Re: multi-dimensional logical index assignment?

[Markus Mützel]

Am 02. Mai 2020 um 19:36 Uhr schrieb "Nicholas Jankowski":
> I'm trying to do a 3+ dimensional version of what is shown here:
> https://stackoverflow.com/questions/18129298/how-to-use-double-indexing-to-redefine-matrix-values
>  
>  
> and the syntax eludes me. The example shown is clear enough for a 2D matrix, 
> e.g.,:
>  
> >> A = [1 2; 3 4];
> >> A(2,:)((A(2,:)>3)) = 0
> error: () must be followed by . or close the index chain
> >> A(2,A(2,:)>3) = 0
> A =
>    1   2
>    3   0
> 
> but if I have a 3D array and I want to change the values of 'page' 2 based on 
> the values of 'page' 4, can that be done in a similar one line assignment 
> without using an intermediate variable?  e.g., 
> 
> >> a = rand(2,3,4);
> >> a(:,:,4)>0.5
> ans =
>   1  1  1
>   1  1  0
> >> a(:,:,2)(a(:,:,4)>0.5) = 0
> error: () must be followed by . or close the index chain
> >> a((a(:,:,4)<0.5),2) = 0
> error: Invalid resizing operation or ambiguous assignment to an out-of-bounds 
> array element
>  
> I feel like I'm missing something obvious. 

I think the problem here is that it is not possible to combine logical indexing 
with subscript indexing.
Maybe converting both to linear indices could do the trick:

a = rand (2, 3, 4);
sz_a = size (a);

src_pg = 4;
target_pg = 2;
a(find (a(:,:,src_pg)<0.5) + (target_pg-1)*prod (sz_a(1:2))) = 0;


"find" effectively converts the logical indices to linear indices. The rest is 
an offset to index into the second page.

HTH,
Markus