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Re: simple question about removing elements from a table

• To: mathgroup at smc.vnet.net
• Subject: [mg122405] Re: simple question about removing elements from a table
• From: Pete Boardman <pete.boardman at mac.com>
• Date: Thu, 27 Oct 2011 06:33:28 -0400 (EDT)
• Delivered-to: l-mathgroup@mail-archive0.wolfram.com
• References: <201110262137.RAA29967@smc.vnet.net>

On 26 Oct 2011, at 22:37, Michael B. Heaney wrote:

> data={{-0.5, 0.0321787}, {-0.489, 0.0321691}, {-0.478,
> 0.032211}, {-0.467, 0.0321723}, {-0.456, 0.0322448}, {-0.444, 0.0321498},
> {-0.433, 0.032132}, {-0.422, 0.0321804}, {-0.411, 0.0322158},
> {-0.4, 0.0320886},  {-0.233,  0.0320596},  {0.567, 0.00286087}, {0.578,
> -0.00143633}, {0.589, -0.00592979}, {0.6, -0.0113609}}
> I want to remove all of the elements that have a positive first number, to
> get:
> 
> datasubset={{-0.5, 0.0321787}, {-0.489, 0.0321691}, {-0.478,
> 0.032211}, {-0.467, 0.0321723}, {-0.456, 0.0322448}, {-0.444, 0.0321498},
> {-0.433, 0.032132}, {-0.422, 0.0321804}, {-0.411, 0.0322158},
> {-0.4, 0.0320886},  {-0.233,  0.0320596}}
> 
> How do I do this?

I'm a newcomer to Mathematica, but this might be something I can answer... :)

>From the help:

  Select[list,crit] 
     picks out all elements Subscript[e, i] of list for which crit[Subscript[e, i]] is True. 

so you supply the list, followed by a function that selects the elements you want.

Select[data, #[[1]] < 0 &]

where  [[1]] < 0 says the first element is less than 0, and & defines a function that is applied to every 
element of the list, ie the #. 

hth

pete

• References:
  • simple question about removing elements from a table
    • From: "Michael B. Heaney" <mheaney@gmail.com>