Variables & Strings

Goals

• Introduce the use of variables• Introduce a new data type: strings• Introduce print statements

Exercise Review: Making Change

• Let’s make change for 91¢Number of quarters: 91 // 25 = ?Number of dimes: (91 % 25) // 10 = ?Number of nickels: ((91 % 25) % 10) // 5 = ?Number of pennies: ((91 % 25) % 10) % 5 = ?

Exercise Review: Making Change

• Let’s make change for 91¢Number of quarters: 91 // 25 = ?Number of dimes: (91 % 25) // 10 = ?Number of nickels: ((91 % 25) % 10) // 5 = ?Number of pennies: ((91 % 25) % 10) % 5 = ?

• This is a bit clunky and the same amount is represented multiple times

Total owed 91¢

Amount owed after quarters: 91 % 25 = 16¢

Amount owed after quarters and dimes: (91 % 25) % 10 = 6¢

Amount owed after quarters, dimes, and nickels: ((91 % 25) % 10) % 5 = 1¢

Variables

• A cleaner way is to use a variable and update it• The = sign is used to assign a value to a variable• Use an informative name to keep code readable

totalAmount = 91

• Variables can be defined using other variablesnumberQuarters = totalAmount // 25

• Variables can be updatedtotalAmount = totalAmount % 25totalAmount %= 25

• Try finishing the rest!

Variables

numberDimes = totalAmount // 10totalAmount %= 10

numberNickels = totalAmount // 5totalAmount %= 5

numberPennies = totalAmount

Variables

numberPennies = totalAmount

• These two variables have the same value, but they are different objects; modifying one does not affect the otherx = 14y = xx += 2x = ?y = ?

Naming Variables

• Variable names must start with a letter

Naming Variables

• Variable names can only contain letters, numbers, and underscores

Naming Variables

• Variable names are case-sensitivenum = 3NUM = 2num – NUM = ?NUM – num = ?

• It’s important is to always use informative names• Read more about naming style here

https://www.python.org/dev/peps/pep-0008/

Assigning multiple variables

• Multiple variables can be assigned on one linex, y = 1, 2

• This makes swapping values very simplex, y = y, x

• How would you swap values without this?

Assigning multiple variables

• Multiple variables can be assigned on one linex, y = 1, 2

• This makes swapping values very simplex, y = y, x

• How would you swap values without this?• You would need to use a temporary variable

temp = xx = yy = temp

Assigning multiple variables

Exercise: Computing the Fibonacci sequence• The nth term is defined as the sum of the previous

two terms: Fn = Fn-2 + Fn-11, 1, 2, 3, 5, 8, 13, 21, …

Assigning multiple variables

Exercise: Computing the Fibonacci sequence• The nth term is defined as the sum of the previous

two terms: Fn = Fn-2 + Fn-11, 1, 2, 3, 5, 8, 13, 21, …

• Initialize two variablesx, y = 1, 1

• Update the variablesx, y = y, x + y

• Run the line multiple times to iterate

Review: Data types

• So far, we’ve seen three data typesIntegers …, -3, -2, -1, 0, 1, 2, 3, …Floats 3.14159, 1.41421, 18.0, …Booleans True, False

• Now we add strings• Strings are enclosed within single or double quotes

“this is a string”‘so is this’

Strings

• You can put unlike quotes in strings“Mary’s dog”‘John said “hello” quietly’

• Or use the backslash to escape quotes‘Mary\’s dog’“John said \“hello\” quietly”

• Backslash also encodes other special characters\t tab\n linebreak\\ backslash

Strings

• Strings can be concatenated using +• Unlike adding numbers, order matters

a = ‘race’b = ‘car’a + b = ?b + a = ?

• Python doesn’t add spaces automatically!• Strings can be compared using == and !=

a + b == b + ab != ‘CAR’

Strings

Exercise: Generating palindromes• A palindrome is a word that reads the same

forwards and backwards, like racecar• Generate a long palindrome using a’s and b’s

Strings

Exercise: Generating palindromes• A palindrome is a word that reads the same

forwards and backwards, like racecar• Generate a long palindrome using a’s and b’s• Initialize the string

palindrome = ‘a’

• Add to both sidespalindrome = ‘a’ + palindrome + ‘a’palindrome = ‘b’ + palindrome + ‘b’

• Run these lines multiple times to grow the string

Ambiguous operators

• + means different things for numbers and strings• Different types cannot be combined

Type casting

• Data types can be cast into other data typesstr() returns a stringint() returns an integerfloat() returns a floatbool() returns a Boolean

• bool() has special mappings• 0 returns False, non-zero returns True• Empty string “” returns False, other strings return True

• type() returns the type of an object

Type casting

• What will these lines return?type(‘a’)bool(2) == bool(‘’)str(3.0>2.8)str(3)+str(2)str(3+2)type(True) != type(‘True’)

Type casting

• Functions like str() take an input and return an output -- they do not necessarily change the inputx = ‘2’y = str(x)type(x) = ?

z = 4bool(z)type(z) = ?

Organizing output

• How can we combine these variables into a string?numberQuarters = 2numberDimes = 1numberNickels = 1numberPennies = 2

Organizing output

• How can we combine these variables into a string?numberQuarters = 2numberDimes = 1numberNickels = 1numberPennies = 2

str(numberQuarters) + “quarters,” + str(numberDimes) + “dimes,” + str(numberNickels) + “nickels, and” + str(numberPennies) + “pennies.”

Organizing output

• How can we combine these variables into a string?numberQuarters = 2numberDimes = 1numberNickels = 1numberPennies = 2

str(numberQuarters) + “ quarters, ” + str(numberDimes) + “ dimes, ” + str(numberNickels) + “ nickels, and ” + str(numberPennies) + “ pennies.”Don’t forget to add spaces!

Organizing output

• Colab is an interactive environment where typing the name of an object will display its valuex = ‘Hello!’x

• Not all coding environments work this way – often a script is written and run in a non-interactive way

• We need a way to tell Python to display an output

Print statements

• The function print() sends output to the screenprint(‘hello’)• Automatically adds a linebreak to its outputprint(‘hello’)print(‘world’)• This can be changed by provided an end argumentprint(‘hello’, end=“ “)print(‘world’)

Print statements

• The function print() sends output to the screenprint(‘hello’)• Can take multiple arguments, automatically separated

by a spacex = ‘hello’y = ‘world’print(x, y)

Print statements

• The function print() sends output to the screenprint(‘hello’)• Arguments do not have to be stringsx = ‘hello’y = ‘world’z = 2print(x + y, z)print(x, y, z)print(x + y + z)

Print statements

Exercise: Printing a tic-tac-toe board• Tic-tac-toe takes place on a 3x3 board, where each

cell can be empty or contain an X or an O• Given a string board with 9 characters in it print

out the corresponding board• You have to use indices to refer to individual

charactersboard[0] board[1] board[2]

board[3] board[4] board[5]

board[6] board[7] board[8]

Print statements

Exercise: Printing a tic-tac-toe boardboard = “XO OX X”print(board[0],board[1],board[2])print(board[3],board[4],board[5])print(board[6],board[7],board[7])

Print statements

Exercise: Printing a tic-tac-toe boardboard = “XO OX X”print(board[0] + “|” + board[1] + “|” + board[2])print(“-----”)print(board[3] + “|” + board[4] + “|” + board[5])print(“-----”)print(board[6],board[7],board[7])

Let’s add the lines on the board in

More printing options

• You can use string formatting to align text, truncate floats, and many other useful formatting options

• Read about string formatting here:https://www.learnpython.org/en/String_Formattinghttps://realpython.com/python-string-formatting/

+ and * by type

Exercise

• What do these evaluate to• If error explain why• Variables must be defined first • X = • “python”*2*3• “python”*2+3• Things that depend on type