DoUntil

DoUntil is a control-flow construct used in many imperative programming languages to repeat a block of statements until a specified condition becomes true. It is typically implemented as a do-until or post-test loop: the loop body executes first, after which the termination condition is evaluated.

In this post-test form, the loop continues to execute as long as the condition remains false, and it stops when the condition evaluates to true. Because the condition is checked after each iteration, the loop body is guaranteed to run at least once. The precise syntax and keywords vary by language, but the general behavior is consistent across many environments.

Common variations include forms such as do { ... } until condition or the equivalent Do Until condition ... Loop in languages like Visual Basic or BASIC dialects. Other languages express the same idea with repeat-until or, in effect, a pre-test do-while where the condition is checked before the next iteration. The distinction between post-test (do-until) and pre-test (do-while) loops is important for understanding when the loop’s body may execute zero times.

Typical uses involve scenarios where some work must be performed at least once before a termination condition can be evaluated, such as reading user input until a valid value is entered, performing iterative calculations until a target accuracy is achieved, or processing data until end-of-file or a sentinel value is reached. Pitfalls include the risk of an infinite loop if the condition is never satisfied and the need to ensure variables updated within the loop body eventually allow the condition to become true.