subvec
==============================================
Purpose
----------------
Extracts an Nx1 vector of elements from an NxK matrix.
Format
----------------
.. function:: y = subvec(x, ci)
:param x: data
:type x: NxK matrix
:param ci: column indices
:type ci: Nx1 vector
:return y: contains the elements in *x* indicated by *ci*.
:rtype y: Nx1 vector
Examples
----------------
::
// Create an additive sequence from 1-12, i.e. 1, 2, 3,...12
x = seqa(1, 1, 12);
// Reshape the sequential vector 'x' into a 4x3 matrix
x = reshape(x, 4, 3);
// The column indices (one per row of 'x') indicating which
// values to extract from 'x'
ci = { 2, 3, 1, 3 };
// Extract subvector from 'x' and assign it to 'y'
y = subvec(x, ci);
After the above code, *x* and *y* are equal to:
::
1 2 3
x = 4 5 6
7 8 9
10 11 12
2
y = 6
7
12
Remarks
-------
Each element of *y* is from the corresponding row of *x* and the column set
by the corresponding row of *ci*. In other words, :math:`y[i] = x[i, ci[i]]`.