Getting the average of only nonzero elements of a matrix

[Jude]

Hi!

Probably a quite basic question but was wondering if someone knows a quick way to get the average of theh positive elements of a matrix, eg, If I have an m*n matrix and want to get the average for each m so have m*1 but only want to work with non-zero elements and ignore the zero elements and negative elements

I guess I could using indexing and pull out the indexes corresponding to the positive elements but just wondering if anyone knew quicker ways!

Thanks!

[Stefan Karpinski]

You mean quicker as in faster or as in easier? Using indexing is probably easiest but not fastest:

julia> M = randn(5,5)

5x5 Array{Float64,2}:

 -1.51134     1.2331    -0.186083   0.412282  -1.13114

 -0.0411132   0.124684   0.377426   0.622427  -0.278162

  0.788182   -0.834092   1.09506    1.45196   -0.620717

  1.45856    -0.889277  -0.768049  -0.95953   -0.340098

  1.69666     0.695566   0.706905   0.63089    0.958474

julia> mean(M[M .> 0])

0.8751552351448673

If you want the fastest, loop through the matrix and keep totals and counts:

julia> function meanpos(a)

           t = zero(eltype(a))

           c = 0

           for x in a

               x > 0 || continue

               t += x

               c += 1

           end

           return t/c

       end

meanpos (generic function with 1 method)

julia> meanpos(M)

0.8751552351448673

This should be fairly efficient for any non-sparse collection.

[Tom Breloff]

I would just write the loop explicitly:

julia> function meanpositives(mat::Matrix)

           nc = size(mat,2)

           sums = zeros(nc)

           counts = zeros(nc)

           for c in 1:nc

               for r in 1:size(mat,1)

                   v = mat[r,c]

                   if v > 0

                       sums[c] += v

                       counts[c] += 1.0

                   end

               end

           end

           Float64[(counts[i]>0 ? sums[i]/counts[i] : 0.0) for i in 1:nc]

       end

meanpositives (generic function with 1 method)

julia> x

3x3 Array{Float64,2}:

 10.0  5.0  0.0

  5.0  0.0  0.0

  0.0  0.0  0.0

julia> meanpositives(x)

3-element Array{Float64,1}:

 7.5

 5.0

 0.0

[Stefan Karpinski]

Oh, didn't catch that you wanted row sums, but yeah, same basic idea.

[Tom Breloff]

Heh... neither did I... I gave you code for column sums!  Should be easy enough to convert...

[Jude]

Thanks Stefan and Tom, that's really helpful!  Stefan, I meant quicker as in faster.

Thanks a lot!