• To: mathgroup at christensen.cybernetics.net
• From: wagner at bullwinkle.cs.Colorado.EDU (Dave Wagner)
• Date: Wed, 26 Jul 1995 00:48:08 -0400
• Organization: University of Colorado, Boulder

```In article <3ui5a6\$231 at news0.cybernetics.net>,
Samuel H. Cox <insshc at gsusgi2.gsu.edu> wrote:
>The function RealDigits[x, b] returns a list of two items.  For example,
>
>RealDigits[Pi //N]
>{{3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5, 8, 9, 7, 9, 3}, 1}
>
>What is the inverse of this function?  That is, given the base b, a list
>{...} of base b digits, and an interger n, how do we elegantly obtain x for
>which RealDigits[x, b] = {{...},n}?

Here's one:

In[70]:=
Fold[r #1 + #2 &, 0, mantissa] /
r ^ (Length[mantissa]-exponent)
]

In[71]:=
RealDigits[N[Pi]]
Out[71]=
{{3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5, 8, 9, 7, 9, 3}, 1}

In[72]:=
unRealDigits[%]
Out[72]=
3.14159

In[73]:=
RealDigits[N[Pi], 8]
Out[73]=
{{3, 1, 1, 0, 3, 7, 5, 5, 2, 4, 2, 1, 0, 2, 6, 4, 3, 0}, 1}

In[74]:=
unRealDigits[%, 8]
Out[74]=
3.14159

In[75]:=
% == %%%
Out[75]=
True

Dave Wagner
Principia Consulting
(303) 786-8371
dbwagner at princon.com
http://www.princon.com/princon

```

• Prev by Date: Re: Simple problem for math wiz
• Next by Date: Re: Re: projected gradient