How to put even numbers from matrix in separate vector in Julia? - matrix

Question

I am solving some matrix problems in Julia and i need to put numbers that can be divided by 2 (or any other number) from matrix in separate vector. Generally, when I need to separate numbers from matrix that can be divided with 2 or 3 or 4...etc i can't index them properly. Basically, I need Julia equivalent for Matlab command:
vector=matrix(rem(matrix,2)==0)
.
I tried few things mentioned below:
vector=matrix[matrix.%2==0];
vector=(matrix.%2==0);
I expect output to be vector of numbers that can be divided with certain number but in first case I get errors and in second I only get "true" or "false".
This is my first post, so sorry if I made any mistakes or broke any rules.
Thanks in advance!

Answer 1

First of all, welcome to stackoverflow!
One way to get what you want, that you almost got right, is the following:
julia> M = rand(1:10, 3,3)
3×3 Array{Int64,2}:
3 10 7
6 7 8
2 10 6
julia> v = M[M .% 2 .== 0]
6-element Array{Int64,1}:
6
2
10
10
8
6
Note the extra dot in .== which applies the equality comparison elementwise.
A faster version would be to use findall:
julia> M[findall(x->x%2==0, M)]
6-element Array{Int64,1}:
6
2
10
10
8
6
Here, x->x%2==0 is a anonymous function representing the find criterium. In the special case x%2==0 this can simply be replaced by iseven:
julia> M[findall(iseven, M)]
6-element Array{Int64,1}:
6
2
10
10
8
6
You can also utilize array-comprehensions to get what you want as well, which should be even faster:
julia> [x for x in M if iseven(x)]
6-element Array{Int64,1}:
6
2
10
10
8
6
Finally, perhaps the most idomatic option, is to use filter:
julia> filter(iseven, M)
6-element Array{Int64,1}:
6
2
10
10
8
6