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<h4 class="subsection">21.1.2 Creating Permutation Matrices</h4>

<p>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 <var>q</var> is a permutation vector of length <var>n</var>, the expression
</p>
<div class="example">
<pre class="example">  P = eye (n) (:, q);
</pre></div>

<p>will create a permutation matrix - a special matrix object.
</p>
<div class="example">
<pre class="example">eye (n) (q, :)
</pre></div>

<p>will also work (and create a row permutation matrix), as well as
</p>
<div class="example">
<pre class="example">eye (n) (q1, q2).
</pre></div>

<p>For example:
</p>
<div class="example">
<pre class="example">  eye (4) ([1,3,2,4],:)
&rArr;
Permutation Matrix

   1   0   0   0
   0   0   1   0
   0   1   0   0
   0   0   0   1

  eye (4) (:,[1,3,2,4])
&rArr;
Permutation Matrix

   1   0   0   0
   0   0   1   0
   0   1   0   0
   0   0   0   1
</pre></div>

<p>Mathematically, an identity matrix is both diagonal and permutation matrix.
In Octave, <code>eye (n)</code> 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.
</p>
<div class="example">
<pre class="example">  eye (3)
&rArr;
Diagonal Matrix

   1   0   0
   0   1   0
   0   0   1

  eye(3)(1:3,:)
&rArr;
Permutation Matrix

   1   0   0
   0   1   0
   0   0   1
</pre></div>

<p>Some other built-in functions can also return permutation matrices.  Examples
include
<em>inv</em> or <em>lu</em>.
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