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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


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