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Re: finding positions of elements in a list

  • To: mathgroup at smc.vnet.net
  • Subject: [mg86791] Re: [mg86761] finding positions of elements in a list
  • From: Curtis Osterhoudt <cfo at lanl.gov>
  • Date: Fri, 21 Mar 2008 01:53:00 -0500 (EST)
  • Organization: LANL
  • References: <200803200755.CAA29380@smc.vnet.net>
  • Reply-to: cfo at lanl.gov

  Hi, Peter, 

     Select is my first approach. Note that I stuck some values into "mylist" 
for which the y-coordinates are less than 80, just to test:

In[29]:= mylist = {{20, 109}, {20, 110}, {22, 18}, {20, 111}, {21, 
   105}, 
     {21, 106}, {21, 107}, {22, 34}, {21, 12}}

Out[29]= {{20, 109}, {20, 110}, {22, 18}, {20, 111}, {21, 105}, {21, 
  106}, 
   {21, 107}, {22, 34}, {21, 12}}

In[30]:= myNewList = Select[mylist, #1[[2]] > 80 & ]

Out[30]= {{20, 109}, {20, 110}, {20, 111}, {21, 105}, {21, 106}, {21, 
  107}}


              Hope that helps!
                           C.O.

         
On Thursday 20 March 2008 01:55:37 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.



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