Re: Iterate through a list

• To: mathgroup at smc.vnet.net
• Subject: [mg76945] Re: Iterate through a list
• From: David Bailey <dave at Remove_Thisdbailey.co.uk>
• Date: Wed, 30 May 2007 05:28:57 -0400 (EDT)
• References: <f3gqtq\$42r\$1@smc.vnet.net>

```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!
>
This will give you a list of two-element lists. The first element is a
number found, and the second is the number of times it occurs:

ss = {{1, 1}, {2, 1}, {2, x + y}, {5, x^2 + 2}}

Tally[Map[First, ss]]

Output: {{1, 1}, {2, 2}, {5, 1}}

Note that Tally is new in 6.0, but there are other ways if you have an
older version.

David Bailey
http://www.dbaileyconsultancy.co.uk

```

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