How to use if-else in a list comprehension

I wanted to compare execution time for two list comprehensions one which uses if-else keywords and another that uses ternary operator.

arr = [5, 8, 12, 17, 24, 42];

function squares_and_cubes(array)
    return [ x%2 ==0 ?  x^2 : x^3 for x in array]
end

function squares_and_cubes_if_else(array)
    [ x^2 if (x%2==0) else x^3 end for x in array]
end
@time squares_and_cubes(arr)
@time squares_and_cubes_if_else(arr) 

However, I get the following error message ERROR: syntax: invalid comprehension syntax
when trying to execute [ x^2 if (x%2==0) else x^3 end for x in arr]. As a Julia newbie, I can’t figure out what is the issue with this syntax – I am using v 1.7 .

>Solution :

It seems you’re trying to use Python-like if-else syntax here. In Julia it is:

julia> arr = [1, 2, 3, 4];

julia> [if (x%2==0) x^2 else x^3 end for x in arr]
4-element Vector{Int64}:
  1
  4
 27
 16

There should be no difference in time of execution since both get parsed into the pretty much the same abstract code underneath, and will generate the exact same machine code. (The ternary operator is more readable and idiomatic for this purpose though.)

But if you did want to indulge your curiosity, make sure to run the functions once before timing them so as to avoid measuring the compilation time.