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Re: Finding position of an element in a list:

  • To: mathgroup at smc.vnet.net
  • Subject: [mg84015] Re: Finding position of an element in a list:
  • From: Albert Retey <awnl at arcor.net>
  • Date: Thu, 6 Dec 2007 07:25:32 -0500 (EST)
  • References: <fj89m3$a90$1@smc.vnet.net>

Hi,

> (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)

this has presumably absolutely nothing to do with precision...

> 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 *)

I don't think that this function is a good choice for the problem at 
hand: conversion of expressions to string to manipulate them is rarely a 
good idea and I think Peter's function served a very different purpose...

> n1 = Nearest[xlis, 6]
> eval = dropOuterBraces[n1]

If you use InputForm[eval] or FullForm[eval] you will find that 
dropOuterBraces returns a string, not a number, so it is the expected 
behavior that no occurance of the string "5.5" is found within the list 
containing the number 5.5. One solution could be to revert eval to a 
number with ToExpression[eval], but why not just use:

n1=First[Nearest[xlis,6]]

Position[xlis,n1]

which extracts the first element of the list which Nearest returns and 
works perfectly for me. Note that Nearest returns always a list, 
presumably since

1. it is possible to have more than one nearest member, like in:
    Nearest[{1,3},2]
2. it is possible to ask Nearest for the n nearest, like in:
    Nearest[{1,2,3,4,5,6,7,8,9},5,4]

In your case, only the first of these cases could make Nearest return a 
list longer than one. In this case additional information is needed to 
decide which one you want n1 to be set to. Also note that 
dropOuterBraces would return a string "1,3" for case 1, which doesn't 
make sense even if converted back with ToExpression.

Conclusion: don't use ToString when not necessary...

hth,

albert


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