Repeating Code with Loops - Python

A loop repeats a block of code. Python has two kinds: for loops, which go through a collection of items, and while loops, which keep going as long as a condition is true.

The for loop

range(n) produces the numbers from 0 up to, but not including, n. It is the usual way to repeat something a fixed number of times.

Python

Open in playground

for i in range(5):
    print("Round", i)

Output

Round 0
Round 1
Round 2
Round 3
Round 4

A for loop can also walk through a list, a string, or any other collection.

Python

Open in playground

fruits = ["mango", "banana", "guava"]
for fruit in fruits:
    print(fruit.upper())

for letter in "abc":
    print(letter * 3)

Output

MANGO
BANANA
GUAVA
aaa
bbb
ccc

The while loop

How the interest example below runs

A while loop checks its condition before every round. Make sure something inside the loop changes so that the condition eventually becomes false.

Python

Open in playground

balance = 1000
years = 0
while balance < 2000:
    balance = balance * 1.1
    years += 1
print(f"It takes {years} years to double at 10% interest.")

Output

It takes 8 years to double at 10% interest.

break and continue

break leaves the loop straight away. continue skips the rest of the current round and moves on to the next one.

Python

Open in playground

for n in range(1, 10):
    if n % 2 == 0:
        continue   # skip even numbers
    if n > 7:
        break      # stop once we pass 7
    print(n)

Output