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finding positions of elements in a list

• To: mathgroup at smc.vnet.net
• Subject: [mg86761] finding positions of elements in a list
• From: PeterM <metaxas at lps.u-psud.fr>
• Date: Thu, 20 Mar 2008 02:55:37 -0500 (EST)

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.

• Follow-Ups:
  • Re: finding positions of elements in a list
    • From: Curtis Osterhoudt <cfo@lanl.gov>
  • Re: finding positions of elements in a list
    • From: "Thomas Dowling" <thomasgdowling@gmail.com>
  • RE: finding positions of elements in a list
    • From: "David Annetts" <davidannetts@aapt.net.au>