Efficient way to sum an array of integers in Julia

Question

I have an 2D array which I want to modify so as to sum a given element in a row with all the elements before it, so for example, if I have an array:

[1 2; 3 6; 4 7; 4 8]

I want to be able to transform it to

[1 2; 4 8; 8 15; 12 23]

I can do so using the following snippet in julia:

for i in 1:10,
   for k in 2:size(d,1),
          d([k,i] += d[k-1,i)];
   end
end

But I assume there must be a more efficient way to do this?

Answer 1

Yes, there is: cumsum

julia> d = [1 2; 3 6; 4 7; 4 8]
4x2 Array{Int64,2}:
 1  2
 3  6
 4  7
 4  8

julia> cumsum(d)
4x2 Array{Int64,2}:
  1   2
  4   8
  8  15
 12  23

Answer 2

As per @tholy's comment, the awesome thing about julia is that built-in functions aren't special and aren't magically faster than user-defined ones. They are both fast. I modified your function and I got it to perform about the same as the built-in cumsum:

function testA!(arr)
    @inbounds for i in 1:size(arr, 2)
        tmp = arr[1, i]
        for k in 2:size(arr,1)
            tmp += arr[k, i]
            arr[k,i] = tmp
        end
    end
    arr
end

function testB!(arr)
    cumsum!(arr, arr)
end

I constructed test arrays:

arr = rand(1:100, 10^5, 10^2)
arr2 = copy(arr)

and I got the following timings:

@time testA!(arr)
0.007645 seconds (4 allocations: 160 bytes)

@time testB!(arr2)
0.007704 seconds (4 allocations: 160 bytes)

which are basically equivalent.