C While Loops

How it works

A while loop tests before entering its body. A do-while loop tests after its body, guaranteeing one execution. Neither form guarantees progress automatically: the body must move its state toward a stopping condition or leave deliberately. When reading input, a sentinel is a special value that means stop; the input operation must also detect end of file and errors. This fixed countdown isolates loop control before the stream lessons add those separate failure conditions.

Run it locally

Use a hosted C17 compiler in a separate lab folder. The command assumes a driver named cc; substitute gcc, clang or zig cc as appropriate. C does not currently run in Bhylu’s browser playground. Apache, PHP and MySQL are not needed for these C programs.

cc -std=c17 -Wall -Wextra -Wpedantic -Werror while-sentinel.c -o lesson

On Windows, use -o lesson.exe and run .\lesson.exe in PowerShell. On Unix-like systems run ./lesson. Math-library linking differs by toolchain; GCC on many Unix systems needs -lm after the source.

Download the complete C source. File-writing labs must run in an empty private directory, away from existing reports.

Example

#include <stdio.h>

int main(void) {
    int remaining = 3;
    while (remaining > 0) {
        printf("Remaining: %d\n", remaining);
        --remaining;
    }
    int attempts = 0;
    do {
        ++attempts;
    } while (attempts < 1);
    printf("Checks: %d\n", attempts);
    return 0;
}

Expected result

Remaining: 3
Remaining: 2
Remaining: 1
Checks: 1

The decrement creates progress for the first loop. The second loop increments attempts before checking whether another iteration is needed. Its final semicolon is required syntax for do-while. An interactive retry loop should have a policy for cancellation and invalid input, rather than assuming that a user will eventually supply a valid line.