Re: size and display of N-d matrices (octave-3.4.2)

[Ben Abbott]

On Aug 10, 2011, at 3:26 PM, Sergei Steshenko wrote:

> Hello,
> 
> here is a screen session:
> 
> "
> octave:1> a(1,1,1,1) = 1
> a =  1
> octave:2> a(1,1,1,2) = 2
> a =
> 
> ans(:,:,1,1) =  1
> ans(:,:,1,2) =  2
> 
> octave:3> a
> a =
> 
> ans(:,:,1,1) =  1
> ans(:,:,1,2) =  2
> 
> octave:4> size(a)
> ans =
> 
>   1   1   1   2
> 
> octave:5> b(1,1,1,1) = 3
> b =  3
> octave:6> b(2,1,1,1) = 4
> b =
> 
>   3
>   4
> 
> octave:7> b
> b =
> 
>   3
>   4
> 
> octave:8> size(b)
> ans =
> 
>   2   1
> 
> octave:9>   
> ".
> 
> Questions:
> 
> 1) why does 'octave' treat 'a' and 'b' differently ?

For "b" the 3rd and 4th indices do not effect how the data is stored in memory. 
Thus, they are superfluous and are pruned just as they are in Matlab.

> 2) why 'b' is shown to have just two dimensions ?

Because only two dimensions are needed to access the data.

        a = 1:5;
        a(1,5,1,1,1,1,1,1,1)
        ans =  5

> 3) why colons when 'a' is displayed ?


It isn't clear in the context of your example, but if you try something a bit 
more complex, you'll see that 2D arrays are displayed one at a time.

        rand(3,2,3)

        ans(:,:,1) =

           0.876498771074170   0.228249951644748
           0.838197338433857   0.112306693206123
           0.881025682532475   0.835890938593277

        ans(:,:,2) =

           0.359380637345459   0.930091325028269
           0.531543061712508   0.041806801803684
           0.327139516557781   0.199321073527378

        ans(:,:,3) =

           0.294857085276711   0.719111880031177
           0.602939270626089   0.753767065450199
           0.633018499368617   0.063436978647205

Ben