Understanding when code is guaranteed to run at least once
| Loop Type | Condition Evaluated | Min Iterations | Primary Use Case |
|---|---|---|---|
while (condition) { ... } | Before loop body (Entry-controlled) | 0 times | Stream processing, reading till EOF |
do { ... } while (condition); | After loop body (Exit-controlled) | 1 time (guaranteed) | User input menus, password retries |
for (initialization; condition; increment/decrement)
// 1. Standard C99 loop with scoped index variable (i exists only inside loop) for (int i = 0; i < 10; ++i) { printf("%d ", i); } // 2. Two pointer in-place array reversal using multi-variable for loop for (int l = 0, r = len - 1; l < r; ++l, --r) { int tmp = arr[l]; arr[l] = arr[r]; arr[r] = tmp; } // 3. Infinite loop idiom for (;;) { // Runs forever until explicit break or return }
Terminating early vs skipping remaining statements in current iteration
for (int i = 1; i <= 10; ++i) { if (i % 2 == 0) { continue; // Skips printf and jumps straight to ++i step } if (i == 7) { break; // Completely terminates the loop immediately } printf("%d ", i); // Prints 1 3 5 }
Unwinding multiple allocated resources without deep nested ifs
int process_driver(void) { void *resA = malloc(1024); if (!resA) goto err_resA; void *resB = malloc(2048); if (!resB) goto err_resB; // Success logic free(resB); free(resA); return 0; err_resB: free(resA); err_resA: return -1; }
Nested loops are loops within loops. The outer loop runs once for every complete cycle of the inner loop, producing O(ROWS × COLS) time complexity for 2D grids.
do { ... } while(0); execute?
do-while is exit-controlled, meaning the body runs before checking the condition (which evaluates to 0/false, terminating the loop).