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Re: finding positions of elements in a list

  • To: mathgroup at smc.vnet.net
  • Subject: [mg86779] Re: finding positions of elements in a list
  • From: Szabolcs Horvát <szhorvat at gmail.com>
  • Date: Thu, 20 Mar 2008 05:49:38 -0500 (EST)
  • Organization: University of Bergen
  • References: <frt5e9$srg$1@smc.vnet.net>

PeterM wrote:
> 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.
> 

Two possibilties:

Select[mylist, #[[2]] > 80 &]
Cases[mylist, {x_, y_} /; y > 80]


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