# Sparse metrix higher value as input value

3 views (last 30 days)
Frederic Stichler on 13 Nov 2023
Answered: Harald on 13 Nov 2023
I want to create a sparse matrix and then sort it. But after initialisation, the highest value in the sparse matrix is higher than the highest input value.
vertexFaceConnectivity=sparse(F(:),ind,ind,numVertices,numFaces);
vertexFaceConnectivity=sort(vertexFaceConnectivity,2,'descend');
max(vertexFaceConnectivity(:)) % 3658
ans =
(1,1) 3658

Harald on 13 Nov 2023
Hi Frederic,
you may not be using sparse the way you intend. In this syntax, the third input is the values of the sparse matrix. Perhaps, you wanted something more like
vertexFaceConnectivity=sparse(ind,ind,F(:),numVertices,numFaces);
Since ind repeats values, that however would mean that you are setting the same values repeatedly - is that intended?. sparse then sums those values, as you can see in this mini-example:
ind = [1 2 3 1 2 3];
F = 1:6;
sparse(ind, ind, F)
ans =
(1,1) 5 (2,2) 7 (3,3) 9
If I had to guess, I would think that you intended to create a tri-diagonal matrix? I would use spdiags for that.
Best wishes,
Harald

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