Python Loops

A for loop takes each item from a sequence in turn and runs its indented block once per item:

for letter in "cat":
    print(letter)

This prints c, a and t on separate lines. The variable after for, here letter, holds the current item on each pass. You can name it anything, but a clear name makes the loop easier to read: for name in names, for price in prices.

range() produces a sequence of whole numbers, which is perfect for repeating something a set number of times:

The end number is never included. range(1, 6) stops at 5. The optional third number is the step: how much to add each time.

Beginners often loop over range(len(items)) just to get a position number. Python has a cleaner tool for that: enumerate() gives you the position and the item together.

songs = ["Intro", "Skyline", "Outro"]
for number, title in enumerate(songs, start=1):
    print(number, title)

This prints 1 Intro, 2 Skyline and 3 Outro. Without start=1, counting begins at 0.

zip() walks through two or more sequences side by side, pairing up items in the same position:

names = ["Ada", "Linus", "Grace"]
scores = [91, 78, 85]
for name, score in zip(names, scores):
    print(f"{name}: {score}")

zip() stops when the shortest sequence runs out, so mismatched lengths don't cause an error, but extra items are silently ignored.

A while loop checks its condition before every pass and stops as soon as it's false:

countdown = 3
while countdown > 0:
    print(countdown)
    countdown -= 1
print("Lift off!")

Use while when you don't know in advance how many repeats you need, for example "keep asking until the answer is valid". Something inside the loop must eventually make the condition false, or the loop never ends.

• break leaves the loop immediately, skipping any remaining items.
• continue skips the rest of the current pass and jumps to the next item.

for n in range(1, 10):
    if n == 6:
        break        # stop completely at 6
    if n % 2 == 0:
        continue     # skip even numbers
    print(n)

This prints 1, 3 and 5. Both keywords work in for and while loops.

A loop inside another loop runs completely for every pass of the outer loop. Nested loops are how you work through rows and columns:

for row in range(1, 4):
    for col in range(1, 4):
        print(row * col, end=" ")
    print()

This prints a 3 by 3 multiplication grid. The inner loop runs three times for each of the three outer passes, nine passes in total. break only leaves the innermost loop it is in.

Loops in Python can also have an else block. It runs when the loop finishes normally, and is skipped if the loop ends with break. That makes it a neat way to report "not found":

for n in [3, 7, 11]:
    if n % 2 == 0:
        print("Found an even number:", n)
        break
else:
    print("No even numbers")

Here every number is odd, so the loop never breaks and the else block prints No even numbers.

Use a for loop when you are working through something that already exists: a list, a string, a file or a range of numbers. Use a while loop when you are waiting for something to change: valid input, a game ending, or a balance reaching a target. If you catch yourself writing a counter, a while condition and counter += 1, a for loop over range() is usually shorter and can't run forever by mistake.