Loops (for / while / do-while)

Loops are how computers actually do work. Without them, you'd have to write printf("Hello"); a million times to print Hello a million times. With a loop, you write it once.

Anatomy of a for Loop

for (initialization; condition; update) {
    body;
}

Execution order:

1. init runs once 2. Check condition — if false, exit 3. Run body 4. Run update 5. Go to step 2

So for (int i = 0; i < 3; i++) printf("%d\n", i); produces:

0
1
2

The check happens before the body, so if i < 3 is false from the start, the body never runs.

All Three Slots Are Optional

for (;;) { ... }                  // infinite loop (must break out)
for (int i = 0; ; i++) { ... }    // no condition → infinite
int i = 0;
for (; i < 10; ) { i++; }         // ugly but legal — like a while

for (;;) is the idiomatic C way to write "loop forever".

while vs do-while — The Off-by-One Trap

int n = 0;
while (n > 0) {        // n is 0 → false → body NEVER runs
    printf("hi");
}
int m = 0;
do {                   // body runs ONCE before check
    printf("hi");      // prints "hi"
} while (m > 0);       // then exits

Use do-while for things that must happen at least once: prompting the user for input, opening a menu, retrying a network call.

break vs continue

for (int i = 0; i < 10; i++) {
    if (i == 5) break;       // exits loop entirely → prints 0..4
    printf("%d ", i);
}
for (int i = 0; i < 10; i++) {
    if (i % 2 == 0) continue;  // skips even → prints 1 3 5 7 9
    printf("%d ", i);
}

break only exits the innermost loop. To exit nested loops, either set a flag, restructure into a function and return, or use goto (legitimate here):

for (int i = 0; i < N; i++) {
    for (int j = 0; j < M; j++) {
        if (matrix[i][j] == target) {
            goto found;
        }
    }
}
found:
    /* ... */

The Off-by-One Bug

The most common loop bug. Should it be < or <=?

int arr[5];                 // valid indices: 0,1,2,3,4
for (int i = 0; i <= 5; i++)    // ❌ runs 6 times — last iteration writes arr[5] (CRASH)
    arr[i] = 0;
for (int i = 0; i < 5; i++)     // ✅ runs 5 times: indices 0..4
    arr[i] = 0;

Rule of thumb for arrays: for (int i = 0; i < size; i++). Use <, not <=.

Infinite Loops

Easy to write by accident:

int i = 0;
while (i < 10) {
    printf("%d", i);   // forgot i++ — i stays 0 forever
}

Or by design:

while (1) {
    char c = getchar();
    if (c == 'q') break;
    process(c);
}

If your terminal looks frozen, you probably have an infinite loop. Press Ctrl+C to stop.

Loop Variable Scope (C99+)

In modern C, declaring the loop variable inside the for keeps it local:

for (int i = 0; i < 5; i++) {
    /* i visible here */
}
/* i NOT visible here — good */

Always prefer this — keeps your variables tightly scoped.

Nested Loops

for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 3; j++) {
        printf("%d ", i * j);
    }
    printf("\n");
}

Outer loop runs 3 times, inner runs 3 × 3 = 9 times. Used everywhere: matrices, grids, pairs.

When to Choose Which

Master loops and you've unlocked the ability to process data of any size.