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MathGroup Archive 2007

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Re: Iterate through a list help

  • To: mathgroup at smc.vnet.net
  • Subject: [mg77286] Re: Iterate through a list help
  • From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
  • Date: Wed, 6 Jun 2007 07:14:56 -0400 (EDT)
  • Organization: The Open University, Milton Keynes, UK
  • References: <f43gfu$2cq$1@smc.vnet.net>

Nick 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!

Part[lst, All, 1] will do what you want.

In[1]:=
lst = {{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}};

lst[[All,1]]

Out[2]=
{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}

Regards,
Jean-Marc


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