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

[hermann@stamm-wilbrandt.de]

On 2024-11-18 19:21, Bill Allombert wrote:
> > ? atr2
> > (~M)->sum(i=1,#M,M[i,#M+1-i])
> > ? for(i=1,100000,atr2(~M))
> > cpu time = 615 ms, real time = 614 ms.
> > ? op(~M)=trace(Mat(Vecrev(M)));
> > ? for(i=1,100000,op(~M))
> > cpu time = 589 ms, real time = 591 ms.
>
> I would be very happy if that were true, but
>
> M=matrix(100,100,i,j,random(1000));
> atr2(~M)=sum(i=1,#M,M[i,#M+1-i]);
> op(~M)=trace(Mat(Vecrev(M)));
> for(i=1,100000,atr2(~M))
>   ***   last result computed in 1,201 ms.
> for(i=1,100000,op(~M))
>   ***   last result computed in 11,051 ms.
>
> I assume your matrix is very small ?
Sorry, yes, it was a 15x15 Primes square matrix I wrote
a validate.gp script for:
https://www.mersenneforum.org/node/1055343?p=1061101#post1061101


Definitely trace(Mat(Vecrev(~M))) looks nicer, but
should not be used for larger matrices.

Regards,

Hermann.