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.
© 1996–2018 John W. Eaton
Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies.
Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one.Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions.
https://octave.org/doc/interpreter/Explicit-and-Implicit-Conversions.html