Add rows from one matrix to another by index

New to Julia
1

In short I want to index into a matrix and add to each row.

F = zeros(Int64,5)
idx = [2,5,2] # same row repeated several times
fi = [5,2,7]
F[idx] += fi

Desired:

julia> F
5-element Array{Int64,1}:
  0
 12
  0
  0
  2

actual:

 F
5-element Array{Int64,1}:
 0
 7
 0
 0
 2
2

When there is repetition, I would just write a loop:

for (i, f) in zip(idx, fi)
    F[i] += f
end

(not that sorting by idx may improve performance)

1 Like
3

You could also use

view(F, idx) .+= fi

But the way @Tamas_Papp recommended might be more explicit and legible.

2 Likes
4

Just for fun, you can also try broadcasting:

((i,f) -> F[i]+=f).(idx,fi)

Performance-wise, a loop will always be faster, though.

1 Like