go by so quickly. If you have a conflict with the mid-term, and by
conflict, I mean an unmovable academic conflict.
Like, if you have another class at the same time, not like, oh, yeah, Tuesday night, you
know, October 30, yeah, around dinnertime, I was gonna go out with some friends for
dinner, but the mid-term is really cramping my style so I can’t take it then. That’s not an
unmovable academic conflict. So if you have an unmovable academic conflict with the
mid-term time, check the schedule.
Send me email by 5:00 p.m. this Wednesday, so you have a few days to do it, 5:00 p.m.
by this Wednesday. Don’t just tell me you have a conflict. Let me know all the times you
are free next week to be able to take the alternate exam 'cause what I’m gonna do is a
little constraint satisfaction problem. I’m gonna take everyone who’s got a conflict with
the mid-term, find out the time that everyone can take the exam an alternate time.
Hopefully, there will be one, and we'll schedule an alternate time. So only email me if
you have an unmovable academic conflict. Make sure to list all the times you're free, and
let me know by 5:00 p.m. Wednesday 'cause after 5:00 p.m. Wednesday, I figure out
what the time is, and we ask for room scheduling. And that’s it. I can’t accommodate any
more requests after that. So 5:00 p.m. Wednesday, know it, live it, learn it, love it, midterm.
A couple of things about the mid-term you should also know, one of which is it is an
open-book exam. It will be all written so leave your laptops at home. You won’t be using
a laptop. But if you ever get a job in computing where someone says, hey, guess what,
you have to memorize, like, the Java Manual, don’t take that job. So in computer science,
we say, hey, you don’t need to memorize all this stuff 'cause people, actually, look it up
in the real world when they need it.
So it’s an open-book, open-note exam. You can bring in printouts of your programs, and I
would encourage you to do that. You can bring in your book, only the book for the class,
by the way. Don’t bring in a whole stack of books. And the reason why we make that
restriction is because we – actually, in the past, there has been bad experiences with
someone who brings in a whole stack of books. And they spend so much time trying to
look things up in a whole stack of books that they just run out of time on the exam. It
doesn’t make any sense.
All the information you're gonna need for the exam will be in your book, and in the Karel
Course Reader. So open book, that’s course text, open Karel Course Reader, open any
handouts or notes you’ve taken in the class, and printouts of your own programs. So you
can bring all that stuff in. Another thing about the mid-term is you may be wondering,
hey, what kind of stuff’s on the mid-term, so on Wednesday I will give you an actual
sample mid-term and the solutions for it.
I would encourage you to actually try to take the mid-term, the sample that I give you
under sort of time constraint conditions. So you get a sense for 1.) What kind of stuff is
on the mid-term? 2.) How much time you're actually spending thinking about the
problems versus looking stuff up in books. And that will give you some notion of what
you actually need to study to sort of remember quickly versus stuff you might want to
look up.
Another good source of mid-term-type problems is the section problems. So like, the
section handout this week, actually, has more problems than will probably be covered in
section, just to give you some more practice. So section problems are real good. I’ll give
you a practice mid-term. That will give you even more sort of examples.
And if you want still more problems to work through, I’d encourage you to work through
the problems of the exercise at the back of every chapter of the book. Those are also
similar in many cases to the kinds of problems that’ll be on exams. So you'll just have
problems up the wazoo if you want to deal with them, okay. So any questions about the
mid-term next week? We can all talk about it more as the week goes on, next week,
Tuesday.
All right, so with that said, any questions about strings? We're gonna do a bunch more
stuff today with strings and characters. But if there’s any questions before we actually
dive in to things, let me know now. And if you could use the microphone, that would be
great. One more time, take those microphones out, hold them close to your heart. Air and
gear, they're lots of fun, they're your friend. Keep the microphone with you.
Student: Actually, sorry, about the mid-term, is it going – what’s the cutoff of the midterm
in terms of, like, caustes.
Instructor (Mehran Sahami): Right, so the mid-term for stuff you need to know, the
cutoff will be Wednesday’s class. So, basically, you'll have a whole week of material that
you won’t need to be responsible for that will be from this Wednesday up until the midterm.
The other thing, though, is to keep in mind that a few people have asked, well, do I
need the book versus your lectures for the mid-term.
You need to know the lectures, and you need to know all the material from the book that
is covered with respect to lectures, which is most of the material from the book. But
there’s a few cases where we go over something very quickly in class, or I say refer to
this page of the book or whatever. That stuff you're responsible for knowing. Stuff that
I’ve, like, explicitly told you you don’t need to know, like, polar coordinates, aren’t
gonna be on the exam, okay.
So the exam will be more heavily geared towards stuff from lecture, but you still should
know all the stuff from the book that we've kind of referred to in lecture as we've gone
along, allrighty. All right, so let’s dive into our next great topic. Actually, it’s a
continuation of our last great topic, which is strings. And so, if we think about strings a
little bit, one of the things we might want to do with strings, is we want to do some string
processing that also involves some characters.
So how are we gonna do that? One thing we might want to do is let’s just do a simple
example to begin with, which is going through a string, and counting the number of
uppercase characters in the string. And the reason why I’m gonna harp on strings a whole
bunch – we talked about it last time – we’re gonna talk about it this time – guess what
your next assignments gonna be. It’s gonna be all about string processing. So it’s good
stuff to know, okay.
So we might want to have some function. Count uppercase. And that’s a function I’ve
actually given to you in one of the handouts, so you don’t need to worry about jotting
down all my code real quickly, but you might want to pay close attention. And what this
does, is it gets past some string, STR, and it’s gonna count how many uppercase
characters are in that string. So it’s gonna return an int.
And let’s just say this is part of some other program, so we'll call this private, although
you could make it public if it was in some class that you wanted to make available for
other people to use. So if we want to count the number of uppercase characters, what do
we want to think about doing? What’s the kind of standard idiom that we use for strings?
Anyone remember? What? We want to have a foreloop, [inaudible] somewhere over
here.
Yeah, it’s just raining candy on you. We want to have a foreloop that goes through all the
characters of the string, sort of counting through the character. So we can do that by just
saying for N2i equals zero, “i” less than the length of the string, right. So STR.length is
the method we use to get the length of the string, and then i++. And this is gonna loop
through all the characters of the string. Okay, where actually it’s gonna loop through
some number of times, which is the number of characters in the string.
Now we want to pull out each one of the characters, individually, to check to see if it’s an
uppercase character. What method might we use to do that? Get a character out of a
string in a particular position. Come on, I’m begging for it. Char at – it’s like where’d it
go? It’s just gone, char at, and we'll just for the delayed reaction, we'll do it in slow mo.
Anyone remember “The Six Million Dollar Man,” that show? No, all right. Get another
man.
I’m just getting so old, I gotta hang it up. And the thing is, I’m not that much older than
you. But it’s just amazing what big a difference a few years makes. So char CH is going
to be from this string. Were gonna pull out the char at apposition i. So now, we've
actually each. We're gonna loop through each character of the string, pulling out that
character, and we want to check to see if the character’s uppercase.
We could actually have an if statement in here to check to see if that CH is in between
uppercase A and uppercase Z, which is kind of how you saw last time we could do some
math on characters. We're gonna use the new funky way, which is to actually use one of
the methods from the character class, and just say if. And the way we use the methods
from the character class, we specify the name of the class here as opposed to the name of
an object because the methods from the character class are what we refer to as static
method.
There is no object associated with them. There are just methods that you call and pass
into character. Is uppercase because this returns a Boolean, and will pass at CH to see if
CH is an uppercase character, okay. If it is an uppercase character, okay. If it is an
uppercase character, we want to somehow keep track of the number of the uppercase
characters we have. So how might we do that? Counter, right.
So have some int count equals zero, up here, that I want initialized. Who said that? It
came from somewhere over here. Come on, raise your hand. Don’t be shy. It’s a candy
extravaganza. So if character is uppercase, CH then got count, we're just gonna add 1 to.
Otherwise we're not gonna increment the counts. It’s not an uppercase character. And
then, we end the foreloop. So this is gonna go through all the characters of the string. For
every character check seeks the uppercase.
If it is, increment our count, and at the end, what we want to do is, basically, return that
count, which tells us how many uppercase characters were actually in the string, okay. Is
there any questions about this? This is kind of like an example of the sort of vanilla string
processing you might do. You have some string. You go through all the characters of the
string. You do some kind of thing for a character of the string. In this case, were not
creating a new resulting string. We're just counting up some number of characters that
might be in the string.
So we can do something a little bit more funky. This is kind of fun, but it’s sort of like,
yeah, just basic kind of string and character stuff. Let’s see something a little bit more
funky, which is actually to do some string manipulation to break the string up into
smaller pieces. And so what we want to do is replace some occurrence of a substring in a
larger string with some other sting, sort of like when you work on [inaudible], when you
do Find/Replace.
You say, hey, find me some little string, or some little work that’s actually in my bigger
document. I’m gonna replace it with some other word. We're actually gonna implement
that as a little function, okay. So what this is gonna do, we'll call this replace occurrence
just to keep the name short, but, in fact, all we're gonna do is replace the very first
occurrence in a string. So we're gonna get past int some string, STR, and what we want to
do is, basically, have some original string, which is the thing that we want to replace,
with some replacement string.
So we're gonna get past three parameters here. Will call this RPL for replace, okay.
Which is the large string, a piece of text that I want to replace some word in, the original
word that I want to replace, and the thing that I want to replace it with, okay. And so what
I want to do because strings are immutable, right. I can’t change the string in place. I
have to actually return a new string, which has this original replaced by this string.
So, this puppy’s gonna return the string, and we'll just make this private again, although
we could have made it public if we wanted to have it in the library that other people
would use, or a class that other people would use, okay. So how might we think about the
algorithm for replacing this original string with the replacement. What’s the first thing we
might want to think about that we want to do with the original string.
Do, do, do, do, a little concentration music. We want to find it, right. We want to see if
this original string appears somewhere on that string, right because if it doesn’t we're
done. Thanks for playing, right, but that’s actually the good things for playing. It’s sort of
like you got no more work to do. And there’s, actually, some methods from the string
class that we can use to do that.
So there’s a string in the string class called “index of.” And what index of does is I can
pass it some string, like the original string I want to look up, and it will return to me a
number. That number is the index of the position of the first character of this string if it
appears in the larger string. So the larger string is the one that I’m sending the message
to, and I’m asking it do you have this original string somewhere inside you.
If you do, return me the index of its first occurrence. And if you don’t, it returns a
negative 1. So I’m gonna assign this thing to some variable I’ll call index, and first of all,
I want to check to see if I have any work to do. If index is not equal to negative 1, then I
have some work to do. If it is equal to negative 1, that means hey, you know what, you
want it to replace this original string, inside string STR. That original string doesn’t exist,
so I got no work to do.
You just called, like, find and replace in the word processor, and the thing you wanted to
find wasn’t there, okay. So in that case, all I would do is I would just return STR, right.
Sort of unchanged, if I assume that I’m not doing what’s inside the braces. If I do find
that string, though, I’m gonna get some index, which is not negative 1, which is the
position of this original string.
So let’s do a little example just to make this a little bit more clear what’s going on. So if
we were to call this function – do, do, do, do, do – and pass in the string, STR. So here’s
STR that we're gonna pass in. We'll just put it in a big box, and we'll say, at this point in
life, everyone’s just friendly. So we say Stanford loves Cal, right. Sometimes you have to
distort reality in order t make an example. All right, so we have Stanford loves Cal.
That’s our original string, STR, and we might want to say, well, you know, this is, really,
not always the way life is. Really, the way life is, is we want to replace the occurrence on
STR of the word “loves” with kind of a more realistic example, like the word “beats,”
right. So what we want to do – and then we're gonna – this is gonna be some string that
comes back, will find it back to STR.
And the question is, when we call this, what index are we actually gonna find in here of
the original string. So strings we start counting from zero. Zero, 1, 2, 3, 4, 5, 6, 7, 8. The
nine is where the L is at. And it keeps going. And 11, 12, 13, just put these all together –
15, 16, 17 is the L and that would be the end of the string. Sorry, the numbers are a little
bit small. But the key is this L is at 9, okay.
So when I call string index up original, it says there’s the word, or the string, “loves,” up
here somewhere in the larger string. Yeah, it does. It appears at Index 9 so that’s what
you get. So if I’ve just gotten Index 9, and what I want to do is construct some new string
that, essentially, is going to have this portion removed from it, how do I want to do that.
What I want to think about is the way I construct that string, it’s from three pieces. The
first piece is everything up to the word I want to replace. That’s Piece No. 1. The second
piece is the thing that I actually want to replace, the string I’m replacing with, right. So
this becomes Piece No. 2. And then, everything else after the piece I’ve replaced is Piece
No. 3. So if I can concatenate those three pieces together, I’m going to essentially get the
new string, which has this part replaced.
And the question is how do I find the appropriate indexes inside my larger string to be
able to actually do the replacement, okay. So first thing that I’m gonna do here is say get
me the first portion. So what is, essentially, the substring of the original string up to this L
position. So the way I can do that is I can say STR, substring, and I’m gonna get the
substring starting at zero 'cause I want to start at the beginning of the string, and I want to
go all the way up, but not including the L.
That means the last position in substring. Remember, in substring you give it two
indexes. You give it the starting point, and the position up to, but not including that last
chapter. That’s Position 9. Where am I getting Position 9 from this thing? From index,
right. Index says where does love start. It starts at Position 9. I’m, like, hey, that’s
fantastic. So zero up to index, or zero up to 9 is Stanford and the states. It does not
include the L. So I get that portion.
Then I say well, to that – I’m not done yet, so premature [inaudible] in there. Always
gotta watch out for that, bad time. So what we're gonna add to that is we're gonna add the
string that we want to replace in here, “beats,” which happens to be the string called the
replacement, or RPL. And then to that, we want to add one more string. And that’s,
essentially, everything from after “loves” over, to get that third piece, okay.
So what I want to know is what’s the index of the position at which I need to get
characters over to the end. That happens to be Position 14. What is 14 equal to, relative to
the kinds of things I have over here? It’s index 'cause I have to first get over to the 9, then
I need to jump over the length of this thing, which is the length of my original string. So
if I add to index, what’s my original dot link, what that gives me is the index from which
I want to take a substring over to the end of the string.
So if I want to take a substring, this becomes an index to the substring function, or the
substring method. And so from the string, what I do is I take the substring, starting at
Position 14. Notice I haven’t given a second index here. In this case I gave two indexes. I
gave a start and end position. Here I just gave one index, and what happens if I only give
one index? It goes to the end.
So that’s part of the beauty is a lot of times you just say, hey, from this position go to the
end. And so that’s what I get when I put all these string things together. And what I need
to do is these three things are just pieces. I’m concatenating them together. I assigned
them back to STR. And then, when I return STR here, I’ve gotten those three pieces
concatenated together. Is there any questions about that? Un huh.
If love appears more than once, index has just returned the index of the very first
occurrence. There’s actually a version of index sub that takes two parameters. One is the
thing you're looking for, and the second is from which position you should start looking
for it at. And so you could actually say look for love starting at Position, you know, 13,
and then it wouldn't actually find love in the remainder of the string. So there’s a different
version of index of, but index of always returns the index of the very first occurrence of
the string you're looking for in that string.
So let’s actually do a little example of this in a running program. Do, do, do, do, do. And
we'll do replace occurrence. And one thing that actually goes on at Stanford, which I
thought was an interesting thing when I got here professionally, is we don’t like to speak
in full terms. So if we want to Stanfordize some strings, we do all these string
replacements.
We sort of say, you know what, if you have Florence Moore in your string, that’s really
FloMo. And Memorial Church is memchu; AmerSc, [inaudible]; psychology is psyche;
economics, econ; your most fun class, CS 106A. So it’s just what Stanford’s all about.
And so if we go ahead and run this, right. Here’s the function we just wrote. Here’s our
little friend, replace first occurrence. Over here we called it replace occurrence. I’m being
explicit and saying it’s only replacing the first occurrence.
You could think of a way to generalize this to replace all occurrences in a string if you
wanted to. But I didn’t give you that version 'cause I might give you that version on
another problem set at some point. So what we're gonna do is we're gonna ask the user,
enter a line to Stanfordize. Notice I want to put Stanfordize inside double quotes. So I put
it in these characters, /quote, which just means a single, double-quote character. That’s
how I print double quotes.
So it says read line for Stanfordize in quotes. I want to keep reading lines and
Stanfordizing them until the user gives me an empty line. How do I do that? I check to
see if the line the user gives me is equal to a quote-quote. So if it’s equal to a quotequote,
it’s equal to the empty string. That means, hey, you entered in – if we ask the user
for a string, they just hit enter. They didn’t enter any characters. That’s the empty string,
so we would break out the loop. It’s our little loop and a-half concept.
Otherwise, we say at Stanford we say, and we Stanfordize the line. And when someone’s
finally done, we say thank you for visiting Stanford, ha, ha, ha. That’ll be $45,000.00.
[Laughter]. All right, so it’s money well spent, trust me. Really. Okay, so replace
occurrence string we want to run, and we come along, and it’s running, it’s running, it’s
running. Sometimes my computer’s running a little bit slow.
I notice this weird thing last night. I’m gonna tell you a story while the computer’s
actually running. I couldn’t type N’s on my keyboard for some reason. And then I reset
my computer, and I could. So at this point, I don’t know if I can type N’s. So let’s just
hope we can. So I live in – oh, I got the N – Florence – you should have been here last
night. I was like, N, N, and I wasn’t getting it – Florence Moore, major in economics – I
can’t even type today – and spend all my time on my most fun class.
And so, at Stanford we say I live in FloMo, major in Econ, and spend all my time on CS
106A, okay. And now, I hit return, Thank you for visiting Stanford. Go home. All right,
so that’s kind of a simple version of replace first occurrence. And notice you can actually
replace multiple things in the same string, as long as the string that you're doing the
replacement on you assign back to itself. And then we kind of do all bunch of these
replacements in a row, okay. Is there any questions about that? Are you feeling okay
about doing replacement. All right.
So now, it’s time for something completely different. Although it’s not completely
different, it’s just kind of different. And the idea is sometimes – and I always say that –
sometimes you want to do this. Yeah, 'cause sometimes you want to do it, and other times
you don’t. Sometimes you feel like a nut. Sometimes you don’t. Oh, man, I gotta start
watching TV in this decade.
So, tokenizers. What is a tokenizer? A tokenizer is something, as they say it’s a computer
science term. All a tokenizer is, is we have some string of text. What we want to do is
break it up into tokens. That’s called tokenization. So you might say, Marilyn, what is a
token? Like, last time I remember what a token was, is when I gave a dollar at the arcade
and I got back, like, ten tokens instead of quarters. And you're like, yeah, Marilyn, I
never did that. I had an XBox.
All right, so a tokenizer – anyone ever go to an arcade? All right, just checking. All right
a token, basically, is a piece of string – a piece of string – is a string that has on the two
sides of it, white space. So if I say, hello there, Mary, hello there and Mary are tokens.
They are something that we refer to as delimited by what space, which means there is
either spaces, or tabs, or returns, or whatever, in between the individual tokens.
We like to think of tokens as words, but computer scientists say token. Token is a more
general term 'cause if I actually said hello there comma Mary, the “there comma” might
actually be considered one token by itself 'cause it’s just delimited by space. Here’s a
space here and has a space there, so the comma’s in there. And you would think why
comma’s not part of the word. Yeah, that’s why we call them tokens and not words.
So if we want to tokenize, there is a library that we can use in Java that actually has some
fun stuff in it for tokenization. And that’s Java util, so we would import Java.util.*, and
what we get for doing that, is we get something called the string tokenizer, which is a
class that we can use to tokenize text. All right, so we get this thing called the string
tokenizer.
How do I create one of these? Well, I paste string tokenizer as the type 'cause that’s the
class that I have, and I’ll call it tokenizer equals I want to create a new tokenizer. So I say
new string tokenizer, and the question that comes up here is well, what is the string you're
gonna tokenize? That is the string that we passed to the string tokenizer’s constructor
when we create a new one.
So we might have some line here that we passed in. And now, line is just some string that
maybe we got from the user for example by doing a read line. Maybe we were unfriendly
and didn’t give the user a prompt. We just like, if a blinking comes up, and there’s like
oh, I gotta turn and write something. It’s just like when you're writing a paper, right. The
blinking cursor comes up and there’s nothing there. You just gotta fill it in.
So you write some line, and then we can say, hey, string tokenizer, I’m gonna create a
new one of you, and the line I want you to tokenize is this line that I’m giving you to
begin with. So once you get that line, there’s a couple of things you can ask the string
tokenizer. One of them is a method that returns a booleon, which is called has more
tokens.
And the way this puppy works is you just ask this string tokenizer, like you would say
tokenizer dot has more tokens, like; do you have more tokens? Have you processed the
whole string yet? So if you’ve just created the new line, and this line is kind of sitting
here like that, and it’s saying do you have any more tokens. Yeah, I got tokens, man. I got
tokens up the wazoo. You want tokens, I’ll give you tokens. And so, has more tokens
[inaudible] true.
If you process the whole string, when you will see when we get there, it’ll say no, I don’t
have any more tokens. How do you get each token? Well, you ask for next token. And
what next token does, when you call the tokenizer with next token, is it gives you the
next token of the string that it’s processing, as a separate string.
So if I started off the tokenizer with this line, I say hey, do you have more tokens. It says
yeah. Well, give me the next token. So what it will return to you is hello. And it will be
sort of sitting here waiting to give you the next token. You can ask if you have more
tokens. It says yeah, give me the next token.
It will give you “there” and the comma 'cause the default version of the tokenizer, the
only think that delimits tokens – delimit is a funky word for splits between tokens – are
spaces, or tabs, or return characters. But for a single line, you won’t have returns in that.
And then you said you had more tokens. Yeah, give me the next token. It will give you
“Mary” as a token that’s sitting here.
And then when you say do you have more tokens, that’s all, okay. And at that point, you
shouldn’t call next token. You can if you want. You can experiment with this if you want
to experiment with random error messages, but there’s no more tokens to give you. It’s
all out of love. It’s so lost without you. It has no more tokens.
Yeah, Air Supply. Not that I would recommend that you have to listen to Air Supply, but
sometimes you hear a song and you can’t get it out of your head as much as you wish you
could. Sometimes selective brain surgery would not be a bad thing, but that’s important
right now. What is important right now is how do we put all this together at the tokenizer
line. So let me show you an example of the tokenizer. This one’s very simple.
All we're gonna do here is we're gonna ask the user – I’ll just scroll over a little bit. We're
gonna ask the user to enter some lines to tokenize and we're gonna write out the tokens of
the string R, and then we're gonna call the message sprint token. What sprint token’s
gonna do, it’s gonna take in the string you want to tokenize.
It creates one of these string tokenizers – I’m so lost without you. [Laughter]. Can we
make Marilyn snap? No. I know it’s like great fun to listen to when you're, like, 14, and
you just broke up with a girlfriend for the first time. And then, after that, you want to kill
the next time you hear it. Fine, So, tokenizer. I’m glad we're having fun though.
So what I’m gonna do is I’m gonna count through all the tokens. So I’m gonna a foreloop
interestingly enough. Here’s something funky. I’m gonna have a foreloop, but the thing
I’m gonna do in my foreloop, my test, is not to check to see if I’ve reached some
maximum number. But my test is actually gonna be to see if tokenizer has more tokens.
So I have a foreloop that’s just like a regular foreloop, but I start off with a count that’s
equal to zero, and you're like that looks okay. I do a count ++ over here, and you're like
that’s okay, what are you counting up to Marilyn. And I say I’m counting up to however
many tokens you have. And you go, oh, interesting. So my condition’s to leave, or to
continue on with the loop, is tokenizer has more tokens.
If it has more tokens, then I’m gonna do something here to get the next token. I’m gonna
keep doing this loop. But what the counter’s gonna give me is a way to count through all
my tokens. So I can write out token number count, and then a colon, and then write out
the next token that the tokenizer gives me. Is there any questions about there? Let’s
actually run this puppy [inaudible]. Do, de, do. You can feel free to keep singing now if
you want, if you want.
All right, so we're gonna do our friend. What’s our friend called? The tokenizer example.
Do, do, do, do, we're running the tokenizer, interline’s tokenized, so I might say “I, for
one, love CS.” We're very formal here. And it says the tokens of the string are on notice.
It got the “I” and the comma together as one token because as we talked about, spaces are
the delimiter. And so “for” and then “one” with the comma, and “love” and “CS,” and
that’s all the tokens we got.
And so at this point you might be thinking, yeah, man, that’s great, but you know what. I
really don’t like punctuation. And sometimes I don’t like punctuation, but I can’t stop the
user from using punctuation because even though I don’t like to be grammatically
correct, they do. So how do I prevent them from being grammatically correct as well,
which is kind of a fun thing to do.
What you can say is hey, what I want to do is change my tokenizer, so that it not only
stops at spaces, but it’s gonna stop or consider a delimiter, any of this list of
characteristics that I give it. So you give it a list of characteristics as a string. So here I’m
gonna give it a comma and a space, okay. And this version of the string tokenizer
constructor, what it will do is it will actually tokenize the string.
But think of the thing that you're using as your delimiter, or what chops up your
individual tokens as either a comma, or a space, or anything you want to put in that string
there. Each of the individual characters in that string is treated as a potential delimiter. So
if you say “I for one love CS,” ah, no commas. Why? Because commas are considered
delimiter.
So it just gives you everything up to a comma or a space, and you could imagine you
could put in period, and exclamation point, and all that other stuff, if you just want to get
out the non punctuation here. So tokenizing is something that’s oftentimes useful if you
get a bigger piece of text, and you want to break it up into any individual words, and then
maybe do something on those individual words, okay. Any questions about tokenization?
Hopefully, it’s not too painful or scary. All right.
So the next thing I want to do, will just pay for the smorgasbord of string, is I want to
teach you about something that’s really gotten to be an important thing about computer
science these last few years, which is, basically, this idea known as encryption. And
encryption is something that’s been around for thousands of years. All encryption is, is
it’s kind of like sending secret messages.
You have some particular message. You want to send it to someone else, but you want to
send a secret version of that message. And people have been doing this for thousands of
years, actually, interestingly enough. They just didn’t have very good methods of doing it
until about the last, oh, 50 years. But you know they did it for a long time, and people
broke encryption.
As a matter of fact, there’s this really interesting book by Simon Singh. I’ll bring in a
copy, perhaps next class, if you're really interested, about the whole history of encryption.
It goes back thousands of years, and how, like, wars, and queenships, and kingships, and
stuff, were, basically, lost in one on the strength of how well someone could break a
piece of code.
But the basic idea of encryption, and it probably dates back even further than this, but one
of the most well-known ones is something that’s known as the Caesar cipher, not to be
confused with the salad. But the basic idea with the Caesar cipher – I picked up the
wrong newspaper – the Caesar cipher is that what we want to do is, basically, take our
alphabet and rotate it by some number of letters to get a replacement.
What does that mean? That’s just a whole bunch of words. So let me show you a little
slide that just makes that clear. So in Caesar’s day – I will now play the role of Caesar. I
actually considered wearing a toga to class today. I just thought that was fraught with
way too much peril. So I just decided to bring my little Caesar crown. And that’s what
I’m trying to find my little crown of reason stuff, but I couldn’t. So I just got a little hat.
[Laughter].
And so the basic idea – say you are Caesar – well, I did crown myself, actually. I knew
someone here could actually take the crown from [inaudible]. That was Napoleon, a
whole different story. I really like to take history and mix it up. It’s just to see if you're
actually paying attention. All right, the basic way the Caesar cipher works is we take our
original alphabet. Here’s all of our letters from A through Z.
We take that whole alphabet, and we shift it over some number of letters. Like let’s say
we shift it over three letters. So I take this whole thing, I shift it over three letters, so now
the D lines up over here where the A should have been so I’ve shifted over these bottom
characters. And the characters that kind of went off the end here like the A, B, and C,
were kind of like whoa, we're going off the end. Where do we go? We just kind of shuffle
them back around over here.
So the basic idea is we're gonna rotate our alphabet by N letters, and N is 3 in the
example here, and N is called the key. So the key of the Caesar cipher is how many
letters you're actually shifting. And then we wrap around it again. And now, once we've
done this little wraparound, we take our original message that we want to encrypt. That’s
something that’s referred to as the plain text. The plain text is your actual original
message.
And we want to encrypt that or change it to our cipher text, which is what the encrypted
message is, by using this mapping. So every time an A appears in the original, we replace
it by a D. And a D appears in the original, we replace it by G, and a C appears in the
original, we replace it by an F, etc., for the whole alphabet. Is there any questions about
the Caesar cipher?
This is actually an actual cipher that, evidently, historians tell us that Caesar used in the
days of yore. And you know, evidently, he was killed, so it didn’t work that well. But you
know most people, that’s one of the things that when you were a little kid, and you had
like the Super Secret Decoder Ring, you were probably getting a Caesar cipher. All right,
any questions about the basics of the Caesar cipher.
So what we're gonna do is let’s write a program that actually can be able to encrypt and
decrypt text according to a Caesar cipher, and we'll do it doing pop-down design. So we'll
actually just do it on the computer together 'cause it’s more fun that way. And because
I’m Caesar, I will drive. So we're gonna have my Caesar cipher, all right. And I just gave
you a little bit of a run message here. It’s kind of the very beginnings of the program.
But all this does – it’s not a big deal. It says this program uses a Caesar cipher for
encryption. It’s going to ask for the encryption key. That means it’s asking for the
number by which it’s gonna rotate the alphabet to create your Caesar key, or to create
your Caesar cipher, and that’s just our key that’s an integer. So our plain text, that’s the
original message that we want to encrypt. We ask the user for the plain text, so we just
get a line form the user.
And then what we're gonna do is we're gonna create our cipher text, or the encrypted text
by calling a function called encrypt Caesar. We’re sort of giving a directive. It’s kind of
like an inquisitive tape. Encrypt Caesar, and we give it the plain text, and we give it the
number for the key that we want it to encrypt using. And then, hopefully, that will give us
back the encrypted string, and we're just gonna write that out, okay. S
o how do we do this encryption? All right, so at this point, and it should be clear that the
thing we want to write is probably encrypt Caesar. So what we're gonna do is we're gonna
write a pleasant message, and what is this puppy gonna return to us? String, right 'cause
that’s what we're expecting, the encoded version of this particular message as a string. So
we'll call this encrypt Caesar.
And what’s it getting past? It’s getting past the string, which we'll just call STR, and it’s
getting past an integer, which will we will refer to as the key. So if I want to think about
doing the encryption, right, what I’m gonna do is, on a character-by-character basis, I
want to do this replacement. I want to say for every character that I see in my original
string, there is some shifted version of that character that I want to use in my encrypted
string.
So in order to do that, I’m gonna use my standard kind of string building idiom, which
says I start off with a string, which I’ll call results, which starts of empty, right. It says,
quote, quote, empty string. And I’m gonna do a foreloop through my string that I’m
giving to encrypt. So up the string’s length, I’m just gonna count through and get each
character. So I’ll so sort of a standard thing.
I’m gonna say CH and I’m gonna essentially get the character that I want to get from the
string, so I’ll say STR.char@chat@char@I. So I’ve now gotten my character. I want to
figure out how to encrypt that character, okay. So I think to myself, wow, gee, while
encrypting the character involves all this stuff, doing the shift and all that, that’s kind of
complicated. Maybe I should just create a function to do it. All right, that’s the old notion
of pop-down design. Any time you get somewhere, well, you're, like, wow, that’s kind of
complicated.
Maybe I don’t want to stick this all in here and figure it out. But it’s the smaller piece,
which is just dealing with a single character instead of dealing with the whole string. Let
me write a function that will actually do it, or a method that’ll actually do it. So what I’m
gonna do, is I’m gonna append to my results what I get by calling encrypt, a single
character.
So I’ll just call it encrypt char and what I’m gonna pass to it is the character that I want
encrypt, and I need to also pass to it the key so it knows how to do the appropriate
shifting to encrypt that character. And after it does this encryption, I’m just gonna say
hey, if you’ve successfully encrypted all of your strings, what I want to do is return,
RTN, my results, right. That’s your standard string idiom.
I start off with an empty string. I do some kind of loop through every character of the
string. I’m gonna do the processing one character at a time, and return my results.
Everything in that function that you see, or in that method that you see, except for that
one line, should be something you can do in your sleep now. You’ve seen it, like, over
and over. We just did it a couple of times today. We did it a couple times last time. It’s
the standard kind of thing for going through a string one character at a time.
And now, we reduced the whole problem of encrypting a whole string to the problem of
just encrypting a single letter. So what I’m gonna have in here is private, and this is
gonna return a single character called – and this puppy’s called encrypt char. And it’s
gonna get passed in some character to encrypt as well as the key that it’s gonna use to
encrypt it. And now I want to figure out how do I encrypt that single character.
So what’s something I could do to think about how this character actually gets encrypted.
How do I want to do the appropriate shifting of the character. So let’s say I’ve gotten an
uppercase A. Let’s assume for right now all my characters are uppercase. As a matter of
fact, that’s a perfectly fine assumption to make.
The solution you’ve gotten to, it assumes all the characters are uppercase, so assume all
the plain text is uppercase, and I want to return to the encrypted cipher text also in
uppercase. Let’s say I’ve gotten an uppercase A, okay. And my T is 3. So I want to do, is
take that A somehow, and convert it to a D. How do I do that?
Student: [Inaudible].
Instructor (Mehran Sahami): Un huh. I want to add 3 to the character. Now the only
problem is I might go off the end of the character. If I just add 3, and I have a Z, I’m
gonna – if I just have the A and go to D, that works perfectly fine, but if I have a Z, I’m
gonna get something like an exclamation point, or something I don’t know 'cause I go off
the end of the character. So I need to do slightly a little bit more math. And what I’m
gonna do is say take this character, and subtract from it uppercase A.
That’s gonna tell me which character in the alphabet it is, which number character it is,
right. Now, if I add the key, what I get is the number, or the index, of the shifted
character. So if I had an uppercase A, and I subtract off uppercase A, I’m gonna get a
zero. I now add the key, so I get 3. And you might say, well, if you just convert that to a
character, you get a D. That’s perfectly fine. Yeah, but if I had a Z and I subtract off an
uppercase A, I get 25.
If I add 3 to 25, I get 28, which is now outside the bounds of the alphabet. How do I wrap
around that 28 back to the beginning of the alphabet. Mod it by 26, or we do with the
remainder operator by 26, right. So what that does is it says if you’ve gone off the end,
basically, when you divide by 26 and take the remainder, if you’ve gone off the end, it
kind of gets rid of the first 26, and wraps you back around the beginning.
So if I do that, this will actually work to get me the position of the character wrapped
around, and once I’ve gotten the position of the character, here’s the funky thing. I need
to add the A back in because if I have, let’s say, an uppercase A to being with, and I
subtract out uppercase A, that gives me zero. I add the key. That gives me 3. I do the
remainder by 26.
Three divided by 26 as the remainder is still 3. So now I have the number 3. I need to get
that 3 converted to the letter D. How do I do that? I add the letter A to that 3, okay. Is
there any questions about that?
Now, the final funky thing that I need to do, is if I want to assign this to a character, I
can't do this directly. Notice if I try to do this directly, I get this little thingy here. And
you might say Marilyn, what’s going on? Like you told me characters were the same as
numbers, and everything I’ve done so far has to do with numbers, so why can’t I assign
that to a character?
And this little error message comes up. And this has to do with the same thing when we
talked about converting from real values to integers. Remember when we went from a
real value to an integer. We said you'll lose some information if you try to truncate a real
value, like a double to an integer. So you explicitly have to cast it from being a doublesplint
integer. Same thing with characters and integers. The set of possible integers is
huge. It’s like billions and billions. The set of characters is much smaller than that.
So if you want to go from an integer back to a character, you need to explicitly say
convert that integer back to a character. So we need to explicitly do a cast here back to a
character. And if we do that, then we're happy and friendly. Did I get all my friends
right? One, two, three, one two three. All right, why is this still unhappy? Oh, duplicate
variable CH, yeah. Let me call this C.
Actually, let me make my life easier. This is a thing I just want to return, so I’m just
gonna return it. Do, do, do, do, do. I won’t even assign it to any temporary variable. We'll
just return it 'cause now I’m upset. No, I’m really not upset. We're just gonna return it.
So, hopefully, that will give us our little Caesar cipher. So let’s go ahead and run this, and
see if, in fact, it’s working. Any questions about this while this is running?
I’ll sort of scroll this down a little bit so you can see what’s going on for that single
character. So this was my Caesar cipher. So we say, et tu, brute. Illegal number format.
Yeah 'cause that’s not the thing I wanted to encrypt. My encryption here is 3, then I will
give it the plain text I was to – everyone’s like what did he do. Sometimes it’s the
obvious that’s wrong, and you just need to read. All right, there we actually go.
Now, there’s a little problem here. See, the little problem is the spaces actually got
encrypted. We don’t want to encrypt spaces. We only want to encrypt things that are
actually valid characters. So we're not quite done yet. What we need to do is come back
over here and say, hey, you know what, for my encrypt character, I wasn’t quite as bright
as I thought I was. I need to make sure this thing’s actually in uppercase character before
I try to encrypt it.
So we can sort of do that if I just call my little friend character. And the thing we want to
say is, is uppercase, and I’ll pass at CH. So if it’s already – if it’s an uppercase character,
then I’ll return this. Otherwise, what I’ll do – I’ll tab this in – is I will just return CH
unchanged. So if I’ve gotten something that isn’t actually a character, then I’ll return –
do, do – why is this unhappy again. Oh, semicolon, thank you. All right.
Student: [Inaudible]
Now I got an extra one. Notice it doesn’t give me an error on the extra one 'cause,
actually, semicolon without a statement is the emsin statement. It’s perfectly fine, but
thank you for catching the straight semicolon. So we'll go ahead and run this, and we'll
try our friend, et tu, Brute, again. Sometimes it’s all about texting, and so we have et tu,
Brute, and now we're okay 'cause we're not encrypting anything that is not a letter.
So sometimes we think we're okay. We need to go back and just make sure we actually
do the texting. Any questions about this? If this all made sense to you, nod your head. If
this didn’t make sense to you, shake your head. Feel no qualms about shaking your head.
If you're someone in the middle, just stare and stare at me. No, if you're someone in the
middle, shake your head. Okay, un huh.
Student: [Inaudible].
Instructor (Mehran Sahami): Why don’t I need an L statement, like, say here? 'Cause if
I hit the return, I return from the function immediately and I never, actually, get it down
to this return. So if I hit this return statement, I’m done with the method. As soon as I hit
that return, it doesn’t matter if there’s any more lines in the method. I’m done. I actually
return out.
So the one other thing we might like to do with this that doesn’t quite actually work right
now. Let’s actually try running this, then I’ll show you what happens, just to show you
that it’s bad time. If I actually encrypt something like et tu, Brute, and I want to decrypt
it, I might say hey, try to use minus 3 as your key, and if I try to put in the text – I don’t
even remember what the text was that I wanted to encrypt. I guess this funky thing with
questions marks, and it’s just not working to move in the negative directly.
So I want to allow for my Caesar cipher to also be able to decrypt information, which
means if I got a Caesar cipher by encrypting with a key of 3, if I give it the text that’s
been encoded, and I give it the key minus 3, it should shift it back three letters and
actually work for me. So how do I do that? Well, it’s something that has to deal with each
individual character.
If I want to encrypt each individual character, I need to figure out what’s the right way of
using the key, okay. Think about a key of minus 3. What’s a key of minus 3 equivalent
to? A key of 23, right. A few people mumbled it, so we'll just throw out some candy. If I
want to go 3 in the opposite direction, if I want to go 3 sort of this way, as opposed to this
way. It’s the same thing as going 23 characters in the opposite direction.
So if I want to think about doing that, I can say, if my key is a negative number, so if my
key is less than zero, there’s some shifting I need to do of the key to actually get this
puppy to work. So if my key is less than zero – as a matter of fact, I’m gonna do this once
down here. So rather than doing it, and encrypting each character, I’m gonna do it over
here by saying you know what, once I shift my key over, I want to use that same key to
encrypt all my characters.
So I want to do the shifting just once up here. It makes sense to do it once for the string,
and then I’ll use my updated version of key. So here’s what I’m gonna do. I’m gonna say
take the key, and the way I’m gonna update key is I’m gonna say it’s 26. And this looks a
little bit funky, but I’ll explain it to you in just a second – modded by 26.
And you might say Marilyn, why do you need all this math to actually pull it off 'cause if
you do something, you could say why can’t you just take 26 and subtract from it your
key. So if you want to say minus – or add toward your key. So if you want to have a key
of minus 3, isn’t that just the same as adding minus 3 to 26. You'll get 23, aren’t you fine.
Yeah, that’s fine for sufficiently small values of key.
So if this thing actually is minus 3, minus, minus 3 gives me 3, and if I were to – oh,
missing a minus in here. Sorry, my bad. I had two minuses. I want to have another minus
right there. So if key is minus 3, and I take a negative of minus 3, that gives me 3.
Twenty-six minus 3 by itself would give me 23, which is the value I care about and that’s
perfectly fine.
But what happens if this key that someone gives me is something, for example, that’s
larger than 26. That’s kind of bad time because if I subtract a number that’s larger than 26
from the 26, so if this happens to be minus, let’s say 27, and I say minus minus 27 is
positive 27. And I subtract 27 from 26, I get minus 1. That’s bad time.
So the reason why I have this 26 in here, is it says first take the key. They gave me some
negative value. Take the negative of that, which gives you some positive value. When
you mod it by 26, you will guarantee that that value they’ve given you is less than 26
'cause if it was 26, 26 mod, 26 is zero. Something larger than 26 gives me a remainder.
So as long as I mod by 26, I will always get back the appropriately mapped value, less
than 26, and then I will subtract that [inaudible]. So just to make sure this actually works,
what I’m gonna do is in my main program, I’m gonna say encrypt Caesar using this key.
And then, do, do, do, so I have some cipher text.
I’m going to now – well, actually, let me write out the cipher text so I’ll still use this print
link. And then, I’m going to have some other string, new plain. A new plain is just going
to be doing encrypt Caesar on my cipher text, so that should be my encrypted text with
the negative of the key. So I want to, essentially, switch back to what I’ve got. And so I’ll
have print link new plane quote dot whatever the new – man, I cannot type to save my
life – L print link. Thank you.
So now I run this puppy in our final moment together, 3 et tu, Brute. Well, at least I got it
back even though I misspelled it. I got my mixed-up characters, and then I got my new
plain text, which is the same as my original text, which I got just by, essentially, shifting
in the negative direction. So any questions about that. Allrighty, then we're done with
strings for the time being, and I’ll see you on Wednesday.
[End of Audio]
Duration: 50 minutes