Functions

return-type function-name(parameter-list);

First we need function prototype:

float area(float radius);

Then we introduce function body:

float area(float radius)
{
        return pi * radius * radius;
}

Now we can start using our sweet function. Call it in your program like so:

#include <stdio.h>

static float pi = 3.14159;

float area(float radius);
float area(float radius)
{
        return pi * radius * radius;
}

int main()
{
        float radius;

        printf("Input circles radius: ");
        scanf("%f", &radius);

        printf("Circles area is: %.2f", area(radius));

return 0;
}

Your terminal should say:

Input circles radius: 12 Circles area is: 452.39