Re: How Do You Reduce Multiple Elements in a List?

• To: mathgroup at smc.vnet.net
• Subject: [mg33875] Re: How Do You Reduce Multiple Elements in a List?
• From: bghiggins at ucdavis.edu (Brian Higgins)
• Date: Sun, 21 Apr 2002 06:12:22 -0400 (EDT)
• References: <a9r3ve\$ltc\$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```Bob, Here are some possibilities:

mylist = {1, 1, 1, 1, 1, 2, 2, 2, 4, 4, 4, 4, 4, 4};
In[32]:=Map[{#[[1]], Length[#]} &, Split[mylist]]

Out[32]={{1, 5}, {2, 3}, {4, 6}}

In[31]:=Split[mylist] /. {x__Integer} :> {First[{x}], Length[{x}]}

Out[31]={{1, 5}, {2, 3}, {4, 6}}

Cheers,

Brian

Bob Harris <nitlion at mindspring.com> wrote in message news:<a9r3ve\$ltc\$1 at smc.vnet.net>...
> Howdy,
>
> This seems like such a simple thing, yet I've search through the big book
> trying to figure it out.  Perhaps someone can help me...
>
> Given a list of integers that generally contains the same number many times,
> I'd like to reduce it to a list that just gives me the number/count pairs.
> For example, I'd like to convert the list
>    {1, 1, 1, 1, 1, 2, 2, 2, 4, 4, 4, 4, 4, 4}
> into
>    {{1,5}, {2,3}, (4,6}}
>
> Is there a built-in function to do this?  Seems like a histogramming
> operation, but I get no useful matches from ?*Hist*.
>
> Thanks,
> Bob H

```

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