Practical 4(a): The Area of a Square, Using a Function

a = area(s); is the call. The value in s is copied into the parameter side, the body runs, and the value that return produces becomes the value of the expression area(s), which is then assigned to a.

return side * side; computes and hands back. return does two things at once: it supplies the value, and it ends the function immediately. Any statement after it in the same block never runs.

area(3) inside the last printf shows that a call is an expression. It has a value, so it can go anywhere a value can go. The argument here is a constant rather than a variable, which is allowed: the argument is a value, not a place.

What void means, in both positions

void print_line(void);

Before the name, void means the function returns nothing. Such a function is called for what it does, not for what it gives back, and it ends either at its closing brace or at a bare return; with no value.

Inside the brackets, void means the function takes nothing. That is why main is written int main(void).

Empty brackets, float area(), are not the same thing in C as (void), and C23 finally made them mean the same. Write (void) and the question never arises.

Where the parameter lives

The parameter side exists only while the function is running. It is created when the call happens, it holds a copy of the argument, and it is destroyed when the function returns.

#include <stdio.h>

void spoil(float side);

int main(void)
{
    float s = 4.5;

    spoil(s);
    printf("Back in main, s is still %.2f\n", s);

    return 0;
}

void spoil(float side)
{
    side = 99;
    printf("Inside the function, side is %.2f\n", side);
}

Inside the function, side is 99.00
Back in main, s is still 4.50

The function changed its own copy and main never noticed. That is call by value, it is how every C function call works unless you go out of your way, and it is the subject of [Practical 7: Swapping Two Numbers, by Value and by Reference].

What beginners get wrong

Putting a semicolon after the definition's head. float area(float side); followed by a body is a declaration and then a stray block. The definition's head has no semicolon.

Forgetting the return type. In C17 a function with no stated return type does not compile.

Declaring the function inside main. It is legal but pointless and confusing. Declare at file scope, above main.

Writing return; in a function that promises a value, or return x; in a void function. Both are errors.

Expecting a changed parameter to reach the caller. It does not. The parameter is a copy.

Calling the function without using its value. area(s); on a line of its own computes the area and throws it away.

Quick revision

• Declaration: name, return type, parameter types, ends in a semicolon, no body.
• Definition: the same head with a body, and no semicolon after the head.
• Call: area(s), an expression with a value.
• Parameter is in the function; argument is at the call.
• return supplies the value and ends the function at once.
• void before the name means no value returned; void in the brackets means no parameters taken.
• Arguments are passed by value: the function works on a copy.

What goes in your journal

Aim, an algorithm that names the function as a step ("call area with s and store the result"), a flowchart, the program with the declaration, the definition and the call all visible, and the output. In the conclusion define parameter and argument in one sentence each; it is the viva question this practical always carries.

Test yourself

1. What is the difference between a parameter and an argument? The parameter is the variable named in the function definition; the argument is the value supplied at the call and copied into the parameter.

2. Why is a declaration needed when the definition comes after main? Because the compiler reads the file from the top, and in C17 a call to a function it has not yet seen is an error.

3. What does return do, apart from supplying a value? It ends the function immediately. Statements after it in the same block are never reached.

4. What does void mean in int main(void)? That main takes no parameters.

5. A function changed its parameter. Does the caller's variable change? No. The argument's value was copied into the parameter, so the function changed only its own copy.

6. Is area(3) legal when the parameter is a float? Yes. The declaration tells the compiler a float is expected, so the constant 3 is converted to 3.0 before the call.