MATLAB: How to make following part of code without for loop

for loopmatrix

How can I make following part of code without for loop?: 
for i=1:n_values
    pre_matrix(i,J{i})=0
end
J{i} is a cell array that each cell includes some numbers that I want to change those elements in i-th row of pre_matrix. Each cell has different numbers and size of cells are different.
pre_matrix is very huge matrix, therefore it takes so long time to create matrix everytime. Is it possible to do same work without for loop?

Best Answer

I'm having a hard time finding a solution that's faster than the original loop... 
% Build test data
n_values = 10000;
nc = 100;
pre_matrix = rand(n_values,nc);
sz = size(pre_matrix);
 
J = cell(1,n_values);
for ii = 1:n_values
    n0 = randi(nc,1);
    J{ii} = randi(nc,1,n0);
end
 
% The original loop
 
tic
for i=1:n_values
    pre_matrix(i,J{i})=0;
end
toc
 
% Indexing directly, but with loop to get row indices (Rik's suggestion)
 
tic
I = cell(size(J));
for ii = 1:n_values
    I{ii} = ii.*ones(size(J{ii}));
end
idx = sub2ind(sz, cat(2, I{:}), cat(2, J{:}));
pre_matrix(idx) = 0;
toc
 
% Indexing directly, no explicit loops
 
tic
I = cellfun(@(a,b) ones(size(a)).*b, J, num2cell(1:n_values), 'uni', 0);
 
idx = sub2ind(sz, cat(2, I{:}), cat(2, J{:}));
pre_matrix(idx) = 0;
toc
Results:
Elapsed time is 0.017597 seconds. <- loop
Elapsed time is 0.047797 seconds. <- indexing with loop
Elapsed time is 0.119106 seconds. <- indexing with cellfun
My guess here is perhaps the culprit is that missing semicolon in the loop? If you're printing out the huge matrix every iteration, that's definitely going to take some time! But the loop itself is pretty fast.
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