Find indices of non-zero elements from [1,2,0,0,4,0] in julia and create an Arrray with them

Question

I know there is a function that does this, for example:

A = [1,2,0,0,4,0]
find(A)

 3-element Array{Int64,1}:
 1
 2
 5

I am trying to do it on my own way, however, I am stuck here

for i=1:endof(A)
   if A[i] != 0
      []
   end
end

Thanks in advance.

Answer 1

Here's one alternative:

function myfind(c)
    a = similar(c, Int)
    count = 1
    @inbounds for i in eachindex(c)
        a[count] = i
        count += (c[i] != zero(eltype(c)))
    end
    return resize!(a, count-1)
end

It actually outperformed find for all the cases I tested, though for the very small example vector you posted, the difference was negligible. There is perhaps some performance advantage to avoiding the branch and dynamically growing the index array.

Answer 2

I have notice that the question is really confusion (because is poorly formulated, sorry about that). Therefore, there are two possible answers: one is [1,2,4]which is an array with the non-zero elements; the other is [1,2,5] which is an array of the indices of the non-zero elements.

Let´s begin with the first option

A = [1,2,0,0,4,0]
B = []

for i=1:endof(A)
    if A[i] != 0
        push!(B,i)
    end
end
println(B)

The output is Any[1,2,5]

However, the type is not the one I wanted. Using typeof(B) it shows Array{Any,1} so I added this code:

B = Array{Int64}(B)
println(B)
typeof(B)

And the result is the desired

[1,2,5]
Array{Int64,1}

To improve its efficiency, following with the recommendations in the comments, I have specified the type of B with the eltype() function before the loop as follows:

A1 = [1,2,0,0,4,0] #The default type is Array{Int64,1}
B1 = eltype(A1)[] #Define the type of the 0 element array B with the type of A
#B1 = eltype(typeof(A))[] this is also valid
for i=1:endof(A1)
    if A1[i] != 0
        push!(B1::Array{Int64,1},i::Int64)
    end
end
println(B1)
typeof(B1)

Then, the output is again the desired

[1,2,5]
Array{Int64,1}

The simplest way of doing this is using the function find(). However, since I´m a beginner, I wanted to do it in another way. However, there is another alternative provided by @DNF that outperform find() for the cases he has tested it (see below answers).

The second option, which creates an output matrix with the non-zero elements has been provided by other users (@Harrison Grodin and @P i) in this discussion.

Thanks all of you for the help!