repeatuntil

Repeatuntil, or repeat-until, is a programming construct used to implement a post-test loop. In this pattern, a block of statements is executed first, and only after the block finishes is a condition evaluated. If the condition is false, the loop repeats; if it is true, the loop terminates. Because the condition is checked after the body, the loop is guaranteed to run at least once.

Languages that support repeat-until provide different syntax but share the same semantics. In Pascal, the form is:

repeat

{statements}

until condition;

Lua uses a similar structure:

repeat

{statements}

until condition

MySQL includes a REPEAT statement pair:

REPEAT

statements;

UNTIL condition

END REPEAT;

Semantically, a repeat-until loop is the opposite of a pre-test loop such as while or for in their typical forms. It is effectively equivalent to a do-while loop with the condition negated (do { ... } while (!condition)) in languages that offer do-while constructs.

Common use cases include input validation, user prompts, and menus, where at least one iteration is desired and the decision to continue is made after processing input or displaying results. Potential drawbacks include the risk of infinite loops if the condition never becomes true and reduced readability in languages with less familiar syntax.

In summary, repeat-until is a post-test looping mechanism that ensures one execution of the loop body before evaluating the termination condition, making it well suited for tasks requiring at least one pass.