Re: sort indexing

[John W. Eaton]

On 02/15/2016 03:39 PM, Doug Stewart wrote:
> I have a question about indexing.
>
> a=randi(9,5)
> [b i]=sort(a)
> c=a(i)
>
> I would think that c should be the same as b, but it is not.
>
> The index array i has all the correct information in it as can be seen with
>
> for k=1:columns(a)
> w(:,k)=a(i(:,k),k);
> endfor
> now b-w is equal to 0
>
> Is there  some technical reason that if you try and use the index array
> i on an matrix
> of the same dimensions, that it can't work?
> I would think that it should apply each col of the i to a  in a(i)
> as I did in the loop.
>
> I know it can be vectorized:
>
>   a(sub2ind (size(a), i, repmat(1:4, rows(a), 1)))
>   a(i+(0:columns(a)-1)*rows(a))
>
> but I just think that octave should be smart enough to just do a(i)

In your example, the index I is a matrix, but it's just the sort order 
for each individual column.  When indexing an array (not a vector) with 
a single argument, Octave uses it as a linear index into the array and 
returns a result with the same shape as the index.  It works this way 
because that's what Matlab does so we can't change it to mean something 
else.

jwe