Bill Allombert on Wed, 15 Feb 2012 21:25:30 +0100

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Re: Question on subst(), deferred evaluation and trap()

On Wed, Feb 15, 2012 at 10:14:10AM -0800, Jack Brennen wrote:
> Using GP, I'm trying to set up a vector full of expressions on the
> variable x, and then evaluate the vector with each non-computable
> element replaced by the number 0.  (Could be any value, 0 works for
> my purposes...)
> Basically, I'd like to do something like this:
>   V=[trap(,0,1/(x-1)),trap(,0,1/(x-2)),trap(,0,1/(x-3))];
>   N=3;subst(V,x,N) -> [1/2, 1, 0]    ...  doesn't do the right thing.
> However, that doesn't work because the trap() functions are
> evaluated immediately rather than stored in the vector.
> So far, the best I've come up with is:
>   V=[1/(x-1),1/(x-2),1/(x-3)];
> And then instead of a simple subst() on V, I use:
>   N=3;vector(#V,j,trap(,0,subst(V[j],x,N)))
> However, for cases where the trap is not taken, my method is about
> 10 times slower than just doing subst(V,x,N).

So your problem boils down to performance.
What is your OS and PARI/GP version ?

I ask because it seems trap is much slower on your system than on our test systems.