Any command in matlab equivilent to vech() in gauss

The definition of vech() in Gauss is

vech
Purpose Vectorizes a symmetric matrix by retaining only the lower triangular portion of the matrix.

Format v = vech(x);
Input
 Output
 Remarks As you can see from the example below, vech will not check to see if x is symmetric. It just packs the lower triangular portion of the matrix into a column vector in row-wise order.

Example x = seqa(10,10,3) + seqa(1,1,3)’;
v = vech(x);
sx = xpnd(v);
x NxN symmetric matrix.
v (N*(N+1)/2)x1 vector, the lower triangular portion of the matrix 

x
11 12 13
21 22 23
31 32 33
=
v
11
21
22
31
32
33

Something like

mask = tril(true(size(a)),0);
out = a(mask);

can only return like

11
21
31
22
32
33

Answer 1

[dpb]

It's simply row-major vis a vis column-major Matlab storage order --

 >> triu(x')
ans =
    11    21    31
     0    22    32
     0     0    33
>> ans(ans>0)
ans =
    11
    21
    22
    31
    32
    33
>>

[Guillaume]

or as a one liner:

nonzeros(triu(x'))

Both assume that there are no 0 in the original matrix. So, this may be safer:

transx= x'
transx(triu(true(size(transx))))

[dpb]

"assume that there are no 0 in the original matrix"

Trudat; was only showing OP really the only difference is internal storage order.

With your refinement on selection of values to keep, it probably would be clearer to write

tril(x).'

as the selection which at least does use tril instead of triu the latter of which is undoubtedly disconcerting.. :)

It would take some playing, however, to get the testing to match for the original array vis a vis the transposed triangular one; not sure a one-liner is then possible altho I didn't work on it too long...