Chapter 5

The ____ function returns the numeric (ordinal) code of a single character. give example of what this looks like

ord in: ord("A") out: 65

Using ___ and ___ we can convert a string into and out of numeric form.

ord char

In ______ ______ encryption, there are separate keys for encrypting and decrypting the message.

public key

with cease cipher and vegenere cipher In a this system the _____ key is used for encrypting and decrypting messages. Everyone you know would need a copy of this key to communicate with you, but it needs to be kept a secret.

same

Strings are always sequences of characters, but _____ can be sequences of arbitrary values. they can have numbers strings or both

lists

how do you evaluate a string as an expression?

eval(<string>)

how do you encode an algorithm? how would this be done with a for loop?

get the message to encode for each character in the message: print the letter number of the character A for loop iterates over a sequence of objects, so the for loop looks like: for ch in <string>

what is the outline for a decoder?

get the sequence of numbers to decode message = "" for each number in the input: convert the number to the appropriate character add the character to the end of the message print the message

___________________ uses numbers 0 through _____ to represent the characters typically found on an (American) computer keyboard, and control codes that are used to coordinate

ASCII system (American Standard Code for Information Interchange) 127

The process of encoding information for the purpose of keeping it secret or transmitting it privately is called _______.

encryption

_________ is used when transmitting credit card and other personal information to a web site.

encryption

In public key systems, the _______ key is made publicly available, while the ________ key is kept private.

encryption decryption only person with decryption key can decrypt it.

How do we get the sequence of numbers to decode?

Read the input as a single string, then split it apart into substrings, each of which represents one number.

how does slicing work? use the greet= Hello Bob example to show this

The slice contains the substring beginning at position start and runs up to but *doesn't include the position end.* <var>[<start>:<end>:<step>] ex: in: greet[0:3] out: 'Hel' in: greet[2:7:2] out: 'loB' *if you do not put an <end> it will include the whole string(position end)

explain how the split method can be used for decoding

This will split a string into substrings based on spaces. in: "Hello string methods!".split() out: ['Hello', 'string', 'methods!']

explain two ways to get multiple values in a single input string

Turning it into a list using the split method Convert the string numbers into their corresponding numbers using eval

with ASCII: letters A-Z are represented by the values___-____, the lowercase versions have codes ___-____. how much memory is this?

65-90 97-122 1byte(8bits)

how do you get a string as input?

<variable>= input("<>:")

explain how to build a program that gives a three letter abbreviation for a month given its number value (use pos function)

#store names in one string months = "JanFebMarAprMayJunJulAugSepOctNovDec" find each position of the months by sub 1 from month number and multiplying by 3 (ex: Mar is the third month, so (3-1)*3=6)-remember"j"starts at 0 n= int(input("enter a month number...") pos= (n-1)*3 monthabbrev= months[pos:pos+3] print(monthAbbrev)

Unicode is a superset of ASCII, and the numbers __-___ have the same meaning in ASCII as they have in Unicode. ?UTF-8, UTF-16 are _____ length encoding?

0-128 variable

how are strings stored in a computer?

as a sequence of binary numbers (1's and 0's)

The ___ function converts a numeric code to the corresponding character. give example of what this looks like

chr in: chr(97) out: 'a'

"glues" two strings together (+)

concatenation

________ is the study of encryption methods.

cryptography

Let's say we want to enter a date in the format "05/24/2015" and output "May 24, 2015." How could we do that?

dateStr = input("Enter a date (mm/dd/yyyy): ") monthStr, dayStr, yearStr = dateStr.split("/") # convert monthStr to the month name months = ["January", "February", "March", "April","May", "June", "July", "August","September", "October", "November", "December"] monthStr = months[int(monthStr)-1] print("The converted date is:", monthStr, dayStr+",", yearStr) may need to int("dayStr")? so 05=5

how would you create a program that turns the day month and year into the mm/dd/yyyy format?

day, month, year = eval(input("Please enter day, month, and year numbers: ")) date = str(month)+"/"+str(day)+"/"+str(year)

To ________ the message, the receiving end needs an appropriate key so the encoding can be _______. what are two cipher examples

decrypt reversed Eg. Caesar cipher , Vegenere cipher

show how you would get someones username using first initial, first seven characters of last name:

in: #get users first and last name first= input("...:") last= input("...:") #concatenate first initial with 7 chars of last name uname= first[0]+last[:7]

Split can be used on characters other than space, by supplying the character as a ________. show an example of this

in: "32,24,25,57".split(",") out: ['32', '24', '25', '57']

how would you repeat the string "spam" 3 times using repetition?

in: "spam" * 3 out: 'spamspamspam

how would you combine the strings "spam" and "eggs" using concatenations?

in: "spam" + "eggs" out: 'spam eggs'

how do you put spaces between the letters in a string? use the string "Spam!" for an example

in: for ch in "Spam!" print(ch, end=" ") out: S p a m !

explain how appending lists can decode (also using join function)

inString = input("Please enter the Unicode-encoded message: ") # Loop through each substring and build Unicode message chars = [] for numStr in inString.split(): # convert digits to a number codeNum = int(numStr) # accumulate new character chars.append(chr(codeNum)) message= " ".join(chars) print (message)

explain how you would create a program to decode a unicode message

inString= input("enter unicode message...") #create accumulator variable message= " " for numStr in inString.split(): #convert digits to a number codeNum= int(numStr) # concatenate character to message message= message+chr(codeNum) print decoded message...

________ returns a string containing a single character from a larger string.

indexing

how can you access the individual characters in a string? how are they positioned? use example: greet= Hello Bob

indexing numbered from the left starting with 0. h-0 e-1 l-2 etc. input: print(greet[0]),greet[2],greet[4]) output: H l o

2 useful operations for chopping strings into smaller pieces:

indexing slicing

how do you find the length of a string?

len("<>")

when programming a decoder: The variable ______ is an accumulator variable, initially set to the empty string, the string with no characters (""). Each time through the loop, a number from the input is converted to the appropriate character and appended to the end of the accumulator.

message

explain how to write a program to convert a textual message into a sequence of numbers, utilizing the underlying Unicode encoding.

message = input("Please enter the message to encode: ") print("\nHere are the Unicode codes:") # Loop through the message and print out the Unicode values for ch in message: print(ord(ch), end=" ") print() # blank line before prompt

how many characters are in Unicode?

million +

how can you use a list in the months abbreviation example to make things simpler?

months = ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"] monthAbbrev = months[n-1] *subtract 1 from n because jan=0 not 1

Lists are ______, meaning they can be changed. ______ can not be changed.

mutable strings

Modern encryption converts messages into _______. Sophisticated mathematical formulas convert these numbers into new numbers - usually this transformation consists of combining the message with another value called the "_____"

numbers key

What is a string object?

objects have both data and operations (they "know stuff," and "do stuff.")

builds up a string by multiple concatenations of a string with itself (*)

repetition

explain these string methods: s.capitalize() s.title() s.center(width) s.count(sub) s.find(sub) s.join(list) s.ljust(width) s.rjust(width)

s.capitalize() - Copy of s with only the first character capitalized s.title() - Copy of s' first character of each word capitalized s.center(width) - Center s in a field of given width s.count(sub) - Count the number of occurrences of sub in s s.find(sub) - Find the first position where sub occurs in s s.join(list) - Concatenate list of strings into one large string using s as separator. s.ljust(width) - Like center, but s is left-justified s.rjust(width) - Like center, but s is right-justified

explain these string methods: s.lower() s.lstrip() s.replace(olds, newsub) s.rfind(sub) s.rstrip() s.split() s.upper()

s.lower() - Copy of s in all lowercase letters s.lstrip() - Copy of s with leading whitespace removed s.replace(oldsub, newsub) - Replace occurrences of oldsub in s with newsub s.rfind(sub) - Like find, but returns the right-most position s.rstrip() - Copy of s with trailing whitespace removed s.split() - Split s into a list of substrings s.upper() - Copy of s; all characters converted to uppercase

strings are really a special kind of _______.

sequence

The ______ method can be used to add an item at the end of a list. give an example

squares = [] for x in range(1,11): squares.append(x*x) We start with an empty list ([]) and each number from 1 to 100 is squared and appended to it ([1, 4, 9, ..., 100]).

how do you convert a number to a string? why might you need to do this?

str(<>) to add things like periods and commas, if we use "+" with interest python will not be able to interpret because it will think you are trying to add, so need to make into string

Text is represented in programs by the _____ data type.

string

a sequence of characters enclosed within quotation marks (") or apostrophes (').

string

The encoding/decoding programs we wrote use a ________ _______, where each character of the original message, known as the ________, is replaced by a corresponding symbol in the cipher alphabet. the resulting code is known as the _______

substitute cipher plaintext ciphertext

We can also access a contiguous sequence of characters, called a ____, through a process called _____.

substring slicing

a much larger standard that includes support for the characters of nearly all written languages

unicode

how do you index from the right side?

using negative indexes In a string of n characters, the last character is at position n-1 since we start counting with 0. sooo <var>[-1]= the last letter in the string

The most common use of personal computers is ____________.

word processing

what is the string format method?

{} within the template-string mark "slots" into which the values are inserted. Each slot has description that includes format specifier telling Python how the value for the slot should appear.