Inline for loops in Julia

Question

I think you are encouraged to use these kind of for loops in Julia:

b = [i+1 for i in [1,2,3]]

but I wonder why, apart from more compact syntax which leads to better readability. Are these inline for loops more efficient to evaluate expressions? If yes, why?

Answer 1

These for loops are called array comprehensions. They are a simple notation for a common programming pattern of allocating an array and then filling each element with a value based on some calculation.

Doing this without a comprehension is possible and probably about as fast if done correctly (removing unnecessary bounds checks with @inbounds). For the example in the question, the following code does the same:

let tmp = [1,2,3], len = length(tmp)
    b = Vector{Int}(len)
    @inbounds for (n,i) in enumerate(tmp)
        b[n] = i+1
    end
    b
end

In most cases using the form in the question is a win for both performance and readability.