Assignment

• Variables can be assigned values

x = 1
y = 1 + 2

Updating Variables

• Sometimes we want to update the values in variables
• We can set variables to expressions that include those variables

x = x + 1

Increment and Decrement

• Increment is increase by 1
• Decrement is decrease by 1

x = x + 1 # Increment
x = x - 1 # Decrement

Augmented Assignment

We can shorten common reassignment using augmented assignment:

x += 1 # Increment
x -= 1 # Decrement

Recursion

• Functions can be use to create repetition in our programs

Example

def count_to_10_from(n):
    if n > 10:
        return

    print(n)
    count_to_10_from(n + 1)

count_to_10_from(0)

while

• Repetition in programs is a common task
• We introduce while to perform operations multiple times

Example

while True:
    answer = input("What is the capital of France?")

    if answer == "Paris":
        print("That's correct!")
        exit()
    else:
        print("Not quite. Try again.")

Indefinite iteration

• We do not specify how many times a while loop will execute in advance
• This makes iteration indefinite

Controlling Iteration

• while accepts a conditional that will stop iteration when false
• This can be used to control how many times we iterate

Counting

i = 0

while i <= 10:
    print(i)
    i += 1

Infinite Loop

• We must be careful to avoid looping forever
• A loop that never stops is called an infinite loop
• This is a common type of bug

Infinite Loop

i = 0

while True:
    print(i)

break

• break can be used to terminate iteration
• Control moves to after the loop body

Example

i = 0

while True:
    if i > 10:
        break
    print(i)
    i = i + 1

continue

• continue can be reused to skip the remainder of an iteration
• Control will return to the conditional on the while statement

Example

while True:
    num = input("Enter a number:")

    try:
        square = int(num) ** 2
    except ValueError:
        print("Invalid number")
        continue

    print(square)

Exercise

Stuck in a time loop

Use the following to get problem input:

import sys

count_to = int(sys.stdin.read())