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

• To: mathgroup at smc.vnet.net
• Subject: [mg33874] RE: [mg33863] How Do You Reduce Multiple Elements in a List?
• From: "David Park" <djmp at earthlink.net>
• Date: Sun, 21 Apr 2002 06:12:21 -0400 (EDT)
• Sender: owner-wri-mathgroup at wolfram.com

```Bob,

list = {1, 1, 1, 1, 1, 2, 2, 2, 4, 4, 4, 4, 4, 4};

Split[list]
{First[#], Length[#]} & /@ %

{{1, 1, 1, 1, 1}, {2, 2, 2}, {4, 4, 4, 4, 4, 4}}
{{1, 5}, {2, 3}, {4, 6}}

If your list is not initially in order you will want to use
Split[Sort[list]].

David Park

> From: Bob Harris [mailto:nitlion at mindspring.com]
To: mathgroup 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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