Function Parameters

Now let’s look at an example of a function with parameters.

There are three function declarations in the code below: square, point_in_circle, and main. Each of these performs a number of steps and returns a value.

• The square function accepts a val parameter, and returns the square of this. This is achieved with a single line of code, but helps make the other code more readable.
• In point_in_circle, we accept parameters for the point (pt_x and pt_y) and the circle (c_x, c_y, and c_radius) and return a boolean value indicating if the point is within the circle. To achieve this, we can get the distance from the point to the center of the circle using the Pythagorean theorem, which we store in a local variable called distance, and then compare this with the circle’s radius to get the result.
• The main function is the entry point, and returns a value that indicates if the program completed successfully or not. Returning 0 at the end indicates the program succeeded. Any other value is treated as an error code.

#include "splashkit.h"

using std::to_string;
using std::sqrt;

double square(double val)
{
    return val * val;
}

bool point_in_circle(double pt_x, double pt_y, double c_x, double c_y, double c_radius)
{
    double distance = sqrt(square(pt_x - c_x) + square(pt_y - c_y));
    return distance <= c_radius;
}

int main()
{
    write_line("5 squared is " + to_string(square(5)));

    write_line("A point at 1, 3 is in a circle at 0, 0, with radius 4: " + to_string(point_in_circle(1, 3, 0, 0, 4)) );

    return 0;
}

Tip

To get the distance between the point and the center of the circle we are also using the sqrt function from the math standard library. Don’t worry if the maths here doesn’t make sense — it is not the point of the example! Focus on how the function parameters are used.

Notice we can use the square function in the calculation in point_in_circle. Once you have a function that calculates something, you can call it any time you need that value.

The great thing about parameters is that they really help you generalise what your functions and procedures can do. By accepting a value parameter, the square function can be used to square any double value. Similarly, by accepting parameters for the point and circle, the point_in_circle function can work for any point in any circle.

Summary

• Parameter is the term given to a variable declared to accept the value from an argument passed to a function or procedure when it is called.
• The procedure or function call must provide argument values for each of the function or procedure’s parameters.
• Within the function or procedure, the parameters can be used in the same way as any other variables.
• It is good practice to use parameters to pass values into a function or procedure.