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MathGroup Archive 2005

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Re: Re: FromDigits[{135,21}] -> 1371 (??!!)

  • To: mathgroup at smc.vnet.net
  • Subject: [mg53765] Re: [mg53763] Re: FromDigits[{135,21}] -> 1371 (??!!)
  • From: Bob Hanlon <hanlonr at cox.net>
  • Date: Thu, 27 Jan 2005 05:40:57 -0500 (EST)
  • Reply-to: hanlonr at cox.net
  • Sender: owner-wri-mathgroup at wolfram.com

FromDigits just implements a polynomial in the specified base.

FromDigits[{a1,a2,a3,a4}]

10 (10 (10 a1+a2)+a3)+a4

FromDigits[{a1,a2,a3,a4},b]==
    Fold[b*#1+#2&,0,{a1,a2,a3,a4}] // ExpandAll

True

Consequently, FromDigits is almost an inverse function of CoeffiicientList.  
They use reverse orderings of the coefficients. 

poly = x^4 + 2*x^3*y + 5*x*y^2 + 13*y^4; 

poly==
  FromDigits[Reverse[CoefficientList[poly,x]],x]==
  FromDigits[Reverse[CoefficientList[poly,y]],y]

True


Bob Hanlon

> 
> From: "Astanoff" <astanoff at yahoo.fr>
To: mathgroup at smc.vnet.net
> Date: 2005/01/26 Wed AM 04:37:40 EST
> To: mathgroup at smc.vnet.net
> Subject: [mg53765] [mg53763] Re: FromDigits[{135,21}] -> 1371 (??!!)
> 
> > Can anyone please explain what happens,
> > if "Digits" actually are "integers":
> > FromDigits[{3,2}]
> > 32(!)
> > FromDigits[{13,2}]
> 
> Digit : "One of the ten Arabic number symbols, 0 through 9,
> or such a symbol used in a system of numeration".
> So, without any base specified, FromDigits[{13,2}]
> assumes default base 10 and should output an error message.
> Imho, that's a bug.
> 
> v.a.
> 
> 


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