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do-while is while with the condition moved to the bottom. The body executes first, unconditionally, and then the condition decides whether to repeat. One structural difference. One semantic guarantee: the body runs at least once. That guarantee is the entire reason this construct exists.
The closing parenthesis of the condition must be followed by a semicolon. This is the only loop in C that requires a semicolon after its closing parenthesis. Forget it and you get a compile error that points at the next line, which is confusing because the actual mistake is on the previous one.
#include <stdio.h>int main(void) {int choice;do {printf("\n--- Menu ---\n");printf("1. Say hello\n");printf("2. Say goodbye\n");printf("3. Exit\n");printf("Choice: ");scanf("%d", &choice);if (choice == 1) {printf("Hello!\n");} else if (choice == 2) {printf("Goodbye!\n");}} while (choice != 3);printf("Program ended.\n");return 0;}
Input validation is the canonical use case. You need to prompt the user, read their response, and then check if it was valid. The prompt must appear before you can validate anything. A while loop would require you to either duplicate the prompt or restructure the logic awkwardly. do-while maps directly to the problem.
#include <stdio.h>int main(void) {int age;do {printf("Enter your age (1-120): ");scanf("%d", &age);} while (age < 1 || age > 120);printf("Valid age recorded: %d\n", age);return 0;}
Two invalid attempts, then a valid one. The loop body ran three times. With a plain while, you would need to prime the variable or duplicate the printf before the loop. do-while keeps it in one place.
You will encounter do { ... } while(0) in macro definitions throughout C codebases. It is not a loop. It is a scoping trick that lets a multi-statement macro behave like a single statement in if-else chains. We cover this in depth in the preprocessor module, but know that when you see it, nobody is actually iterating.
do-while is niche but irreplaceable for its specific job. If you find yourself forcing a while loop to behave like a do-while by duplicating code before the loop, stop. Use the right construct. Next: for loops, which pack initialization, condition, and update into one line and introduce their own set of off-by-one traditions.
Finish the lesson once you have worked through the material. This awards ★ 20 XP.