While loops in Python

Looping until a condition changes, not a fixed count.

n = 27
steps = 0
while n != 1:
    if n % 2 == 0:
        n //= 2
    else:
        n = 3 * n + 1
    steps += 1

guesses = [50, 25, 37, 42]
target = 42
while guesses:
    guess = guesses.pop(0)
    if guess == target:
        break

How it works

1. The Collatz walk repeats until n reaches 1.
2. The loop body updates the very condition it tests.
3. break exits early the moment the target is found.

Keywords and builtins used here

• break
• else
• if
• while

The run, in numbers

Lines

15

Characters to type

188

Tokens

67

Three-star pace

90 tpm

At the three-star pace of 90 tokens a minute, this run takes about 45 seconds.

Type this snippet

Step 3 of 7 in Flow control, step 18 of 72 in Language basics.

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