remove cat(X...) statements

[odunbar]

For convenience we often use cat(X..., dims=p) statements to concatenate vectors and matrices. During some profile I see these become terribly inefficient at even moderate problems. We should probably remove them. As an example"

X = [zeros(3,3) for i = 1:1000]

# option 1 
xx = cat(X...,dims=3)

# option 2
function squidge(X::AbstractVector) 
       xx = zeros(size(X[1],1),size(X[1],2),length(X))
       for i = 1:length(X)
           xx[:,:,i] = X[i]
       end
       return xx
end
xx = squidge(X)

option 1 gives e.g.:
0.037136 seconds (10.01 k allocations: 13.965 MiB)
option 2 gives e.g.:
0.000088 seconds (2 allocations: 70.375 KiB)

Action

Replace cat(X...,dims=p) with the appropriate loops.

[haakon-e]

Alternative with julia base functions (ref):

function squidge2(x)
    B = reduce(hcat, x);
    sz = size(X[1])
    C = reshape(B, sz[1], sz[2], :);
end

Gets: 0.000073 seconds (6 allocations: 70.516 KiB)