(octave.info)Explicit and Implicit Conversions

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21.1.3 Explicit and Implicit Conversions
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The diagonal and permutation matrices are special objects in their own
right.  A number of operations and built-in functions are defined for
these matrices to use special, more efficient code than would be used
for a full matrix in the same place.  Examples are given in further
sections.

   To facilitate smooth mixing with full matrices, backward
compatibility, and compatibility with MATLAB, the diagonal and
permutation matrices should allow any operation that works on full
matrices, and will either treat it specially, or implicitly convert
themselves to full matrices.

   Instances include matrix indexing, except for extracting a single
element or a leading submatrix, indexed assignment, or applying most
mapper functions, such as “exp”.

   An explicit conversion to a full matrix can be requested using the
built-in function “full”.  It should also be noted that the diagonal and
permutation matrix objects will cache the result of the conversion after
it is first requested (explicitly or implicitly), so that subsequent
conversions will be very cheap.