A loop repeats part of a program. Repetition can continue while a condition holds or can visit each value in a collection.

Condition-controlled loops

A while-style loop repeats as long as its condition remains true:

while balance > 0:
    process_payment()

Something inside the loop must eventually make the condition false if the algorithm is intended to terminate.

Iterating over values

A for-style loop often processes each element of a collection:

sum = 0
for value in values:
    sum = sum + value

After the loop, sum contains the accumulated total.

Loop state

A loop typically maintains state that changes from one iteration to the next: a counter, an accumulated result, a current position or a partially processed structure.

A useful way to reason about a loop is to identify what remains true before and after every iteration. Such a property is called a loop invariant.

Termination

An infinite loop may be intentional, as in a long-running server, or it may be a bug. For algorithms expected to finish, there must be progress toward a terminating condition.

Loops turn finite program text into computations whose number of steps depends on the input.