Re: reverse of digits()

[Bill Allombert <Bill.Allombert@math.u-bordeaux1.fr>]

On Wed, Oct 17, 2012 at 09:01:08PM -0400, Mathieu Carbou wrote:
> Hello,
> 
> I've seen the new digits() and sumdigits() functions. I was wondering if
> there was also the reverse function of digits (i.e.
> vectonum([1,2,3,4,5], 10) == 12345) plus these functions which could be
> useful ;-)

Your function can be done in a single line of GP:

vectonum(v,b=10)=subst(v,variable(v),b)

For ndigits, you can use 
ndigits(n,b=10)=#ndigits(n,b)

for the last one, this is related to
mynumtoperm(n,a)=Vec(Vecsmall(numtoperm(n,a))^-1*vectorsmall(n,i,n+1-i))

Cheers,
Bill.