Re: [Maxima-discuss] submatrix assignment ala Matlab

[Viktor T. T. <vt...@vt...>]

In the case of your example, you could use


  apply(matrix,makelist(M[i],i,41,50));


or even something like


  transpose(apply(matrix,makelist(transpose(apply(matrix,makelist(M[i],i,41,50)))[j],j,11,20)));


to get a submatrix that consists of rows 41-50, columns 11-20.

For what it's worth, there may be more efficient ways to do this, but I 
just tried it with a 1000 x 1000 matrix, and the result was effectively 
instantaneous.

If you use this often, it is of course easy to encapsulate it in the 
form of a function.



Viktor





On 5/2/2024 12:07 PM, 德馬堤 wrote:
>
> I am also missing the feature posted in 
> https://sourceforge.net/p/maxima/mailman/message/35760048/ 
> <https://sourceforge.net/p/maxima/mailman/message/35760048/>.
>
> In Matlab it is very easy to handle matrices.
>
> In Maxima the function /submatrix (i_1, …, i_m, M, j_1, …, j_n) /seems 
> cumbersome if one really wants to go beyond toy problems. Say if I 
> have a matrix that stores 50 rows, and I want to get only the last 10 
> rows, then I would have to write "submatrix(1,2,3,4,5,6,...,40,M)".
>
> In Matlab it would simply be "M(41:50,:)".
>
>
> And what if the matrix has 1000 rows?
>
>
> I really like Maxima, and it helped me quite a lot in my studies. But 
> I also got used to Matlab, and if it comes to numerical computing, I 
> think Maxima should be improved.
>
>
>
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