matrix formation from two given matrix

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

Answer 1

[Walter Roberson]

"for" loop around repmat() in order to build the diagonal vector, and then diag() that.

Or just use a nested "for" loop.

Answer 2

[Andrei Bobrov]

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)]

The old-school (fast) way of run length decoding :-)

Answer 3

[Jos (10584)]

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{:})

[Jos (10584)]

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{:}])

Answer 4

[Osman mamun]

thanks, i was looking for something like repmat.