[R] All the products of common factors
Stavros Macrakis
macrakis at alum.mit.edu
Wed Feb 25 08:01:50 CET 2009
Argh! The second (concise) version should have |, not & !!!
-s
On 2/24/09, Stavros Macrakis <macrakis at alum.mit.edu> wrote:
> "L'esprit de l'escalier" strikes again....
>
> An even simpler statement of your original problem:
>
> Find the factors that A and B have in common.
>
> If A and B are fairly small (< 1e7, say), a very direct approach is:
>
> which( ! (A %% 1:min(A,B)) & !(B %% 1:min(A,B)) )
>
> Is this "brute force"? Well, I suppose, but it is simple and direct
> and fast enough for A=B=1e7 (5 sec). It doesn't involve factorization
> into prime factors, GCDs, or combinations.
>
> If your goal is concision and not clarity or speed, you can do even better:
>
> which( !(A %% 1:B & B %% 1:A) )
>
> which is still practical (though it gives a warning).
>
> How big do your A and B get, and how many different A's and B's do you
> need to run this calculation for?
>
> -s
>
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