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Re: finding positions of elements in a list

  • To: mathgroup at smc.vnet.net
  • Subject: [mg86854] Re: finding positions of elements in a list
  • From: "Dana DeLouis" <dana.del at gmail.com>
  • Date: Sun, 23 Mar 2008 01:04:02 -0500 (EST)

> I also tried Position but it wasn't clear to me if a pattern can be used
for this.

Hi.  You have answers, but I see you were stuck on using "Position."
Here's one way if you ever need to use it:


pts = {{20, 109}, {21, 50}, {20, 110}, {22, 60}};

Position[pts, {___, n_} /; n > 80]

{{1}, {3}}

Extract[pts, %]

{{20, 109}, {20, 110}}

-- 
HTH   :>)
Dana DeLouis


"PeterM" <metaxas at lps.u-psud.fr> wrote in message
news:frt5e9$srg$1 at smc.vnet.net...
> Hi.
> 
> Having a few problems here with what I think should be a simple operation.
I have a list as shown:
> 
> mylist={{20, 109}, {20, 110}, {20, 111}, {21, 105}, {21, 106}, {21,
107},...}
> 
> It's a list of {x,y} co-ordinates of some data points I'm analysing.
> 
> I want to create a new list which contains only those data points whose y
co-ordinate is greater than 80 ie. mylist[[n,2]]>80.
> 
> Problem is, I can't work out how to use Select to do this when each
element in the list has an x and a y co-ordinate. If I split the list into x
and y co-ordinates and searched for those y values greater than 80, I would
have a list of those y-values but would not know the corresponding x-values.

> 
> I also tried Position but it wasn't clear to me if a pattern can be used
for this.
> 
> Thanks in advance, Pete.
>




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