Ifelse() evalutes both cases

General Usage
1

I have encountered an unexpected (for myself) behavior of the ifelse() function. I thought that it would be easier to read if I used ifelse() function instead of an if-block. To see the problem that I had consider the following function and its call:

function foo(x, α)

	ret = ifelse(x < α, log(x), log(-x))

	return ret

end

foo(1.1, 2.0)

The call of this function returns a domain error as it tries to evaluate log(-1.1) which would not be the case if I used a regular if-block. Is it an intended behavior of the ifelse() function? I would prefer to use this function for readability purposes but it is not useful for such instances.

2

This is intentional. If you want something that is more compact, but behaves like if, then use the ternary operator:

x < α ? log(x) : log(-x)
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3

As ifelse is a function, its arguments are always evaluated first, as with any function.

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4

The ifelse function is useful when both expressions are very cheap to evaluate, so cheap that it’s faster to evaluate both than to use an actual brach, which has a certain cost and might even prevent simd vectorization. So it’s primarily used for performance.

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