Return statements and recursion

In this final introductory lesson on functions, we will look at returning under a specific condition. A return leaves a function. For example, return 0; in main returns the value zero and ends the function.

By choosing when to return, we can make a function repeat work without a loop. The following example prints Hello World multiple times by calling the function again from inside itself:

#include <stdio.h>

void test(int num)
{
    if(num < 1)
    {
        return;
    }

    test(num-1);
    printf("Hello World\n");
    
}

int main()
{
    test(10);
    return 0;
}

Follow the calls

main calls test(10). Inside test, the program checks whether num is less than one. If not, it calls test(num - 1), reducing the value from ten to nine and continuing in the same way.

When num reaches zero, the condition is true and the function returns. The waiting calls then resume, each printing Hello World. In total, the message is printed ten times.

This pattern is called recursion: a function calls itself. For suitable problems, recursion can express repeated work compactly and make the overall structure easier to follow.

Editorial note: the recursive call appears before printf, so printing occurs while the calls return, not before descending to zero. Because test returns void, its stopping branch uses return;, not return 0;. The original code already used the correct statement, but its prose mixed them up.