Re: select a range of elements from a nested list
- To: mathgroup at smc.vnet.net
- Subject: [mg5437] Re: select a range of elements from a nested list
- From: "Brian J. Albright" <albright at physics.ucla.edu>
- Date: Sat, 7 Dec 1996 00:26:12 -0500
- Organization: UCLA Department of Physics and Astronomy
- 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!
>
> 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.
>
> I hope I made myself clear enough to you all.
>
> Thanks a lot.
>
> Oliver Schurr
> Department of Chemistry
> Georgetown University
> Washington, DC 20057
Hi Oliver.
Ick. Don't build your new list element-by-element. Instead, use the
Cases function. E.g.:
Input:
Output:
This is much more elegant and executes faster than building your list
one element at a time. Additionally, it works on unsorted data. If for
some twisted reason you want to stick with the Table format, you can do
the following:
Input:
0.3 ] // Length} ]
Output:
Timings:
Input:
Output:
Input:
/; x <= 0.3 ] // Length} ]; ]
Output:
I hope this helps.
-Brian
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Brian J. Albright |
Department of Physics and Astronomy, UCLA | Um, yeah. Uh.... Oh gee.
albright at physics.ucla.edu |
http://bohm.physics.ucla.edu/~albright | [.signature stage fright]
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