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MathGroup Archive 2007

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Re: Iterate through a list

  • To: mathgroup at smc.vnet.net
  • Subject: [mg76951] Re: [mg76886] Iterate through a list
  • From: Murray Eisenberg <murray at math.umass.edu>
  • Date: Wed, 30 May 2007 05:32:04 -0400 (EDT)
  • Organization: Mathematics & Statistics, Univ. of Mass./Amherst
  • References: <200705290902.FAA03917@smc.vnet.net>
  • Reply-to: murray at math.umass.edu

Easy:  Call your list lis.  Then:

   First /@ lis
{1,1,2,2,4,4,4,4,4,4,4,4,4,4,5,5,5,5,5,5,5,5,8,8,8,8,8,8,8,8,8,8}

Explanation: For a list a = {stuff,morestuff,stillmorestuff,andmore} of 
whatever length, First[a] gives the first entry (stuff) of the list. 
You want to do that with each element of your list lis, and what does 
this is to "map" the function First onto that list, like this:

   Map[First,lis]

An abbreviated way to do the same thing is:

   First /@ lis

Nick Hoffman wrote:
> Hello! I'm (somewhat) new to Mathematica and I  need help to iterating
> through a list and counting the occurrence of a number.
> 
> My list looks like:
> 
> {{1,0},{1,1},{2,x+x^2+x^4},{2,1+x+x^2+x^4},{4,x+x^2},{4,1+x+x^2},{4,x+x^4},{4,1+x+x^4},{4,x^2+x^4},{4,1+x^2+x^4},{4,x^3+x^4},{4,1+x^3+x^4},{4,x+x^2+x^3+x^4},{4,1+x+x^2+x^3+x^4},{5,x+x^3},{5,1+x+x^3},{5,x^2+x^3},{5,1+x^2+x^3},{5,x+x^3+x^4},{5,1+x+x^3+x^4},{5,x^2+x^3+x^4},{5,1+x^2+x^3+x^4},{8,x},{8,x^2},{8,x^3},{8,x^4},{8,1+x},{8,1+x^2},{8,1+x^3},{8,x+x^2+x^3},{8,1+x+x^2+x^3},{8,1+x^4}}
> 
> 
> 
> The number that I would need to keep track of is the first in each sub-
> list.
> 
> So for the first couple {{1,0},{1,1},{2,x+x^2+x^4},{2,1+x+x^2+x^4},{4,x
> +x^2},........
> 
> I would need to keep track of 1,1,2,2,4........
> 
> It will not always be 1,2,4,5,8's.  The numbers may be completely
> different on each pass.  Each sublist may be composed of more items
> but the number that I need to keep track of will always be the 1st in
> the sub-list.  Any help or suggestions that you can give me will be
> greatly appreciated.  Thanks so much!
> 

-- 
Murray Eisenberg                     murray at math.umass.edu
Mathematics & Statistics Dept.
Lederle Graduate Research Tower      phone 413 549-1020 (H)
University of Massachusetts                413 545-2859 (W)
710 North Pleasant Street            fax   413 545-1801
Amherst, MA 01003-9305


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