Loops

for loops and range

for i in range(5):
    print(i)

This prints 0, 1, 2, 3 and 4. range(5) produces the numbers from 0 up to, but not including, 5. You can also give a start, a stop and a step:

range(1, 6)      # 1, 2, 3, 4, 5
range(0, 10, 2)  # 0, 2, 4, 6, 8
range(5, 0, -1)  # 5, 4, 3, 2, 1

A for loop can walk over any sequence, including the characters of a string:

for ch in "code":
    print(ch.upper())

The accumulator pattern

A very common job is building up a result one step at a time. Start with an empty value, then update it in the loop:

total = 0
for n in range(1, 101):
    total += n
print(total)  # 5050

total += n is short for total = total + n.

while loops

A while loop keeps going as long as its condition is true. Use it when you do not know in advance how many times to loop:

n = 1
while n < 1000:
    n *= 2
print(n)  # 1024

If the condition never becomes false, the loop runs forever. Press Ctrl + C in the terminal to stop a stuck program.

break and continue

break leaves the loop immediately. continue skips to the next round.

while True:
    word = input("Word (or 'quit'): ")
    if word == "quit":
        break
    if word == "":
        continue
    print(word[::-1])

Nested loops

A loop inside a loop runs the inner loop completely for every round of the outer one:

for row in range(1, 4):
    line = ""
    for col in range(1, 4):
        line += f"{row * col:4}"
    print(line)

This prints a 3 by 3 multiplication table. :4 pads each number to four characters so the columns line up.

Exercises

for i in range(10, 0, -1):
    print(i)
print("Liftoff!")

text = input("Sentence: ")
count = 0
for ch in text.lower():
    if ch in "aeiou":
        count += 1
print(count)

import random

secret = random.randint(1, 100)
guesses = 0
while True:
    guess = int(input("Guess: "))
    guesses += 1
    if guess < secret:
        print("higher")
    elif guess > secret:
        print("lower")
    else:
        break
print(f"Got it in {guesses} guesses.")