Re: Some function like Position[] but uses criteria, not pattern?

*To*: mathgroup at smc.vnet.net*Subject*: [mg99439] Re: Some function like Position[] but uses criteria, not pattern?*From*: Roland Franzius <roland.franzius at uos.de>*Date*: Tue, 5 May 2009 06:08:29 -0400 (EDT)*References*: <gtp182$jm1$1@smc.vnet.net>

Nasser Abbasi schrieb: > V 7.0 > > Position[] works by returning position of elements which matches a pattern. > This is all good and well. > > But sometimes, I need to find the positions (i.e index) of elements that > matches a criteria, not a pattern. > > For example, using pattern > > a = {1, 0, 4, -5, 8} > Position[a, 0] > Out[15]= {{2}} > > But what if I wanted the position of the elements which, say, are larger > than 3? this is not a pttern, but a criteria. Which in this example should > find elements which are in position 3 and 5 in list 'a' above. > > I can't type > Position[a, #1 > 3 & ] > > Now, Select[] does take a criteria, good, so I could type > > Select[a, #1 > 3 & ] > {4, 8} > > But I wanted the positions of the elements, not the elements themselves. > > I could use Select to find the elements, then use Position to find the > position of these elements as follows > > c = Select[a, #1 > 3 & ]; > Flatten[(Position[a, #1] & ) /@ c] > > Out[65]= {3, 5} > > But I think this is bit messy. I think Position[] or new function similar, > should have an option to return position of elements based on criteria, not > just pattern. The above gets more messy for me, when I wanted to find > position of elements in a matrix based on some criteria. > > For example, find the position (i.e. row and column) index in a matrix (list > of lists) of those elements which matches some criteria. Say >3. Use PatternTest _? x = Array[RandomInteger[{1, 9}] &, {10, 10}] Position[x,_?(#>3&)] -- Roland Franzius