# How to create a matrix from multiples of submatrice?

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Timon Viola il 11 Giu 2020
Modificato: madhan ravi il 11 Giu 2020
I would like to find a smarter way of building a larger matrix from vectors. The following code works fine:
rv = [1 2 3 4];
alpha =[0.0140;
0.0100;
0.0100;
0.0100];
c = [];
for i = 1:4
c = [c; diag(alpha).*rv(i)];
end
>> c =
0.0140 0 0 0
0 0.0100 0 0
0 0 0.0100 0
0 0 0 0.0100
0.0280 0 0 0
0 0.0200 0 0
0 0 0.0200 0
0 0 0 0.0200
0.0420 0 0 0
0 0.0300 0 0
0 0 0.0300 0
0 0 0 0.0300
0.0560 0 0 0
0 0.0400 0 0
0 0 0.0400 0
0 0 0 0.0400
Essentially I build a submatrix from a vector. Then I multiple the diagonal submatrix with each element of a vector. Then I concatenate these vertically.
Is there a computationally more efficient way of doing this?
Thanks!
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### Risposta accettata

madhan ravi il 11 Giu 2020
Modificato: madhan ravi il 11 Giu 2020
Z = reshape(rv,1,1,[]) .* diag(Alpha);
Wanted = reshape(Z,numel(rv),[]).'
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Michael Soskind il 11 Giu 2020
Even more confusing but with no tansposes and in one line:
repmat(diag(alpha),4,1).*reshape(repmat(rv, 4,4),[],4)
madhan ravi il 11 Giu 2020
Modificato: madhan ravi il 11 Giu 2020
" and in one line"
Wanted = reshape(reshape(rv,1,1,[]) .* ...
diag(Alpha),numel(rv),[]).'
"Even more confusing"
Why?
N = 1e6;
tic
for k = 1:N
Z = reshape(rv,1,1,[]) .* diag(Alpha);
Wanted = reshape(Z,numel(rv),[]).';
end
% Elapsed time is 4.847994 seconds.
toc
tic
for k = 1:N
repmat(diag(alpha),4,1) .* reshape(repmat(rv, 4,4),[],4);
end
toc
% Elapsed time is 5.865444 seconds.

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