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A while loop checks its condition before executing the body. Zero iterations is a valid outcome. That single fact distinguishes it from do-while and determines when you reach for each. If the body must run at least once regardless of input, while is the wrong tool. If the body might not run at all, while is correct.
#include <stdio.h>int main(void) {int count = 5;while (count > 0) {printf("T-minus %d\n", count);count--;}printf("Liftoff.\n");return 0;}
Forget the decrement and you loop forever. That is the bug version. The tool version is while (1), an intentional infinite loop used in embedded systems, server main loops, and event-driven programs. The difference between the two is whether there is a break or return inside the body that provides an exit path.
#include <stdio.h>int main(void) {int input;while (1) {printf("Enter a number (0 to quit): ");if (scanf("%d", &input) != 1) {printf("Invalid input. Exiting.\n");break;}if (input == 0) {break;}printf("You entered: %d\n", input);}printf("Loop terminated.\n");return 0;}
The while (1) pattern is standard in production C. Linux kernel subsystems use it. Network daemons use it. The key is that every intentional infinite loop has at least one documented exit condition. If you cannot point to the line that breaks out, you do not have a server loop. You have a bug.
A common idiom: read characters or lines until end-of-file. The condition lives in the while header, and the body processes each unit. This pattern appears in text processors, lexers, and file parsers throughout real codebases.
#include <stdio.h>int main(void) {int ch;int line_count = 0;while ((ch = getchar()) != EOF) {if (ch == '\n') {line_count++;}}printf("Lines read: %d\n", line_count);return 0;}
ch must be declared as int, not char. getchar() returns EOF (typically -1), which cannot be represented in a char on platforms where char is unsigned. Declaring ch as char silently converts EOF to 255, and your loop never terminates. This is a classic bug that appears in textbook code from the 1980s and still gets copied today.
while is the workhorse loop. You will use it for input validation, linked list traversal, and any situation where the iteration count is unknown ahead of time. Next lesson flips the check order with do-while, which guarantees at least one execution.
Finish the lesson once you have worked through the material. This awards ★ 20 XP.