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Re: Finding position of an element in a list:

  • To: mathgroup at smc.vnet.net
  • Subject: [mg84008] Re: Finding position of an element in a list:
  • From: dh <dh at metrohm.ch>
  • Date: Thu, 6 Dec 2007 07:21:51 -0500 (EST)
  • References: <fj89m3$a90$1@smc.vnet.net>


Hi Gopinath,

with version 6 we have the function Nearest:

Nearest[xlis -> Automatic, 6]

hope this helps, Daniel





Gopinath Venkatesan 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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