MATLAB: Lower triangular semidefinite matrix

Question

how can i build a sparse matrix in this form for a given semidefinite matrix? i only need the sparse lower triangular of semidefinite matrix

for example a semidefinite matrix as Q0:

2 0 -1

Q0 = 0 0.2 0

-1 0 2

is written as:

i = [1 2 3 3 ];

j = [1 2 3 1 ];

c = [2 0.2 2 –1 ];

where i shows number of row

j shows number of column

and c is the elemnt that belong to (i,j) that is not zero and belong to lower triangular of semidefinite matrix

Answer 1

[i, j, c] = find(tril(yourmatrix))

seems to be what you're after.