Parameters and Return Values

Big idea

Parameters pass values into a function. return sends one value back out.

The idea

int add(int a, int b) {
    return a + b;
}

• int (first) — the return type: this function gives back an int.
• (int a, int b) — two parameters: inputs the function receives.
• return a + b; — ends the function and hands the result back to the caller.

When you call add(3, 4), the arguments 3 and 4 are copied into a and b, the body runs, and the call itself becomes the returned value — so int total = add(3, 4); stores 7.

#include <stdio.h>

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

int main(void) {
    printf("%d\n", square(5));
    printf("%d\n", square(12));
    int x = square(3) + square(4);
    printf("%d\n", x);
    return 0;
}

output
25
144
25

Watch it

Parameters are copies

A function gets its own copies of the arguments. Changing a parameter doesn't change the caller's variable:

To get a result out, return it: x = double_it(x); with int double_it(int n) { return n * 2; }. (To let a function change the caller's variable directly, you'll need pointers — coming up.)

return ends the function

Nothing after a return runs. That makes early exits easy:

#include <stdio.h>

int max(int a, int b) {
    if (a > b) {
        return b;
    }
    return a;
}

int main(void) {
    printf("%d %d\n", max(3, 9), max(8, 2));
    return 0;
}

output
9 8

Think of it like

A juice machine. You put oranges in (parameters), it does its work, and one glass of juice comes out (the return value). Your fruit bowl at home is untouched — the machine got its own oranges.

🕷️ Spidey-sense: I hand Ned the evidence, he runs the analysis, and returns one answer. He never touches the original — it's a copy. Forensics 101.

Key terms

• Parameter — A variable in the function's header that receives a value.
• Argument — The actual value passed in the call.
• Return value — What the function gives back with return.
• Pass by value — C copies arguments into parameters.