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MathGroup Archive 1996

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Re: select a range of elements from a nested list

  • To: mathgroup at
  • Subject: [mg5427] Re: select a range of elements from a nested list
  • From: haberndt at (Harald Berndt)
  • Date: Sat, 7 Dec 1996 00:25:59 -0500
  • Organization: DNAI ( Direct Network Access )
  • Sender: owner-wri-mathgroup at

In article <57ush1$3cb at>, Oliver Schurr
<schurro at> wrote:

> Hi MathGroup
> Say you have a nested list like that {{x1,y1},{x2,y2},{x3,y3},...{xn,yn}}
> basically containing X and Y values for a XY plot in each element. The X
> values span a range from say 0 to 0.4 and you ONLY want to select the
> elements (x,y values) from the list for which the x value spans say 0 to
> 0.3. BUT you don't want to COUNT the elements (visually on the screen) and
> figure out the position at which the x value is just greater than 0.3 and
> then subtract 1 from this position.
> I still want do define the list by using
> Table[list[[x,1]],list[[x,2]],{x, 1, ZZZ}]
> here ZZZ means the number for which the x value <= 0.3, this number needs
> to be found by MATHEMATICA!
> I like to know if there is a general procedure to select elements from a
> nested list based on the values of one member of the elements of that
> nested list.

There is, and it's quite simple. A quick look into the documentation turns
up the function Select[], which even sounds like the right thing to try.
So, let's say you have a list



Simply select all pairs whose first part has the right properties:

Select[t1, First[#]<=0.3&]


It doesn't even matter whether the list is ordered or not. It's reaonably
fast, too ...
> I hope I made myself clear enough to you all.

Well, and I hope I got it.


Harald Berndt, Ph.D.                            Research Specialist,
Voice: 510-652-5974                                      Consultant
FAX:   510-215-4299

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