Re: finding positions of elements in a list

*To*: mathgroup at smc.vnet.net*Subject*: [mg86807] Re: [mg86761] finding positions of elements in a list*From*: Bob Hanlon <hanlonr at cox.net>*Date*: Fri, 21 Mar 2008 01:56:03 -0500 (EST)*Reply-to*: hanlonr at cox.net

There are many different ways mylist = {{20, 109}, {20, 110}, {20, 111}, {21, 105}, {21, 106}, {21, 107}}; threshold = 108; Select[mylist, #[[2]] > threshold &] Cases[mylist, _?(#[[2]] > threshold &)] Cases[mylist, {_, _?(# > threshold &)}] DeleteCases[mylist, _?(#[[2]] <= threshold &)] mylist /. {_, _?(# <= threshold &)} -> Sequence[] Pick[mylist, Thread[mylist[[All, 2]] > threshold]] Bob Hanlon ---- PeterM <metaxas at lps.u-psud.fr> 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. >