Speed up comparing two arrays and write into new array

Hello,

I compare values of two arrays, and write the values of an associated array into a new one. Unfortunately, this comparison is quite slow. I know vectorization is a way to do it, but I don't know how in this case. Below are smaller arrays for testing, it can go up to 1'000'000 x T or so. Looking at the profiler data, the comparison takes 99% of the time for below code snippet with large arrays.

I got three initial arrays:

coordIndex: 514x4 double (integer), with most values occurring several times. Same values as in Vert_ID_mat, so 218 different values. Array can contain NaN-Values

Vert_ID_mat: 218x1 double (integer), every value only once

Colors_Res: 514x4 cell (strings). Can contain zeros, which are associated to the NaN-values

Resulting array:

Assoc_Colors: 218x1 cell (strings)

I use following code right now:

for k = 1:size(coordIndex,1)
    for kk = 1:size(coordIndex,2)
        for i = 1:size(Vert_ID_mat,1)
            if coordIndex(k,kk) == Vert_ID_mat(i)        %isequal
                Assoc_Colors{i} = Colors_Res(k, kk);
            end
        end
    end
end

Assoc_Colors = Assoc_Colors.';

How can I write a faster code and achieve a faster processing time?

Best regards

Daniel

Answer 1

[Bruno Luong]

For simplification I use numerical data for Colors_Res/Assoc_Colors adatp tou your data type

clear
% Dummy test data
coordIndex=randi(9,8,9);
Colors_Res=rand(size(coordIndex));
Vert_ID_mat=randi(9,10,1);

% Your method
for k = 1:size(coordIndex,1)
    for kk = 1:size(coordIndex,2)
        for i = 1:size(Vert_ID_mat,1)
            if coordIndex(k,kk) == Vert_ID_mat(i)        %isequal
                Assoc_Colors{i} = Colors_Res(k, kk);
            end
        end
    end
end
Assoc_Colors = Assoc_Colors.';
Assoc_Colors

Assoc_Colors = 10×1 cell array
    {[0.2511]}
    {[0.6004]}
    {[0.0595]}
    {[0.0595]}
    {[0.3949]}
    {[0.2511]}
    {[0.0595]}
    {[0.0937]}
    {[0.8265]}
    {[0.0935]}

% Vectorized method
coordIndexTrans = coordIndex.';
Colors_ResTrans = Colors_Res.';
[tf,loc]=ismember(Vert_ID_mat(:), coordIndexTrans(:),'legacy');
Assoc_Colors = cell(size(Vert_ID_mat));
Assoc_Colors(tf)=num2cell(Colors_ResTrans(loc(tf)))

Assoc_Colors = 10×1 cell array
    {[0.2511]}
    {[0.6004]}
    {[0.0595]}
    {[0.0595]}
    {[0.3949]}
    {[0.2511]}
    {[0.0595]}
    {[0.0937]}
    {[0.8265]}
    {[0.0935]}

[Daniel Rohrer]

Thank you very much for your fast response. Your solution works perfectly fine!

Edit: Runtime of an array of 150'000 x T went down from 720s to only 0.17s. That's a significant improvement =)