Bytebook Compiler

Functions

Functions

Split programs into named, reusable pieces: declarations, definitions, parameters, return values and call by value.

5 min read6 runnable examples4-question quiz

A function is a named block of code that does one job. You have been calling functions since lesson one — printf and scanf are functions written by someone else. Now you will write your own.

Why bother?

  • Reuse — write the logic once, call it many times.
  • Readability — isPrime(n) explains itself; forty lines of loop do not.
  • Testing — a small function is easy to check on its own.

Defining and calling a function

C
#include <stdio.h>

// definition: return type, name, parameters, body
int square(int x) {
    return x * x;
}

int main(void) {
    int a = square(7);           // call: 7 is the argument
    printf("7 squared is %d\n", a);
    printf("12 squared is %d\n", square(12));
    return 0;
}
Output
7 squared is 49
12 squared is 144
Try it Yourself
Part In the example
Return type int — the type of value sent back
Name square
Parameter list (int x) — inputs the function receives
Body { return x * x; }
return sends a value back and ends the function

A function that returns nothing has return type void. A function that takes nothing has (void) as its parameter list.

C
#include <stdio.h>

void printLine(int n, char ch) {
    for (int i = 0; i < n; i++) putchar(ch);
    putchar('\n');
}

int main(void) {
    printLine(20, '=');
    printf("  REPORT CARD\n");
    printLine(20, '-');
    return 0;
}
Output
====================
  REPORT CARD
--------------------
Try it Yourself

Declarations (prototypes)

C reads your file top to bottom. If main calls a function defined below it, you must declare the function first with a prototype — the header line followed by a semicolon:

C
#include <stdio.h>

int isPrime(int n);          // prototype: tells the compiler the function exists

int main(void) {
    printf("Primes up to 30: ");
    for (int i = 1; i <= 30; i++)
        if (isPrime(i)) printf("%d ", i);
    printf("\n");
    return 0;
}

int isPrime(int n) {         // definition comes later
    if (n < 2) return 0;
    for (int d = 2; d * d <= n; d++)
        if (n % d == 0) return 0;
    return 1;
}
Output
Primes up to 30: 2 3 5 7 11 13 17 19 23 29
Try it Yourself

Call by value

When you pass a variable to a function, C copies its value into the parameter. The function works on the copy; the original is untouched.

C
#include <stdio.h>

void tryToChange(int x) {
    x = 100;
    printf("Inside the function: x = %d\n", x);
}

int main(void) {
    int x = 5;
    tryToChange(x);
    printf("Back in main:        x = %d\n", x);
    return 0;
}
Output
Inside the function: x = 100
Back in main:        x = 5
Try it Yourself

So how can a function swap two variables in main? It needs their addresses — that is call by reference, which you will learn in Pointers, Arrays & Functions.

Functions calling functions

Good programs are built from small functions that call each other:

C
#include <stdio.h>

int gcd(int a, int b) {
    while (b != 0) {
        int r = a % b;
        a = b;
        b = r;
    }
    return a;
}

int lcm(int a, int b) {
    return a / gcd(a, b) * b;
}

int main(void) {
    int a, b;
    printf("Two numbers: ");
    scanf("%d %d", &a, &b);
    printf("GCD = %d, LCM = %d\n", gcd(a, b), lcm(a, b));
    return 0;
}
Output
Two numbers: 12 18
GCD = 6, LCM = 36
Sample input is includedTry it Yourself

gcd uses Euclid's algorithm: gcd(a, b) = gcd(b, a mod b), stopping when b is 0.

Library functions

C ships with many ready-made functions. Include the right header to use them:

Header Useful functions
stdio.h printf, scanf, fgets, puts, fopen
math.h sqrt, pow, fabs, ceil, floor, sin, log
string.h strlen, strcpy, strcmp, strcat
stdlib.h malloc, free, abs, rand, atoi, exit
ctype.h isdigit, isalpha, toupper, tolower
C
#include <stdio.h>
#include <math.h>

int main(void) {
    printf("sqrt(144) = %.0f\n", sqrt(144));
    printf("pow(2, 10) = %.0f\n", pow(2, 10));
    printf("ceil(4.1) = %.0f, floor(4.9) = %.0f\n", ceil(4.1), floor(4.9));
    printf("hypotenuse of 3,4 = %.1f\n", sqrt(3 * 3 + 4 * 4));
    return 0;
}
Output
sqrt(144) = 12
pow(2, 10) = 1024
ceil(4.1) = 5, floor(4.9) = 4
hypotenuse of 3,4 = 5.0
Try it Yourself

Test yourself

4 questions · pick an answer to check it instantly

  1. 1

    What is printed?

    C
    void f(int n) { n = n * 2; }
int main(void) { int n = 4; f(n); printf("%d", n); }
  2. 2

    Why do we write a function prototype?

  3. 3

    What is the return type of a function that does not return a value?

  4. 4

    In int add(int a, int b), what are a and b?

Esc
↑↓ move↵ open