How to find the indices of non-zero elements in a matrix
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Hello,
I have an Na-by-Nt matrix which is sparse, i.e.: most elements are zeros. I want to find the indices of the non-zeros elements in the form of (i,j) where i is the row and j is the column.
I have then these two Na-by-1 vector a and Nt-by-1 vector tau. i corresponds to the ith element in a and j the jth element in tau, and I want to find them as well.
How can I do the above in an efficient way without the need for for loops?
Thanks
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Cedric
2014-6-27
编辑:Cedric
2014-6-27
If it is for storing only non-zero elements, MATLAB supports SPARSE matrices. If you need to get row/column indices of non-zero elements of A:
[rId, cId] = find( A ) ;
and if you need values as well:
[rId, cId, val] = find( A ) ;
If you just need values, you can get them simply with
vals = A(A ~= 0) ;
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Cedric
2014-6-30
编辑:Cedric
2014-6-30
I meant a simple/minimal numeric example showing which triplets you need to ID in which data structure, because I am not sure that anybody will have time to understand your code/algorithm.
If it is working but too slow, the first thing that comes to my mind is to replace the last double FOR loop with
A = complex( zeros( numel(s), Np * Ntau )) ;
colId = 0 ;
for pp=1:Np
theta_p=fc.*kron(1+Doppler_Tentative_Set(pp),ones(N))-fr;
Gamma_p=exp(1i*pi*T.*theta_p).*sinc(T.*theta_p);
for tt=1:Ntau
Lambda_p_diag = exp(-1i*2*pi.*(f0+(0:N-1)/T).*Tau_Tentative_Set(tt));
colId = colId + 1 ;
A(:,colId) = bsxfun( @mtimes, Gamma_p, Lambda_p_diag ) * s ;
end
end
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