MATLAB: How to make a lower triangular matrix from a column vector

Question

I am trying to make a lower triangular matrix from a column vector to then produce a fully populated 24×24 symmetric matrix.

The column vector I'm using (A as a simple example) has 300 elements and I need to populate the triangular matrix (B as a simple example) as follows:

A = [i1 i2 i3 i4 i5 i6 i7 i8 i9 i10 …]

B = [i1; i2 i3; i4 i5 i6; i7 i8 i9 i10; ….]

Any help would be appreciated. It may be that it's easier to make the symmetric matrix straight away, if so I'm open to suggestions. Thanks in advance.

Answer 1

Create the symmetric matrix directly:

n   = 24;
Ind = zeros(n, n);
Ind((1:n) >= (1:n).') = 1:300;
Ind = Ind + Ind.' - diag(diag(Ind));
B = A(Ind);