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MathGroup Archive 2007

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Re: Using Select

  • To: mathgroup at smc.vnet.net
  • Subject: [mg76227] Re: [mg76162] Using Select
  • From: Darren Glosemeyer <darreng at wolfram.com>
  • Date: Fri, 18 May 2007 06:08:44 -0400 (EDT)
  • References: <200705170954.FAA00930@smc.vnet.net>

Mark Coleman wrote:
> Greetings,
>
> I'm working on a small application and I'm searching for a way to  
> Select a subset of rows from a list based upon a list of criteria  
> chosen by the user.  Here is small (hypothetical) example. Say one  
> has a data set of consisting of a list of as follows:
>
> { {1,A,Blue,"Hello",10.5},{7,D,Green,"Goodbye",9.4}, 
> {6,S,Yellow,"Hello",6.9},{3,A,Blue,"Hello",8.0}....}
>
> The user will specify a letter and a color and the program should  
> Select[ ] the appropriate rows, e.g., if I pick Color=Blue and Letter  
> = A, then it will return
>
> { {1,A,Blue,"Hello",10.5},{3,A,Blue,"Hello",8.0}....}, etc.  Thus one  
> can enter Select[myData,(#[[2]]==A && #[[3]]==Blue)&] and get the  
> appropriate records.Note that The actual data set has about 20,000  
> records (lists), each with about 20 fields, and can select 7 or 8  
> different attributes, so the Select statements get fairly long.
>
> My question involves an efficient way to code the Select statement if  
> a user wants to chose records that correspond to any value of a  
> particular field, e.g., All of the Color=Blue, regardless of the  
> Letter choice,e.g.,
>
> Select[myData,(#[[3]]==Blue)&]
>
> More precisely, is there a way to pass Select the user choice of any  
> letter using some sort of 'wildcard' value that represents any value  
> for the specific attribute, i.e.,
>
>   Select[myData,(#[[2]]=='Wildcard' && #[[3]]==Blue)&]
>
> My motivation is that if I have may possible selection criteria, I  
> can still use a single Select statement.
>
> Thanks,
>
> -Mark
>   

You could use MatchQ in the second argument to Select. The pattern to 
match will be a list of length equal to the number of columns in your 
data and will have Blank[]s for each attribute without a specified 
value.  In the example given with data representing your data set,

Select[data, MatchQ[#, {_, A, Blue, _, _}] &]

will select all rows where the second element is A and the third is 
Blue, while

Select[data, MatchQ[#, {_, _, Blue, _, _}] &]

will select all rows with Blue as the third element.

Darren Glosemeyer
Wolfram Research


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