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Think Julia by Allen B. Downey, Ben Lauwens

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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 multiple ...

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