C

Loops

A loop repeats a block of code while some condition holds — the tool for "do this 10 times," "keep reading until there's no more input," or "process every element in this array." C has three loop forms; this chapter is about picking the right one, not just knowing the syntax.

for

A for loop bundles the three parts of "repeat N times" into one line: an initializer, a condition, and an update.

for (int i = 0; i < 5; i++) {
    printf("%d\n", i);
}
// 0 1 2 3 4

while

A while loop checks its condition before every pass, including the first — if the condition starts false, the body never runs at all.

int count = 0;
while (count < 5) {
    printf("%d\n", count);
    count++;
}
// 0 1 2 3 4

This is the right shape whenever you don't know the number of iterations ahead of time — reading input until a sentinel value appears, or polling a hardware flag until it's set.

while (!data_ready()) {
    // spin, waiting
}
process(read_data());

do-while

A do-while checks its condition after the body — guaranteeing the body runs at least once, even if the condition is false from the start:

int choice;
do {
    printf("Enter 1-5: ");
    scanf("%d", &choice);
} while (choice < 1 || choice > 5);

This shape fits menus and input-validation loops exactly — you always need to ask at least once before you have anything to validate.

Which one to reach for

break and continue

break

Exits the loop immediately, skipping every remaining iteration — execution resumes at the first statement after the loop.

continue

Skips the rest of the current iteration only and jumps straight to the next one's condition check — the loop itself keeps running.

for (int i = 0; i < 10; i++) {
    if (i == 3) continue;   // skip printing 3, keep looping
    if (i == 7) break;      // stop the loop entirely at 7
    printf("%d\n", i);
}
// 0 1 2 4 5 6

Real-world applications

• Array/buffer processing — a for loop over a fixed-size array or ring buffer is the single most common loop shape in any C program.
• Polling a hardware flag — a tight while loop waiting on a register bit or a "data ready" flag, common in bare-metal drivers before interrupts are involved.
• The main firmware loop itself — most non-RTOS embedded firmware is structurally one giant while (1) { ... } loop that never exits for the life of the device.

Gotchas