Cumsum reversed?

General Usage
3

If you want performance you probably need to use a custom function. This should be close to optimal:

julia> function rev_cumsum!(c,x)
           cumsum = zero(eltype(x))
           @inbounds for i in lastindex(x):-1:firstindex(x)
               cumsum += x[i]
               c[i] = cumsum
           end
           return c
       end
rev_cumsum! (generic function with 1 method)

julia> function rev_cumsum(x)
           c = similar(x)
           return rev_cumsum!(c,x)
       end
rev_cumsum (generic function with 1 method)

julia> x = rand(1:10,1000);

julia> @btime reverse(cumsum(reverse($x)));
  2.516 μs (3 allocations: 23.81 KiB)

julia> @btime rev_cumsum($x);
  570.379 ns (1 allocation: 7.94 KiB)

julia> @btime rev_cumsum!(c,$x) setup=(c=similar(x));
  364.524 ns (0 allocations: 0 bytes)

(edit: added @inbounds, which increases significantly the performance)

2 Likes