matrix formation from two given matrix

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i want to form a diagonal matrix from two matrix, one contains the value of diagonal elements and other contains how many times it should occur.For example: a=[5 10 15] and b=[2 2 2] so the resultant matrix should look like r=[5 0 0 0 0 0;0 5 0 0 0 0;0 0 10 0 0 0;0 0 0 10 0 0;0 0 0 0 15 0;0 0 0 0 0 15]; But the problem is a and b can vary depending upon the situation or user input.
can i form such a matrix? I will highly appreciate any kind of suggestion.
Thanks, Mamun

Accepted Answer

Walter Roberson
Walter Roberson on 18 Apr 2014
"for" loop around repmat() in order to build the diagonal vector, and then diag() that.
Or just use a nested "for" loop.

More Answers (3)

Andrei Bobrov
Andrei Bobrov on 18 Apr 2014
Edited: Andrei Bobrov on 18 Apr 2014
a=[5 10 15 20];
b=[2 1 4 3];
x = cumsum(b);
v = zeros(x(end),1);
v(x-b+1) = 1;
out = diag(a(cumsum(v)));

Jos (10584)
Jos (10584) on 18 Apr 2014
A = [5 10 15 20] ;
B = [2 1 4 3] ;
C = arrayfun(@(k) A(k)*eye(B(k)),1:numel(A),'un',0) ;
result = blkdiag(C{:})
  1 Comment
Jos (10584)
Jos (10584) on 18 Apr 2014
Edited: Jos (10584) on 18 Apr 2014
or using REPMAT and DIAG (which is therefore also, probably, more memory efficient):
A = [5 10 15 20] ;
B = [2 1 4 3]
C = arrayfun(@(k) repmat(A(k),1,B(k)), 1:numel(A),'un',0) ;
result2 = diag([C{:}])

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Osman mamun
Osman mamun on 18 Apr 2014
thanks, i was looking for something like repmat.

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