In computer science, a for-loop (or “for loop”) is used to run specified lines of code repeatedly. In different languages, there are different keywords to specify this statement—ways to identify a for loop—but they all do the same task. Most languages use either “for” or “do” as keywords.

A for-loop has two parts: a header, and a body of code. The body consists of a set of instructions (lines of code) that run for each repetition of the loop. The header often declares how many loops to complete, generally indicated by either a loop counter or a loop variable. For-loops are typically used when the number of repetitions are known. For-loops are similar to “while-loops”, but usually have a known number of repetitions.

Sample for-loop in Python:

for i in range(10):
    print('Hello, world!')

This for loop prints outs "Hello, world!" ten times.

The name for-loop comes from the English word for, which is the direct translation of the earlier German für, used by Heinz Rutishauser, who also helped define ALGOL 58 and 60.

A for-loop is available in most common programming languages. While for-loops in different languages will all fulfill the same task, there are differences on how each programming language deals with for loops.

In computer science, a loop counter is a variable that controls how many repetitions the loop will do. Often, the loop counter variable is an integer value that increments by 1 for each completed loop. However, loop counters can also decrement, and can have step sizes other than 1.

Loop counters change each iteration. With each repeat, there is a unique counter value. The loop counter is the deciding factor for when a loop should end and go onto the next section of code.

Common identifier names for loop counters are i, j, and k (and so on, if needed).

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