Indices after accumarray in order to do N-dimensional histograms
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Quentin Garçon
le 6 Juin 2019
Commenté : Quentin Garçon
le 10 Juin 2019
Hello everyone,
I am using an accumarray function in my script in order to computer N-dimensional histograms. This is the line:
ht = accumarray(sub, 1, nBins+1 .* ones(1, nFeat));
where nBins stands for the number of Bins in my histogram and nFeat the number of features (so the dimension of my histogram).
If I use for example this set of data:
[1 2 ; 2 3 ; 1 2 ; 1 3 ; 1 1 ; 3 2]
My 2-dimensional histogram will be:
H = [1 2 1;
0 0 1;
0 1 0]
The first value for example says that there is 1 occurrence of 1 1, the second value 2 occurrences of 1 2...
However I want to keep the indices. In this example, I would like to have something like that:
Hind = {5}, {[1 3]}, {4};
{ }, { }, {2};
{ }, { 6 }, { }
I don't really care about it being in cells or something else.
The number is the cell are just the linear indices of the values that contributed to the histogram.
Basically, I want the indices that could allow me to generate my histogram for example by doing:
H = cell2mat(cellfun(@numel, Hind, 'Un', false));
I hope this was clear.
Thanks a lot,
Quentin
4 commentaires
Steven Lord
le 9 Juin 2019
So nFeat could be up to 10. What is an upper bound on nBins? If it's too large, even storing "only the indices" could require more memory than is available on the planet or in the known universe.
Réponse acceptée
Walter Roberson
le 9 Juin 2019
ht = accumarray(sub, (1:size(sub,1)).', nBins+1 .* ones(1, nFeat), @(v) {v});
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