Re: A list of numbers without "73"
- To: mathgroup at smc.vnet.net
- Subject: [mg63027] Re: A list of numbers without "73"
- From: Peter Pein <petsie at dordos.net>
- Date: Sun, 11 Dec 2005 04:56:52 -0500 (EST)
- References: <dnee1r$1ep$1@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
Thomas Schmelzer schrieb:
> Experts,
>
> S(i) should be a list of positive integers without the substring "73".
>
> My code seems to be very inefficient. I multiply all numbers with 0 if they
> contain "73", sort the new list, make the union (remove double elements) and
> delete the first element. Therefore it is necessary to use an if statement
> in order not to delete the first element of the numbers 1-9. Any more
> sophisticated ideas?
>
> L := Table[k, {k, 1, 9}]
>
> S[i_] := S[i] =
> If[i == 1, L,
> Delete[Union[
> Table[k*(1 - Boole[StringMatchQ[ToString[k], "*73*"]]), {k,
> 10^(i - 1), 10^i - 1}]], 1]]
>
>
>
> Best
>
> Thomas
>
>
>
>
Hi Thomas,
it seems to be more efficient to construct only numbers without the
sequence 73 instead of testing already existing ones:
s2[1] = Range[1, 9];
s2[i_] := Flatten[(10*#1 + If[Mod[#1, 10] == 7,
{0, 1, 2, 4, 5, 6, 7, 8, 9},
Range[0, 9]] & ) /@ s2[i - 1]]
Timing[Length[#1[6]]]& /@ {S, s2}
--> {{7.765*Second, 854569}, {0.953*Second, 854569}}
Ceers,
Peter