Programming Fundamentals › Programming Basics

Loop

Repeating code with for, while, do-while or for-each.

Also known as: for loop, while loop, repeat

A loop repeats a block of code. Use one when you need to do the same thing many times, or until something becomes true.

# for: repeat for each item or a fixed number of times
for i in range(3):
    print(i)              # 0, 1, 2

# while: repeat as long as a condition holds
attempts = 0
while attempts < 3 and not connected():
    attempts += 1

for (let i = 0; i < 3; i++) { ... }
while (cond) { ... }
do { ... } while (cond);      // runs at least once

Controlling a loop

• break leaves the loop.
• continue skips to the next round.

Common bugs

• Infinite loop. The condition never becomes false because nothing in the loop changes it. Check that every path moves toward the exit.
• Off-by-one. Running one time too many or too few. Remember that range(3) is 0, 1, 2.
• Changing the thing you’re looping over, which can skip or repeat items (iteration).
• Heavy work inside, such as a network call per item.
• Forgetting to reset a counter or accumulator that should start fresh each round.

for or while?

Use for when you know what you’re looping over (a collection, or N times). Use while when you’re waiting for a condition. Often a built-in like sum, map or any says what you mean more clearly than a hand-written loop (map, filter, reduce).

Loops and recursion can solve the same problems.