# 6.10. Composition¶

As we have already seen, you can call one function from within another. This ability to build functions by using other functions is called composition.

As an example, we’ll write a function that takes two points, the center of the circle and a point on the perimeter, and computes the area of the circle.

Assume that 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’ve just written a function, `distance`, that does just that, so now all we have to do is use it:

```radius = distance(xc, yc, xp, yp)
```

The second step is to find the area of a circle with that radius and return it. Again we will use one of our earlier functions:

```result = area(radius)
return result
```

Wrapping that up in a function, we get:

We called this function `area2` to distinguish it from the `area` function defined earlier. There can only be one function with a given name within a module.

Note that we could have written the composition without storing the intermediate results.

```def area2(xc, yc, xp, yp):
return area(distance(xc, yc, xp, yp))
```