Services & Resources / Wolfram Forums / MathGroup Archive
-----

MathGroup Archive 2009

[Date Index] [Thread Index] [Author Index]

Search the Archive

Re: Some function like Positionbut uses criteria, not pattern?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg99483] Re: Some function like Positionbut uses criteria, not pattern?
  • From: Bill Rowe <readnews at sbcglobal.net>
  • Date: Wed, 6 May 2009 05:28:03 -0400 (EDT)

On 5/5/09 at 5:37 AM, nma at 12000.org (Nasser Abbasi) wrote:

>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.

Here is one way to accomplish what you want

In[5]:= Pick[Range[Length@a], a, _?(# > 3 &)]

Out[5]= {3,5}

Alternatively, you could use Position as follows:

In[6]:= Position[a, _?(# > 3 &)]

Out[6]= {{3,5}}




  • Prev by Date: Functions in partial differential equations with different number of
  • Next by Date: Re: defining consecutive variables
  • Previous by thread: Functions in partial differential equations with different number of
  • Next by thread: Given a matrix, find position of first non-zero element in each row