Python Loops: for, while, break, continue and range()

The for loop

A for loop repeats a block once for each item in a sequence — a list, string, range, dictionary, file, anything iterable:

for fruit in ["apple", "kiwi", "pear"]:
    print("I like", fruit)

for ch in "abc":
    print(ch.upper(), end=" ")
print()

Unlike C-style languages there is no counter to manage; Python hands you each item in turn.

range()

When you need numbers, range(start, stop, step) generates them lazily. stop is exclusive:

for i in range(5):        print(i, end=" ")   # 0..4
print()
for i in range(2, 7):     print(i, end=" ")   # 2..6
print()
for i in range(10, 0, -3): print(i, end=" ")  # 10 7 4 1
print()
print(list(range(0, 20, 5)))

enumerate() and zip()

Need the index as well as the item? Use enumerate. Walking two lists side by side? Use zip:

names = ["Ada", "Linus", "Grace"]
langs = ["Analytical Engine", "Linux", "COBOL"]

for i, name in enumerate(names, start=1):
    print(f"{i}. {name}")

for name, lang in zip(names, langs):
    print(f"{name} -> {lang}")

The while loop

while repeats as long as a condition stays true. Use it when you do not know in advance how many iterations you need:

import random
secret = random.randint(1, 10)
guess = None
tries = 0
while guess != secret:
    guess = int(input("Guess 1-10: "))
    tries += 1
print(f"Correct! It took {tries} tries.")

Every while loop needs something inside it that eventually makes the condition false, otherwise it runs forever. If that happens on the phone, tap the stop button in the console.

break, continue and else

for n in range(2, 20):
    if n % 7 == 0:
        print("First multiple of 7:", n)
        break            # leave the loop entirely

for n in range(1, 8):
    if n % 2 == 0:
        continue         # skip to the next iteration
    print(n, end=" ")
print()

for n in [3, 5, 9]:
    if n % 2 == 0:
        print("Found an even number")
        break
else:
    print("No even numbers")   # runs only if loop was NOT broken

The else on a loop trips people up: think of it as "no break happened".

Nested loops

for row in range(1, 4):
    for col in range(1, 4):
        print(f"{row * col:3}", end="")
    print()

Nested loops multiply work: two loops of 1,000 iterations run one million times. That is fine for learning, but keep it in mind when data gets big.

Run the loops in the app's built-in FizzBuzz example — the Coding Python app runs real Python 3 on your phone, with examples, quizzes and challenges built in.

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Generators: making your own iterable

Everything above loops over something that already exists. A generator produces values on demand instead: write yield where you would have appended to a list, and the function hands back one value at a time, remembering where it left off.

def countdown(n):
    while n > 0:
        yield n
        n = n - 1

for x in countdown(3):
    print(x)

Nothing runs until you iterate, and only one value exists at a time. That is what makes a generator able to describe a sequence too large to hold in memory — or one with no end at all:

import itertools

def naturals():
    n = 1
    while True:
        yield n
        n = n + 1

print(list(itertools.islice(naturals(), 5)))

A generator expression is the same idea in one line — a comprehension with round brackets. Use it when you only want to consume the values once:

nums = [1, 2, 3, 4, 5]

total = sum(n * n for n in nums)   # no list is built
print(total)

The rule of thumb: build a list when you need to keep the values, index them or go through them more than once; use a generator when you are passing straight through. yield from delegates to another generator, which is how pipelines are chained together.

Exercise: FizzBuzz

Print the numbers 1 to 30, but print "Fizz" for multiples of 3, "Buzz" for multiples of 5 and "FizzBuzz" for both. This is the classic interview warm-up and one of the 30 built-in examples in the Coding Python app.

for n in range(1, 31):
    if n % 15 == 0:
        print("FizzBuzz")
    elif n % 3 == 0:
        print("Fizz")
    elif n % 5 == 0:
        print("Buzz")
    else:
        print(n)

Loops and lists go together, so Python lists are next.

Frequently asked questions