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Re: Finding position of an element in a list:

  • To: mathgroup at smc.vnet.net
  • Subject: [mg84005] Re: [mg83986] Finding position of an element in a list:
  • From: Bob Hanlon <hanlonr at cox.net>
  • Date: Thu, 6 Dec 2007 07:20:15 -0500 (EST)
  • Reply-to: hanlonr at cox.net

nearestPosition[xlis_List, val_?NumericQ] :=
  Flatten[Position[xlis, #]] & /@
   Nearest[xlis, val];

xlis = {0, 1, 3, 10, 4, 5, 5.5, 10.25};

nearestPosition[xlis, 6]

{{7}}

xlis = {0, 1, 6.5, 10, 4, 5, 5.5, 10.25};

nearestPosition[xlis, 6]

{{3}, {7}}


Bob Hanlon

---- Gopinath Venkatesan <gopinathv at ou.edu> wrote: 
> Hello Friends,
> 
> (This is an easy problem, but I was unable to get it work in Mathematica, probably because of the precision - I tried setting precision too, still got stuck)
> 
> Suppose we have a list, xlis, having some elements, and I have to find one nearest value I am interested in that list. In the below example, I wanted to find the position of a number belonging to list and close to 6 (which is 5.5 located at 7th).
> 
> I can do this:
> 
> xlis = {0, 1, 3, 10, 4, 5, 5.5, 10.25};
> dropOuterBraces[lst_List] := 
>  StringTake[#1, {2, StringLength[#1] - 1}] &[
>   ToString[lst]]; (* took from previous help posted by Peter Pein *)
> n1 = Nearest[xlis, 6]
> eval = dropOuterBraces[n1]
> Position[xlis, eval]
> Position[xlis, 5]
> 
> As you see, the last but one command returns nothing, and we have to explicitly say the number to return the position.
> 
> Or should some other command be used to extract the position of an arbitrary number in a list?
> 
> Same with the case of MemberQ command, it gives True for some and False for some. For example, consider this list,
> 
> xlis = Table[N[((1 - Cos[(j - 1)/9 \[Pi]])/2) 1.32], {j, 1, 10}];
> dropOuterBraces[lst_List] := 
>  StringTake[#1, {2, StringLength[#1] - 1}] &[ToString[lst]]
> 
> eval = Nearest[xlis, 0.32];
> xlis
> elem = dropOuterBraces[eval]
> MemberQ[xlis, 0.545392]
> MemberQ[xlis, 0.33]
> Position[xlis, 0.33]
> Position[xlis, elem]
> 
> Please hint where I am wrong.
> 
> Thanks,
> 
> Gopinath Venkatesan
> Graduate Student
> University of Oklahoma
> 



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