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matlabfor-loopmatrixindexingvectorization
Nishant
Nishant

Reputation: 2619

Matlab: Access matrix elements using indices stored in other matrices

I am working in matlab. I have five matrices in ,out, out_temp,ind_i , ind_j, all of identical dimensions say n x m. I want to implement the following loop in one line.

out = zeros(n,m)
out_temp = zeros(n,m)
for i = 1:n
    for j = 1:m
        out(ind_i(i,j),ind_j(i,j)) = in(ind_i(i,j),ind_j(i,j));
        out_temp(ind_i(i,j),ind_j(i,j)) = some_scalar_value;              
    end
end

It is assured that the values in ind_i lies in range 1:n and values in ind_j lies in range 1:m. I believe a way to implement line 3 would give the way to implement line 4 , but I wrote it to be clear about what I want.

Upvotes: 1

Views: 85

Answers (1)

Divakar
Divakar

Reputation: 221544

Code

%// Calculate the linear indices in one go using all indices from ind_i and ind_j
%// keeping in mind that the sizes of both out and out_temp won't go beyond
%// the maximum of ind_i for the number of rows and maximum of ind_j for number
%// of columns
ind1 = sub2ind([n m],ind_i(:),ind_j(:))

%// Initialize out and out_temp
out = zeros(n,m)
out_temp = zeros(n,m)

%// Finally index into out and out_temp and assign them values 
%// using indiced values from in and the scalar value respectively.
out(ind1) = in(ind1);
out_temp(ind1) = some_scalar_value;

Upvotes: 1

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