Lesson 5

Loops — repeating yourself

When you need to do something more than once, reach for a loop. Python has two: for (when you know what you're looping over) and while (when you keep going until something changes).

for — looping over a sequence

names = ["Sam", "Pat", "Riley"]

for name in names:
    print(f"Hello, {name}!")
Hello, Sam! Hello, Pat! Hello, Riley!

Read it as: "for each name in names, do this".

range() — looping a fixed number of times

range(n) produces the numbers 0, 1, 2, … n-1:

for i in range(5):
    print("Tick", i)
Tick 0 Tick 1 Tick 2 Tick 3 Tick 4

range can take a start and a step too:

print(list(range(2, 10, 2)))   # [2, 4, 6, 8]

Looping over a string

for letter in "hello":
    print(letter)
h e l l o

while — keep going until…

A while loop runs while its condition is true:

count = 3
while count > 0:
    print(f"{count}...")
    count = count - 1
print("Lift off!")
3... 2... 1... Lift off!
Avoid infinite loops Make sure something inside the loop eventually makes the condition false. Otherwise your program will hang. (If that happens, hit Ctrl+C in the terminal to stop it.)

break and continue

break exits the loop immediately. continue skips to the next iteration:

for n in range(10):
    if n == 5:
        break           # stop the whole loop at 5
    if n % 2 == 0:
        continue        # skip even numbers
    print(n)
1 3

A practical example: a guessing game

secret = 7

while True:
    guess = int(input("Guess (1-10): "))
    if guess == secret:
        print("Got it!")
        break
    elif guess < secret:
        print("Too low.")
    else:
        print("Too high.")
Quick recap for loops over a sequence (list, string, range). while loops as long as a condition holds. break stops a loop, continue skips to the next iteration.
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