C Functions

A function is a named block of code that does one job. You have used functions from the start: main, printf, scanf. Writing your own lets you name a job, use it many times and keep each part of the program small enough to understand.

Defining and calling a function

A function definition gives the type of the value it returns, its name, its parameters in brackets and its body. Calling it runs the body.

#include <stdio.h>

void greet(void) {
    printf("Hello from a function!\n");
}

int main(void) {
    greet();
    greet();
    return 0;
}

Hello from a function!
Hello from a function!

void before the name means the function returns nothing; void in the brackets means it takes nothing.

Returning a value

A function can compute a value and give it back with return. The call then stands for that value.

#include <stdio.h>

int square(int x) {
    return x * x;
}

double average(int a, int b, int c) {
    return (a + b + c) / 3.0;
}

int main(void) {
    printf("square(6) = %d\n", square(6));
    int s = square(3) + square(4);
    printf("3*3 + 4*4 = %d\n", s);
    printf("average = %.2f\n", average(81, 77, 90));
    return 0;
}

square(6) = 36
3*3 + 4*4 = 25
average = 82.67

return ends the function at once. A function that returns a value must reach a return on every path; with -Wall the compiler warns when it might not.

A function with a decision

#include <stdio.h>

int isPrime(int n) {
    if (n < 2) return 0;
    for (int d = 2; d * d <= n; d++) {
        if (n % d == 0) return 0;
    }
    return 1;
}

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

Primes below 30: 2 3 5 7 11 13 17 19 23 29

Why use functions?

• Reuse: write the code once, call it many times.
• Readability: if (isPrime(n)) reads like a sentence; the details are hidden in the function.
• Testing: a small function with one job is easy to check on its own.
• Teamwork: different people can write different functions.

A good rule: if a block of code needs a comment to say what it does, it may want to be a function with that name.

Practice exercises

1. Write int maximum(int a, int b) and use it to find the largest of three numbers.
2. Write double celsiusToFahrenheit(double c) and print a table from 0 to 100 degrees in steps of 10.
3. Write void printLine(int n) that prints n dashes, and use it to draw a box.
4. Write int digitSum(int n) that adds up the digits of a number.