Re: Condensing indices in a matrix

On 12/8/20 6:10 AM, Brett Green wrote:
> Is it possible to construct a matrix of the following form without for 
> loops?
> 
> Start with a multidimensional array
> M( T1 , n1 , m1 , T2 , n2 , m2 )
> where we have the same size in the T1 and T2 dimensions, the n1 and n2 
> dimensions, and the m1 and m2 dimensions.
> 
> Then, for each n and m, set
> M( 1 , n , m , 2 , n , m ) = a;
> M( 2 , n , m , 1 , n+1 , m ) = b;
> M( 2 , n , m , 1 , n , m+1 ) = c;
> for some numbers a, b and c. I can do this with e.g.
> M( 1 , : , : , 2 , : , : ) = a;
> so this is not a problem.
> 
> Then condense the multidimensional array to a 2D array
> M( u(T1,n1,m1) , u(T2,n2,m2) )
> where u is some function that condenses the indices from tuples of 3 to 
> single indices. The function u can be anything that condenses our 
> indices this way, though it'd be especially nice if I could define it 
> myself.
> 
> I am familiar with using reshape to condense all indices. Is there also 
> a way to condense only certain indices? Or, is there another way to 
> construct this matrix besides reshaping a multidimensional array?
> 
> - Brett Green
> 


Was it possible to send some minimal working example how array M is 
created?  For u(x,y,z), maybe `sub2ind` [1] can be helpful.

HTH,
Kai

[1] https://octave.org/doc/v6.1.0/Advanced-Indexing.html