Next: Explicit and Implicit Conversions, Previous: Creating Diagonal Matrices, Up: Creating and Manipulating Diagonal/Permutation Matrices [Contents][Index]
For creating permutation matrices, Octave does not introduce a new function, but rather overrides an existing syntax: permutation matrices can be conveniently created by indexing an identity matrix by permutation vectors. That is, if q is a permutation vector of length n, the expression
P = eye (n) (:, q);
will create a permutation matrix - a special matrix object.
eye (n) (q, :)
will also work (and create a row permutation matrix), as well as
eye (n) (q1, q2).
For example:
eye (4) ([1,3,2,4],:) ⇒ Permutation Matrix 1 0 0 0 0 0 1 0 0 1 0 0 0 0 0 1 eye (4) (:,[1,3,2,4]) ⇒ Permutation Matrix 1 0 0 0 0 0 1 0 0 1 0 0 0 0 0 1
Mathematically, an identity matrix is both diagonal and permutation matrix.
In Octave, eye (n)
returns a diagonal matrix, because a matrix
can only have one class. You can convert this diagonal matrix to a permutation
matrix by indexing it by an identity permutation, as shown below.
This is a special property of the identity matrix; indexing other diagonal
matrices generally produces a full matrix.
eye (3) ⇒ Diagonal Matrix 1 0 0 0 1 0 0 0 1 eye(3)(1:3,:) ⇒ Permutation Matrix 1 0 0 0 1 0 0 0 1
Some other built-in functions can also return permutation matrices. Examples include inv or lu.