For this, I think I would have gone with:
selPrime=: #~ 1&p:
seed=: selPrime digits=: 1+i.9
step=: selPrime@,@:(,&.(10x&#.inv)/&>)@{@;
digits&step&.>^:a: ,<seed
Which, granted, is not particularly fast. (That last line takes about
15 seconds on my current laptop.)
Not sure if this helps, though...
--
Raul
On Sun, Nov 20, 2022 at 8:37 PM 'Pascal Jasmin' via Programming
<[email protected]> wrote:
>
> based on Raul's rosettacode link, but building an expaning list of left
> truncable primes, such that further search is possible on future "iterations"
>
>
> selPrime=: #~ 1&p:
>
> ltrunc =: (] , ((1+i.9) (10 #. ,)"0 1 (10&#.inv))"0(,@:)(selPrime@:) each@:{:)
>
> # &> sofar =: ltrunc^:6 < 3 7x
>
> 2 11 39 99 192 326 429
>
> # &> sofar =: ltrunc^:6 sofar
>
> 2 11 39 99 192 326 429 521 545 517 448 354 276
>
> the number of truncable primes decreases substantially as the digits
> increase. (last answer is for 1 to 13 digits, with single digit primes
> wrongly listed (4 is right number of single digit primes)
>
> the full list eventually goes to 0
>
> # &> sofar =: ltrunc^:1 sofar (NB. iterations skipped)
>
> 2 11 39 99 192 326 429 521 545 517 448 354 276 212 117 72 42 24 13 6 5 4 3 1 0
>
> which I guess we knew from example number
>
> > _2 { sofar
>
> 357686312646216567629137
>
>
> On Saturday, November 19, 2022 at 07:04:25 p.m. EST, Raul Miller
> <[email protected]> wrote:
>
>
>
>
>
> On Sat, Nov 19, 2022 at 6:47 PM 'Skip Cave' via Programming
> <[email protected]> wrote:
> > Now what is the J verb that will find an n-digit integer that is still
> > prime when each of the digits are removed?
>
> I'd probably go with https://rosettacode.org/wiki/Truncatable_primes#J for
> that.
>
> Thanks,
>
> --
> Raul
>
> ----------------------------------------------------------------------
> For information about J forums see http://www.jsoftware.com/forums.htm
> ----------------------------------------------------------------------
> For information about J forums see http://www.jsoftware.com/forums.htm
----------------------------------------------------------------------
For information about J forums see http://www.jsoftware.com/forums.htm
