C Function Return Values

So far your functions have printed their results. But often you don't want a function to print anything; you want it to give you an answer that you can store, compare or calculate with. That's what C function return values are for. In this lesson, you'll learn how the return statement works, how to choose a return type, how to leave a function early, and what to do when you need more than one result.

What is a return value in C?

A return value is the result a function sends back to its caller. Two parts of the function work together to make it happen:

• The return type, written before the function name, says what type of value comes back (int, double, char, and so on).
• The return statement, return expression;, ends the function and sends the value of the expression back.

#include <stdio.h>

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

int main(void) {
    int best = larger(8, 3);
    printf("Best score: %d\n", best);
    printf("Higher bid: %d\n", larger(250, 310));
    return 0;
}

Best score: 8
Higher bid: 310

Think of the call larger(8, 3) as a question. When the function returns, the whole call is replaced by its answer, so the line effectively becomes int best = 8;.

Choosing the return type

The return type should match the kind of answer the function gives. A tax calculator works with money, so it returns a double:

#include <stdio.h>

double price_with_tax(double price, double tax_rate) {
    return price + price * tax_rate;
}

int main(void) {
    double headphones = price_with_tax(40.00, 0.08);
    double book = price_with_tax(19.99, 0.20);

    printf("Headphones: $%.2f\n", headphones);
    printf("Book:       $%.2f\n", book);
    printf("Together:   $%.2f\n", headphones + book);
    return 0;
}

Headphones: $43.20
Book:       $23.99
Together:   $67.19

The book costs 19.99 + 3.998 = 23.988, which %.2f rounds to 23.99 for display. The stored value is still 23.988, which is why the total rounds to 67.19.

Using returned values in expressions

Because a call becomes a value, you can use it anywhere a value of that type is allowed: in arithmetic, as an argument to another call, or inside a condition.

#include <stdio.h>

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

int main(void) {
    int total = larger(3, 7) + larger(10, 4);
    printf("Sum of winners: %d\n", total);

    int best_of_three = larger(larger(4, 9), 6);
    printf("Best of three: %d\n", best_of_three);

    int age_a = 15;
    int age_b = 19;
    if (larger(age_a, age_b) >= 18) {
        printf("At least one adult\n");
    } else {
        printf("No adults\n");
    }
    return 0;
}

Sum of winners: 17
Best of three: 9
At least one adult

In larger(larger(4, 9), 6), the inner call runs first and returns 9, so the outer call becomes larger(9, 6), which returns 9.

Returning early from a function

A return statement ends the function immediately, even in the middle of the body. Nothing after it runs. You can use this to return as soon as you know the answer, instead of building a long if/else chain:

#include <stdio.h>

char letter_grade(int score) {
    if (score >= 90) {
        return 'A';
    }
    if (score >= 80) {
        return 'B';
    }
    if (score >= 70) {
        return 'C';
    }
    return 'F';
}

int main(void) {
    printf("95 -> %c\n", letter_grade(95));
    printf("83 -> %c\n", letter_grade(83));
    printf("70 -> %c\n", letter_grade(70));
    printf("61 -> %c\n", letter_grade(61));
    return 0;
}

95 -> A
83 -> B
70 -> C
61 -> F

For a score of 95, the first return 'A'; ends the function, so the other checks never run. That's why there's no need for else here.

Can a void function use return?

Yes. A void function returns no value, but it can still use a plain return; (with no value) to stop early. A common use is rejecting bad input:

#include <stdio.h>

void print_discount(int age) {
    if (age < 0) {
        printf("Invalid age: %d\n", age);
        return;
    }
    if (age >= 65) {
        printf("Age %d: senior discount 20%%\n", age);
        return;
    }
    printf("Age %d: no discount\n", age);
}

int main(void) {
    print_discount(70);
    print_discount(30);
    print_discount(-5);
    return 0;
}

Age 70: senior discount 20%
Age 30: no discount
Invalid age: -5

Remember that %% inside printf prints a single percent sign. When a void function reaches its closing brace, it returns automatically, so a final return; is never needed.

Can a C function return more than one value?

No. A return statement sends back exactly one value. If you write return a, b;, C uses the comma operator and returns only b. This is a classic surprise for people coming from Python functions, which can return several values at once.

When you need several results, you have three options:

• Write separate functions, such as smallest() and largest().
• Pass the addresses of variables for the function to fill in, using pointers (see pass by reference).
• Group the results into a single structure and return that (see C structs).

Common mistakes with return values

1. Forgetting to return a value

int larger(int a, int b) {
    if (a > b) {
        return a;
    }
    /* missing: return b; */
}

When a is not larger, the function reaches its end without returning anything. GCC with -Wall warns that control reaches end of non-void function, and if the caller uses the result, it gets a garbage value. Make sure every path through the function ends in a return.

2. Losing decimals because of integer division

#include <stdio.h>

double average_wrong(int a, int b) {
    return (a + b) / 2;
}

double average_right(int a, int b) {
    return (a + b) / 2.0;
}

int main(void) {
    printf("Wrong: %.1f\n", average_wrong(7, 8));
    printf("Right: %.1f\n", average_right(7, 8));
    return 0;
}

Wrong: 7.0
Right: 7.5

A double return type doesn't help if the calculation is done with integers: 15 / 2 is already 7 before it's converted. Dividing by 2.0 fixes it (see type conversion).

3. Using the result of a void function

Writing int x = print_menu(); when print_menu returns void is an error: GCC says void value not ignored as it ought to be. There is no value to store.

4. Calling a function and ignoring its result

larger(5, 9);              /* the 9 is thrown away */
int winner = larger(5, 9); /* keep it */

This compiles without a warning, but the answer is lost. If you need the result, store it or use it directly.

Practice exercise

#include <stdio.h>

double shipping_cost(double order_total) {
    if (order_total >= 50.0) {
        return 0.0;
    }
    return 4.99;
}

int main(void) {
    double cart = 35.50;
    printf("Cart $%.2f, shipping $%.2f, total $%.2f\n",
           cart, shipping_cost(cart), cart + shipping_cost(cart));

    cart = 72.00;
    printf("Cart $%.2f, shipping $%.2f, total $%.2f\n",
           cart, shipping_cost(cart), cart + shipping_cost(cart));
    return 0;
}

Cart $35.50, shipping $4.99, total $40.49
Cart $72.00, shipping $0.00, total $72.00

What's next?

You can now write complete functions with inputs and outputs. Next, learn how to keep main at the top of your file and organize larger programs with C function prototypes.