The C++Course provides a general introduction to programming in C++. It is based on A.B. Downey's book, How to Think Like a Computer Scientist. Click here for details.  ## Composition of Expressions

As you should expect by now, once you define a new function, you can use it as part of an expression, and you can build new functions using existing functions. For example, what if someone gave you two points, the center of the circle and a point on the perimeter, and asked for the area of the circle?

Let's say the center point is stored in the variables xc and yc, and the perimeter point is in xp and yp. The first step is to find the radius of the circle, which is the distance between the two points. Fortunately, we have a function, distance, that does that.

double radius = distance (xc, yc, xp, yp);

The second step is to find the area of a circle with that radius, and return it.

return result;

Wrapping that all up in a function, we get:

double fred (double xc, double yc, double xp, double yp) {
double radius = distance (xc, yc, xp, yp);