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MathGroup Archive 1996

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Re: select a range of elements from a nested list

  • To: mathgroup at smc.vnet.net
  • Subject: [mg5456] Re: select a range of elements from a nested list
  • From: Clemens Frey <Clemens.Frey at uni-bayreuth.de>
  • Date: Sat, 7 Dec 1996 00:26:34 -0500
  • Organization: uni-bayreuth.de
  • Sender: owner-wri-mathgroup at wolfram.com

Oliver Schurr 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!

Hi,

what about using the CASES-function to get a list of all pairs matching 
your condition ? Just look at this example:

L = {{0.1,1},{0.2,2},{0.3,3},{0.4,4}};

For the upper list L use Cases to get a list M of all pairs having a 
first value <= 0.3 :

M = Cases[ L, {x_Real,_} /; x<=0.3 ]

Then you should have Length[M]==ZZZ of your example.
_/; is a conditional pattern.
I used x_Real instead of x_ alone to beware of errors
if the list L contains only two pairs of numbers.

Hope I could help you
Clemens
(-: Clemens.Frey at uni-bayreuth.de :-)


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