Control flow determines execution order. C provides if/else, switch, and three loop forms — all driven by integer expressions.

If / Else 

  #include <stdio.h>
int main(void) {
    int score = 85;
    if (score >= 90)
        printf("A\n");
    else if (score >= 80)
        printf("B\n");
    else
        printf("C or below\n");
    return 0;
}

Always brace multi-line blocks. Single statements may omit braces but this causes bugs during maintenance.

Switch 

  switch (command) {
case 'q':
case 'Q':
    quit = 1;
    break;          /* essential — fall-through otherwise */
case 'h':
    show_help();
    break;
default:
    fprintf(stderr, "Unknown: %c\n", command);
}

C23 adds case 1 ... 5: range labels. Use default to catch unexpected values.

For Loop 

  for (int i = 0; i < 10; i++) {
    printf("%d ", i);
}

/* C99: declare loop variable in for-init */
int arr[] = {3, 1, 4, 1, 5};
for (size_t i = 0; i < sizeof arr / sizeof arr[0]; i++) {
    printf("%d ", arr[i]);
}

While and Do-While 

  int n = 10;
while (n > 0) {
    printf("%d ", n);
    n--;
}

int ch;
do {
    ch = getchar();
    putchar(ch);
} while (ch != '\n');   /* runs at least once */

Break, Continue, Goto 

  for (int i = 0; i < 100; i++) {
    if (i % 2 == 0) continue;  /* skip evens */
    if (i > 20) break;         /* exit loop */
    printf("%d ", i);
}

/* goto: acceptable for centralized cleanup in C */
if (error) goto cleanup;
cleanup:
    free(buffer);

Common Pitfalls

  • Treating compiler warnings as optional rather than actionable feedback.
  • Skipping error checks on library and system calls.
  • Copy-pasting examples without adapting to your project’s conventions.

Best Practices

  • Enable strict compiler warnings and fix them before merging.
  • Write small, testable units with clear input/output contracts.
  • Document non-obvious invariants and preconditions.
  • Use version control and code review for every change.

Memory and Performance Notes

Branch prediction affects loop performance. Prefer predictable branches in hot loops.

Exercise

Print all prime numbers between 2 and 100 using nested loops. Count how many primes you found.

Hint: A number is prime if no integer from 2 to sqrt(n) divides it evenly.

Summary

Apply these concepts in small programs before moving to larger projects. Combine with adjacent topics in the learning path for deeper mastery.

Real-World Application

These concepts appear in production codebases — from operating system kernels to embedded firmware. Study open-source projects that use this topic extensively to see idiomatic patterns at scale.

Debugging Checklist

  1. Reproduce the issue with the smallest possible input.
  2. Enable compiler warnings and sanitizers.
  3. Use a debugger to inspect state at the failure point.
  4. Verify assumptions about types, sizes, and return values.
  5. Compare working and broken code paths side by side.
  6. Write a regression test once the bug is fixed.

Further Reading

Consult the ISO C standard, Effective C by Robert C. Seacord, and your compiler documentation for platform-specific behavior.

Quick Reference

Review the code examples on this page before starting the exercise. Type them manually to build muscle memory.