MATLAB: How to get elegantly an upper triangle random matrix (0 elsewhere)

matrixmatrix manipulation

I have seen the triu and tril functions, but I can't use them to set to 0:

A = rand(4);
tril(A)=0; % won't work

any idea, other than using 2 for loops?

Best Answer

triu(A,1)

Related Solutions

MATLAB: Converting Upper triangle to lower triangle

If the question consists of converting lower triangular binary matrix into upper triangular one, then is it possible to use logical operation xor, there are two possible values in a={1,0}, the kernel is b=1, so xor(a=1,b)=0 and xor(a=0,b)=1.

 n=10;
 A=ones(n);
 A=tril(A);
 B=xor(A,ones(n));

diagonal elements can be adjusted with options of tril function.

MATLAB: Randomise the upper triangle of a matrix

If you are going to do this randomization 10000 times, you would want it to be as efficient as possible. Here is a possible way. Let the matrix be of size n x n. (Note that I am assuming by the upper triangular part of a matrix you mean the part above the main diagonal but not including the diagonal.)

Do this just once and save f and n2:

 f = find(triu(ones(n),1)==1);
 n2 = length(f);

Then each of the 10000 times if your n x n matrix is M, do this:

 M(f) = rand(n2,1); % Or use randn or any other random number function

You accomplish the randomization in one line and with only n*(n-1)/2 calls on the random number function.

Best Answer

Related Solutions

MATLAB: Converting Upper triangle to lower triangle

MATLAB: Randomise the upper triangle of a matrix

Related Question