Re: Using Select

• To: mathgroup at smc.vnet.net
• Subject: [mg97459] Re: Using Select
• From: Szabolcs Horvát <szhorvat at gmail.com>
• Date: Fri, 13 Mar 2009 04:54:25 -0500 (EST)
• References: <gpacsh\$ms3\$1@smc.vnet.net>

```Karsten W. wrote:
> Hello,
>
> here is a problem I have solved but I am not happy with the method
>
> I have a data matrix DATA, a list of m ordered n-tuples. I run a
> function on the data matrix which returns a list DATAEVAL with m
> elements. The i-th element of this list corresponds to the i-th tuple
> of the data matrix. I think of this DATAEVAL list as an additional
> column of the data matrix.
>
> Now I am only interested in those tuples of the data matrix, where the
> corresponding DATAEVAL list has a value greater than zero.
>
> My current approach is the following: I add the DATAEVAL list to the
> data matrix and call Select afterwards:
>
> data = Import[...];
> dataEval = f[data, ...];
> filteredData = Select[Transpose[Join[Transpose[data], {dataEval}] ],
> Last[#] > 0 &];
>
> This works. However, calling Transpose two times seems not to be the
> most efficient way to solve the problem.
>
> As a newbie to Mathematica (and functional programming), I wonder if
> you can point me to a simpler solution?
>

Hello Karsten,

Use Pick[].

Pick[DATA, DATAEVAL, n_ /; n > 0]

```

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