Re: Is transpose costless?

[John W. Eaton]

On 7/15/19 11:47 AM, Rik wrote:

> I had a question about optimization in the parser.  If I write
>
> x = x.';
>
> is that effectively a costless operation that simply swaps the first two
> values of the dimension vector?  Or does Octave first create a temporary
> intermediate (which is costless because of COW), but then the transpose
> operator forces a deep copy to the intermediate, and then the final
> assignment takes place (also potentially another deep copy).

Transpose for Array<T> objects, which includes numeric arrays, cell 
arrays, and values of the fields of struct arrays (because they are 
stored as cell arrays) is handled in liboctave/array/Array.cc. 
Transpose only works for 2-dimensional arrays and there is a special 
case for empty arrays and vectors that just swaps the dimensions and 
shares the data array.  Otherwise, if the number of rows and columns are 
both greater than 1, we have to copy data.

jwe