lec09, Tue 10/29

String Processing and Cryptography

Lecture 09: String Processing and Cryptography

A la carte

string

• indexing

  s = "Halloween"
    #012345678  positive index
    #        -1 negative index
  

• iteration (using range()) ```python s = “Halloween”

  print index values

  for i in range(len(s)): print (i)

print each letter in s

for c in s: print (c)

use string index

for i in range(len(s)): print (s[i]) #print each letter of s

### iclicker question

* How to print a string backwards?
```python
for i in range("???"):
    print (s[i])

Answer: "???" => (-1, -len(s)-1, -1) # start from the last one, to the first one

start = 2
end = 9 #the loop will never reach the end (i + step < end)
step = 3
for i in range(start, end, step):
    print (i)

module

other useful functions for string:

• find ()

  str.find(item) #return the index of the first occurence if valid, or return -1 if item is not in str
  str.rfind(item) #return the index of the last occurence
  

• count ()

  str.count(item) #count how many times item occurs
  

cryptography

• plaintext -> encryption -> ciphertext
• ciphertext -> decryption -> plaintext

# declare all strings
message = "hello world"
encrypted = ""
decrypted = ""

# generate an alphabet with space
from string import ascii_lowercase
alphabet = ascii_lowercase + " " 
print ('"' + alphabet + '"')

# generate the key
key = alphabet[::-1]
# loop through every letter of message
# find a letter from the key to replace it with 
# assign a new letter to an encrypted message
for letter in message:
    idx = alphabet.find(letter)
    encrypted += key[idx]
print(encrypted)

# look through every letter of the encrypted 
for letter in encrypted:
    idx = key.find(letter)
    decrypted += alphabet[idx]
print(decrypted)

Full Menu

On the menu today: fruity looped strings and indices, a dash of imported modules, and a cipher sundae. But before we begin…

Some words of wisdom

Try not to hard-code values into your programs. Write code that can be reused in many different situations without modification. You can do this by keeping automation in mind.

For example, instead of specifying that there are 26 letters in a string, use the len() function to get that same number. That way, your program can quickly and easily adapt to different cases, such as a string with 33 letters.

Plus, when writing a program, it’s always nice to start with pseudocode and work your way up to algorithms and then actual code. That way, you’ll have an easier time keeping track of your main objective rather than getting bogged down by syntax.

Oh, and run your code every few lines that you write. It’s best to catch errors early on when they’re not as much of a hassle to fix.

Now, a bit of review (or, the appetizers)

(Froot) Loops

A for loop is a count-controlled loop that repeats a specific number of times based on how many items are in a collection, such as a list, a tuple, or a range. As such, you don’t need to tell it specifically when to end, because it ends by itself when it feels like it when it has gone through all of the items in the collection. Here’s the rough format of a for loop:

for VARIABLE in COLLECTION:
    STATEMENT(s)

Compare this to a while loop, which is a condition-controlled loop that does require you to specify an end condition, such as when a counter reaches a certain value.

This lecture will exclusively use for loops. while loops will be saved for another meal.

Strings (and string cheese)

Happy Halloween! Soon. Let’s define a string:

s = "Halloween"

As you (hopefully) recall, individual characters in a string can be accessed with their index. Here are the indicies, both positive and negative, of each of the characters of our string s.

The syntax for accessing characters is

c = s[i]

where c is a variable for storing the retrieved character, s is the variable of the string "Halloween", and i is the index.

Home on the range()

The range() function generates an incremental set of numbers from one number to another number. It sounds simple, but there’s a lot that’s going on. So let’s unpack everything.

Here’s range() written out, with all three possible arguments and their positions represented by placeholder values:

range(start, end, step)

Let’s go through each of those arguments one by one.

Messing around with strings (or, the main course)

String indexing and iteration

Remember the len() function? It’s amazing.

Let’s combine everything we’ve done so far to print every character in string s using a for loop. Here’s what that would look like:

s = "Halloween"
for i in range(len(s)):
    print(s[i])

Here’s what the Python shell returns when we run the code:

H
a
l
l
o
w
e
e
n

If you’re a bit confused about the syntax of the Python code above (especially with the range() part), here’s the same code but with the optional and default range() arguments included:

s = "Halloween"
for i in range(0, len(s), 1):
    print(s[i])

Here, range() is telling the for loop to assign i a value of 0. When the loop finishes running once, range() tells the for loop to increase i by a value (step) of 1. These iterations continue until i is equal to a value of 8, which is one less than the value of len(s), which is equal to 9.

Of course, you don’t need to include 0 at the beginning of range(), nor do you need to include 1 at the end. But these are the implied and unseen values at work here, and sometimes it helps to make the unseen seen.

So if i is first equal to 0, then the for loop is basically telling print(s[i]) to print(s[0]), or to print('H'). Then, since there are no more statements within the loop, a new iteration begins, and i becomes 1. Now, print(s[i]) becomes print(s[1]) becomes print('a') on a new line. And on it goes until print(s[8]) becomes print('n') and the final character, n, is outputted onto the shell.

Bigger steps (or, more range() shenanigans)

Now that you’re (hopefully) more confident about range(), let’s start messing around with the start, stop, and end values and changing their defaults. Perhaps it’ll be easier for you if you actually represent start, end, and step as variables, like so:

s = "Halloween"

start = 2
end = len(s)
step = 3
 
for i in range(start, end, step):
    print(s[i])

And the output of the Python shell:

l
w
n

Printing a string backwards (or, even more range() shenanigans)

You’re not out of the woods yet. You’ve printed all of the characters in a string, and you’ve even printed some of the characters in a string based on a step value. But what if you wanted to print a string backwards?

Well, here you go:

s = "Halloween"

start = -1
end = -len(s) - 1
step = -1

for i in range(start, end, step):
    print(s[i])

Remember those negative indices? Time to put them to use.

All of this outputs:

n
e
e
w
o
l
l
a
H

Edge cases (or, the side dishes)

You’ve done for loops, string indexing, string iteration, and lots of range()-related stuff. Time for some more advanced topics.

Handling index overflow

String s = "Halloween" has positive indices of 0 to 8 and negative indices of -1 to -9, so statements like print(s[6]) and print(s[-3]) work perfectly fine.

But what happens if you choose an index that doesn’t exist in this case, like 42 and -1729, and use them in statements like print(s[42]) and print(s[-1729])? You run into an IndexError and your day is ruined.

Well, not quite. What if there was some way you could “loop” an index back to one of the valid indices? You’re in luck.

s = "Halloween"

i = input("Enter an index: ")
i = int(i)

if i > len(s) - 1:
    i %= len(s)
elif i < -len(s):
    i %= len(s)
    
print(s[i])

As usual, we first define a string s = "Halloween" with which to work with.

Then, the program prompts the user to type in an index and stores that value in variable i. Let’s assume the user types 42 as the input. Because input() normally gives values in the string data type, the program must convert 42 into an integer type using int() so that mathematical operations on the value are possible. (You can also combine the first two lines into something like i = int(input("Enter an index: ")) if you prefer.)

Now comes the index checking part. The first if statement checks to see if the index exceeds the upper indexical bound of string s, which in this case is 8. Again, in the interest of automation, len(s) - 1 becomes 9 - 1 becomes 8 by itself without programmer interference, which is the ideal scenario.

Since we earlier defined i to be equal to 42, i is indeed greater than len(s) - 1. The if statement successfully proceeds to the next statement i %= len(s), which is just a shorthand way of writing i = i % len(s). Recall that the % symbol, or modulo, divides a number on the left side by the number on the right side and returns the remainder of the operation. In this case, i = i % len(s) becomes i = 42 % 9 becomes i = 6.

Because the if statement has been satisfied, the program skips the elif statement during execution and moves on to print(s[i]). Now that i = 6 instead of i = 42, print(s[i]) properly returns the character e instead of a scary IndexError.

If you had a very big negative index (very small index?) like -1729, the elif statement runs instead of the if statement. Notice that the run condition for elif is different than the run condition for if: It’s -len(s), not len(s) - 1. That’s because elif is checking to see if the index you entered exceeds the lower, not upper, indexical bound of string s, which is -9 instead of 8.

Also, if you want, you can condense the if and elif statements into a single line, like so:

if (i > len(s) - 1) or (i < -len(s)):
    i %= len(s)

Parentheses aren’t technically required here, as everything else in the if statement already has a higher precedence than the or keyword, but parentheses help improve readability in more complex lines of code and reduce the chance that you’ll actually run into issues later down the line.

Accessing substrings and string slicing

Let’s, for a moment, step away from all of this if and len() craziness and focus specifically on chopping up and and sticking different parts of strings together. To do that, we’ll need to once again use indices.

Bring up your Python shell and type in a string:

>>> 'mississippi'

Just like in lists and tuples, you can access specific sets of characters, or substrings, within a string by using square brackets, numbers, and separating colons. The first number is where you start, the second number is where you end plus one, and the optional third number is how many numbers you count by. Sound familiar? Yep, it’s pretty much just like range()!

>>> 'mississippi'[0:4]
'miss'
>>> 'mississippi'[0:4:2]
'ms'

However, there are some differences between specifying substrings and specifying a range().

>>> 'mississippi'[4] # Access character at index 4.
'i'
>>> 'mississippi'[:4] # Access characters from the beginning of the string (index 0) to index 3 (4 - 1 = 3).
'miss'
>>> 'mississippi'[4:] # Access characters from index 4 to the end of the string.
'issippi'

You can assign a string to a variable and use the exact same syntax for accessing substrings.

>>> example = 'mississippi'
>>> example[0:4]
'miss'
>>> example[4:]
'issippi'

All of this work so far has been in service to string slicing, which is, well, slicing up a string into smaller pieces, or substrings. You can join these sliced fragments into larger pieces.

>>> 'mississippi'[:4] + 'mississippi'[4:]
'mississippi'

Useful modules (or, the condiments)

Generating an alphabet

If you ever find yourself typing out abcdefghijklmnopqrstuvwxyz on your keyboard because

1. you’re bored (don’t worry, everyone’s done it at some point) and/or
2. you need a string of characters for a cryptography assignment,

simply import the string module and the ascii_lowercase constant at the top of your program file. Voilà! Now you don’t have to worry about typos. ascii_uppercase is also available in case you want, well, uppercase letters.

from string import ascii_lowercase
from string import ascii_uppercase

lowercase_letters = ascii_lowercase
uppercase_letters = ascii_uppercase

Note that since ascii_lowercase and ascii_uppercase are constants, not functions, there aren’t any parentheses that go at the end. You’d treat them like variables, except these “variables” are immutable.

YOu also have the option of importing the entire string module and not just ascii_lowercase and ascii_uppercase. However, in order to access those two constants, you would have to prefix them with string. so that Python knows where you’re getting these values from.

import string

lowercase_letters = string.ascii_lowercase
uppercase_letters = string.ascii_uppercase

Whether you only import select functions or values from a module or import the entire module is up to you.

Generating random numbers

Alright, alright, they’re not actually random. They’re pseudorandom. But we’re not running a lottery or anything, so pseudorandomness is good enough for our purposes.

To get (somewhat) random values in your program, import the randint() function from the random module. As described before, you can accomplish this in two ways:

# Tactic 1
from random import randint

# Tactic 2
import random

Assuming you employed the second tactic, here’s how you would use the randint function:

random.randint(start, end)

start establishes the lower bound of your random-ish number generation and end establishes the upper bound. Note that, unlike the range of numbers in range(), the range of numbers in randint() will go up to and include the final value. So if you had random.randint(0, 1), it would be possible to get 1 as a result instead of just 0.

Analyzing a string

Oh look, more strings. This time you’ll be taking functions from the str class (not to be confused with the string module from above). Since the str class is built into Python, you don’t need to import anything this time around.

Here are some str functions that might come in handy:

• str.find(item) returns the index of the first occurrence of substring item in string str, or -1 if item isn’t found.
• str.rfind(item) returns the index of the last occurrence of substring item in string str, or -1 if item isn’t found.
• str.count(item) returns the number of times substring item appears in string str.

When you’re using these functions, replace str at the beginning of the function with the string or the name of the variable that you’re searching in. For example, if you’re trying to find the index that the character 'e' appears in the string "alphabet", you could do one of the following:

>>> "alphabet".find('e') # Directly look for a character in a string.
6
>>> word = "alphabet"
>>> word.find('e') # Using a variable to look for a character in a string.
6

Cryptography (or, the dessert)

You’ve sliced strings, you’ve combined them, you’ve printed them, you’ve for-looped them, you’ve range()d them, and you’ve indexed them. Now it’s time to apply those skills to something practical.

Lots of vocabulary

If you’re sending a message to someone, you probably don’t want someone else to snoop in and read what you’ve written. This is where cryptography comes in. Cryptography is the process of making communication secure so that your message is understood only by your intended recipients.

Encryption is one of the ways that cryptography works, by converting plaintext, your original message, into ciphertext, a (hopefully) secure and mostly unintelligible version of your original message. Decryption works the opposite way around, by converting ciphertext into plaintext so that your intended recipient(s) can read what you’ve written.

In both cases, you’ll need a cipher—an algorithm for performing encryption and decryption—with which to convert messages back and forth between plaintext and ciphertext. One type of cipher is a substitution cipher, where each character of your original message is mapped to another character in a scrambled but systematic key.

original_message = "abcdefghijklmnopqrstuvwxyz"
key = "zyxwvutsrqponmlkjihgfedcba"

Here, in the process of encryption, 'a' would become 'z', 'b' would become 'y', and so on and so forth. Note that key corresponds to each letter of original_message, so this cipher has a 1:1 mapping. However, this does not always have to be the case.

original_message = "abcdefghijklmnopqrstuvwxyz"
key = "zzzzzzzzzzzzzzzzzzzzzzzzzz"

Here, because key is just a string of 26 z’s original_message would just be converted into, well, a string of z’s.

Example program

From class, here’s an example of what a cryptography program could look like.

from string import ascii_lowercase

def generate_key():
    # return "abcdefghijklmnopqrstuvwxyz "
    return "opqrstuvwxyz abcdefghijklmn"

# Given a message, encrypt it using the key.
message = "hello, world!"
encrypted = ""
decrypted = ""

# Generate an alphabet.
alphabet = ascii_lowercase + " "
print('"' + alphabet + '"')

# Generate the key.
key = generate_key()

# Loop through every letter of message.
# Find a letter from the key to replace it with.
# Assign a new letter to an encrypted message.
for letter in message:
    index = alphabet.find(letter)
    encrypted += key[index]
print(encrypted)

# Loop through every letter of encrypted.
for letter in encrypted:
    index = key.find(letter)
    decrypted += alphabet[index]
print(decrypted)

Acknowledgement

Big thanks to Yiu-On Li and Kevin Wang for their help with creating these notes and adding the narrative.