ï¿½ï¿½Chapter 18-19 List, For Loops · 2019. 10. 1. · Title: ï¿½ï¿½Chapter 18-19...

Strings and Random Numbers

Caesar and Vigenère CiphersJohn K. Bennett

IWKS 2300Fall 2019

String Special CharactersSpecial characters can be inserted in a string using an escape

sequence, which is a backslash (\) followed by another character.

Here are some common escape sequences:

\" Double Quote \\ Backslash

\n Newline \t Horizontal Tab

print("Favorite Color:\n\t\"Glow in the Dark\"")

Favorite Color:

"Glow in the Dark"

Single or Double Quotes: Your Choice

Strings can be written using either single or double quotes, your choice.

primary = 'Python'

secondary = "English"

Using single quotes means no need to escape double quotes:

Using double quotes means no need to escape single quotes:

print('So you must be "the one"?')

print("Margaret's house is blue.")

Strings Are Like Lists (almost)

Strings are like lists containing characters:

my_name = "Jack"

print(my_name[0])

J

my_name = "Jack"

my_name[0] = "T" # produces item assignment error

But unlike lists, strings cannot be modified

Strings Are Iterables

You can iterate over each character of a string.

for char in "IWKS 2300"

print(char)

I

W

K

S

2

3

0

0

Spliting strings with .split()To separate the words in a string into a list, call .split() on the string.

my_str = "Python is really cool"

word_list = my_str.split()

# wordlist will be ["Python", "is", ... ]

for word in word_list:

print(word)

Python

is

really

cool

Note: The dot (".") above is called the accessor operator (though most programmers simply call it the dot operator). This syntax allows us to use a function that is specific to a certain data type on the object.

Using .split()

The input() function returns a string containing the line that the user typed. Often you need to split that input, e.g.:

line = input("What is your full name?")

words = line.split()

firstname = words[0]

lastname = words[1]

Joining Strings With .join().join() does the opposite of .split(). It also allows you to specify

what you want to use as a separator.

separator = "@"

word_list = ["jkb", "ucdenver.edu"]

email = separator.join(wordlist)

print(email)

[email protected]

String Formatting - Old StyleThree basic formatting specifiers (C-Style) are:● %s - String● %d - Integers● %f - Floatsname = 'John'

i_age = 21

f_age = 21.5

print('Hello, %s. Are you %d or %f?' %(name, i_age, f_age))

print()

print("We can print %f or %.2f. How?" %(f_age, f_age))

Hello, John. Are you 21 or 21.500000?

We can print 21.500000 or 21.50. How?

String Formatting - New StyleThe "new style" string formatting gets rid of the %-operatorsyntax, and instead calls the .format() function on a string object.

name = 'John'

i_age = 21

f_age = 21.5

print('Hello, {}. Are you {} or {}?'.format(name, i_age, f_age))

print()

print('Hello, {name}. You are {age}.'.format(name=name, age=i_age))

Hello, John. Are you 21 or 21.5?

Hello, John. You are 21.

New New String Formatting: f-strings

Python 3.6+ offers a new formatting string approach: "f-strings". This new way of formatting strings lets you use embedded Python expressions inside string constants.

name = 'John'

i_age = 21

f_age = 21.5

print(f"Hello, {name}. Aren't you really {i_age - 3}?")

Hello, John. Aren't you really 18?

ASCII Character Representation

Character Manipulation in Python

The Python function ord() returns the ordinal ASCII number of a character, e.g.,

print(ord('A'))results in:65

Similarly, the Python function chr() is the inverse of ord(), e.g., print(chr(122))

results in:z

Character Manipulation in Python

The Python character methods isupper()and islower() return Trueif the character is uppercase or lower case, respectively.Running this code:

lchar = 'a'

uchar = 'G'

if lchar.islower():

print (lchar + ' is a lowercase letter')

if uchar.isupper():

print (uchar + ' is an uppercase letter')

will produce the following output:a is a lowercase letterG is an uppercase letter

How to Generate a Random Number

Import the Random Library:import random

To create a random integer that ranges from [a, b]random.randint(a, b)

To create a random float that ranges from [0, 1)random.random() # no parameters, includes 0, excludes 1!

How to generate a random integer that ranges from 1 to 10?

How to generate a random integer that ranges from 0 to 7?

>>> random.randint(1, 10)

>>> random.randint(0, 7)

How to generate a random float that ranges from 0 to 5?

>>> random.random() * 5 # number from [0, 5)

How to generate a random float that ranges from 1 to 6?

>>> random.random() * 5 + 1 # number from [1, 6)

Example of Generating Random Numbers

Practice: How Do You…

• Generate a random integer that ranges from [5, 8]:

• Generate a random float that ranges from [0, 2.5]:

• Generate a random float that ranges from [3, 7]:


• Generate a random float that ranges from [2.5 to 3.75]:

Practice: How Do You…

• Generate a random integer that ranges from [5, 8]:





random.randint(5, 8)

Practice: How Do You…• Generate a random integer that ranges from [5, 8]:






random.random() * 2.5








random.random() * 4 + 3









random.random() * 4.5 + 3









random.random() * 4.5 + 3

random.random() * 1.25 + 2.5

Each letter in the plaintext message is replaced by a letter some

fixed number of positions from that letter in the alphabet. For

example, with a left shift of three (the "key" purportedly used by

Caesar), D would be replaced by G, E would become H, e.g.:Plaintext Alphabet: Cipher Alphabet:

Caesar Cipher

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

X Y Z G H I J K L M N O P Q R S T U V W X Y Z A B C

To encrypt a message, we replace each plain alphabet letter

of the plaintext message with the cipher alphabet letter

corresponding to the plain letter. Example:

Plaintext: WE WILL ATTACK AT DAWN UNLESS ITS RAINING

Ciphertext: ZH ZLOO DWWDFN DW GDZQ XQOHVV LWV UDLQLQJ

To decrypt the message, we apply the opposite shift.

Caesar Cipher Encryption

The "Vigenère Cipher" improves upon the Caesar cipher by

applying it differently for each letter, using the letters of a

"keyword" to determine the specific Caesar shift to use for each

letter. The Vigenère cipher is named for Blaise de Vigenère, who

wrote of it, but the cipher was actually invented by Giovan

Battista Bellaso in 1553, who was building upon the work of

others, most notably, Johannes Trithemius, who created the

tabula recta, on which the Vigenère cipher is based.

The tabula recta is just a table of all possible Caesar ciphers.

The Vigenère Cipher chooses a particular Caesar cipher for

each letter based upon a Key.

Vigenère Cipher

Tabula Recta

A Vigenère cipher will use

only as many Caesar

cipher keys in the Tabula

Recta as there are unique

letters in the Key. If the

message is longer than the

Key, the letters of the Key

will be reused in the same

order

Plaintext: WE WILL ATTACK AT DAWN UNLESS ITS RAINING

KEY: COMPUTERSCIENCEISAWESOME

Ciphertext: YS IXFE EKLCKO NV HION QRDSEW KHE GUBRZFI

Vigenère Cipher Example If the first letter of the plaintext message is W, and the first letter

of the Key is C, row C and column W of the Tabula Recta yields

a Y. So, to encrypt a message, we replace each plain alphabet

letter of the Plaintext message with the Ciphertext letter found

in the tabula recta at the location determined by the Key.

Vigenère Cipher Implementation"Write a Python program that implements the Vigenère

cipher, allowing the user to specify the plaintext and the key

(for encryption), or the ciphertext and the key (for decryption

Make sure you handle both upper case and lower case

letters, but numbers and other characters (like spaces)

should be passed through unmodified." Keys can only

contain letters.

Hint 1: You do not have to build a tabula recta table to do

the encryption/decryption.

Hint 2: Get the Caesar cipher working first, then extend it to

Vigenère.

Vigenère Cipher Implementation

1. For each letter in the plaintext, extract the corresponding

letter from the key (wrapping the key as necessary).

2. Use the ordinal number of the key letter (minus the

ordinal number of ‘A’ or ‘a’, as appropriate), as the

keyshift for that plaintext character.

3. Then just apply the Caesar cipher for the plaintext letter

with that keyshift.

4. Repeat for each letter in the plaintext message.

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ï¿½ï¿½Chapter 18-19 List, For Loops · 2019. 10. 1. · Title: ï¿½ï¿½Chapter 18-19...

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