Karim Belabas on Wed, 08 May 2013 22:16:02 +0200


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Re: Extracting numbers from strings 

* Richard in Reading [2013-05-08 18:40]:
> I'm using forell to search through elliptic curves and I want to extract the numeric value of the conductor preferably without using ellinit and ellglobalred which make it too slow.
> As an example, I'd like to be able to replicate the following
> 
> forell(E,1,100,print(E[1]"\t"ellglobalred(ellinit(E[2]))[1]))
> 
> 11a1    11
> 11a2    11
> 11a3    11
> 14a1    14
> 14a2    14
> 
> just by manipulating E[1] instead of dealing with E[2]. Hopefully this would be faster.

For this particular problem, you can use

  N = ellconvertname(E[1])[1]

( or even [N,isogeny_class,index] = E[1] in 2.6.* )

In general, you would have to use Vecsmall(str) then test ASCII values
(not pretty), or use Unix-specific hacks of the form

  str_to_cond(str) = extern(Str("echo ", str, "| sed 's/[^0-9].*//'"))

Cheers,

    K.B.
--
Karim Belabas, IMB (UMR 5251)  Tel: (+33) (0)5 40 00 26 17
Universite Bordeaux 1          Fax: (+33) (0)5 40 00 69 50
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