Re: Inverse of IntegerDigits? and "Index" function?

*To*: mathgroup at smc.vnet.net*Subject*: [mg3958] Re: Inverse of IntegerDigits? and "Index" function?*From*: espen.haslund at fys.uio.no (Espen Haslund)*Date*: Sat, 11 May 1996 23:53:31 -0400*Organization*: Universitet i Oslo*Sender*: owner-wri-mathgroup at wolfram.com

In article <4menl1$jva at dragonfly.wolfram.com>, pehowland at taz.dra.hmg.gb says... ;> ;>Excuse me if I'm being particularly inept this morning, but can anyone offer ;>any help on the following problems please? ;> ;>Inverse of IntegerDigits ;>======================== ;>I'm playing around with some binary numbers, and am using IntegerDigits ;>to convert from a decimal integer to the binary notation. eg. ;> In[1]:= IntegerDigits[10,2] ;> Out[1]= {1,0,1,0} ;> ;>What is the function to convert back? eg. I want to do ;> In[2]:= DigitsToInteger[{1,0,1,0},2] ;>and get ;> Out[2]= 10 ;>I've scoured the manual and can't find the function. This led me to ;>try to write my own function, which identified another problem... ;> ;>"Index" Function ;>================ ;>Writing a function to convert back is easy in principle, eg. ;> ;> DigitsToInteger[x_,n_] := Apply[Plus[Map[(#*n^(Index[#]-1))&, Reverse[x]]]] ;> ;>would do the trick, but I seem to require a function, that I've called ;>Index[] here, that when Map[]'d across a list returns the position of ;>each element. Thus I want a function that works as follows: ;> ;> In[3]:= Map[Index[#]&, {2,6,3,9,10,2,4}] ;> Out[3]= {1,2,3,4,5,6,7} ;> ;>But I can't seem to find this function either! ;> ;>For the moment I'm using the following more Fortran-like function, which ;>works, but disturbs my sensibilities: .... ;>Any suggestions? ;> Hi, Paul: I think your Index function is impossible to implement. I hope that the function below does what you want: DigitsToInteger[x_,n_] := Plus @@ (Table[n^i,{i,Length[x],1,-1}]*x) -Espen ==== [MESSAGE SEPARATOR] ====