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MathGroup Archive 1998

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Re: Select x s.t. y>10

  • To: mathgroup at smc.vnet.net
  • Subject: [mg13989] Re: [mg13957] Select x s.t. y>10
  • From: Xah Lee <xah at best.com>
  • Date: Sat, 12 Sep 1998 16:59:13 -0400
  • Sender: owner-wri-mathgroup at wolfram.com

At 7:06 PM -0000 1998.09.11, Jason Gill wrote:
>...
> what I
>am trying to do is take a list of lists of length 2 ie a list of x,y
>coordinates, and Select x values based on some criterion for y.  For
>example I want to select the first x value for which the corresponding
>y is greater than 5.
>...

Use Select or Cases.

In general, there are two ways to extract elements from the list. One is
 based on their position, the other on properties of elements. The
former is
 done with Take or Part, which is useful when you know the positions of
 elements you want. For the latter, there's Select and Cases. In Select,
the
 selection is based on a user defined True/False function (a predicate).
 Cases is more general. It uses pattern matching as criterion, and user
can
 also specify the level to extract at, and applying a transformation on
 elements of matched result.

Note that a pattern matching and predicate are interchangable as
follows:
 Suppose predicateQ is your predicate function, and suppose patternQ is
your
 pattern.

To turn a predicate into a pattern, use

(_?predicateQ)

To turn a pattern into a predicate, use

Function[MatchQ[#,patternQ]]

--

In your example, perhaps one of the following is useful:
 (Last at #>5&) or {x_, _?(#>5&)}->x.

 Xah, xah at best.com
 http://www.best.com/~xah/Wallpaper_dir/c0_WallPaper.html
 Perl: all unix's stupidity in one.




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