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

[hermann@stamm-wilbrandt.de]

On 2024-11-18 16:59, Loïc Grenié wrote:
> > >
> > 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).
Thank you, I have used Vecrev() before, but did not think about it.
And yes and no regarding slower:
While "trace(op(~M))" is slower, moving "trace()" into "op()"
makes it slightly faster than atr2 from Bill:

? 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.
?


Regards,

Hermann.