Print or Return?

Beginners often mix up return and printf, because both seem to give out a value. But they do very different things:

• printf only shows a value to you, in the output. The program can't work with what was printed.
• return gives the value to the program, which can store it in a variable, calculate with it or print it.

Look at what happens when a function prints its result instead of returning it, and the program then wants to calculate with it:

#include <stdio.h>

void print_sum(int a, int b) {
    printf("%d\n", a + b);
}

int main(void) {
    int result = print_sum(2, 3);
    printf("%d\n", result * 2);
    return 0;
}

The program doesn't compile:

main.c:8:9: error: initializing 'int' with an expression of incompatible type 'void'
    8 |     int result = print_sum(2, 3);
      |         ^        ~~~~~~~~~~~~~~~
1 error generated.

The error message says that a variable of type int can't be initialized with an expression of type void. The function print_sum is void, so it returns nothing that could be stored; it would only print 5. print_sum(2, 3) * 2 ends the same way, only with the error invalid operands to binary expression ('void' and 'int'): you can't calculate with nothing.

The fix is to change void to int and to return the value instead of printing it: return a + b;.

It can also happen the other way round: a function returns a value, but nobody uses it. If you write only add(2, 3); in main, the program prints nothing. The function returned 5, but the program didn't store it or print it, so the value was lost. If you want to see it, print it: printf("%d\n", add(2, 3));.

So when should a function print, and when should it return? A function that calculates something should return the result. The code that calls it then decides what to do with it: print it, store it or keep calculating with it. Print inside a function only when printing is its whole job, as in greet or print_row. Such functions often have names that start with print_, so you can tell them apart at a glance.