Think Julia

Chapter 6. Fruitful Functions

Many of the Julia functions we have used, such as the math functions, produce return values. But the functions we’ve written are all void: they have an effect, like printing a value or moving a turtle, but they return nothing. In this chapter you will learn to write fruitful functions.

Return Values

Calling a function generates a return value, which we usually assign to a variable or use as part of an expression:

e = exp(1.0)
height = radius * sin(radians)

The functions we have written so far are void. Speaking casually, they have no return value; more precisely, their return value is nothing. In this chapter, we are (finally) going to write fruitful functions. The first example is area, which returns the area of a circle with the given radius:

function area(radius)
    a = π * radius^2
    return a
end

We have seen the return statement before, but in a fruitful function the return statement includes an expression. This statement means: “Return immediately from this function and use the following expression as a return value.” The expression can be arbitrarily complicated, so we could have written this function more concisely:

function area(radius)
    π * radius^2
end

However, temporary variables like a and explicit return statements can make debugging easier.

The value returned by a function is the value of the last expression evaluated, which, by default, is the last expression in the body of the function definition.

Sometimes it is useful to have ...