Re: Is there a function for sum of square matrix antidiagonal like "trace()" for main diagonal?

[Loïc Grenié <loic.grenie@gmail.com>]

Le lun. 18 nov. 2024 à 16:48, <hermann@stamm-wilbrandt.de> a écrit :

On 2024-11-18 15:57, Bill Allombert wrote:
>
> or
>
> atr(~M)=my(j=#M+1);sum(i=1,#M,j--;M[i,j]);
> atr2(~M)=sum(i=1,#M,M[i,#M+1-i]);
> for(i=1,100000,atr(~M))
> for(i=1,100000,atr2(~M))
>
Thanks Bill,

that atr2 is fastest.


On 2024-11-18 15:34, hermann@stamm-wilbrandt.de wrote:
>
> M~ is transpose matrix M.
> Is there some other operator in GP such that
> atr(M) == trace(op(M)) ?
>
trace(op(~M)) is slower than atr2, but I finally got it:

op(~M)=Mat([M[-i,]~|i<-[-#M..-1]]);

op(M)=Mat(Vecrev(M));

  but I expect it to be slower than the sum (too many copies).

          Loïc