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Re: Selecting Many Things Rather Than Selecting One Thing From Many


m = Table[Random[Integer, 1000], {1000}, {3}];

k = {34, 876, 199};

sel=Select[m, MemberQ[k, #[[1]]]&]

{{876, 551, 965}, {34, 454, 475}, {876, 810, 192}}

pos = Position[m, _?(MemberQ[k, #[[1]]]&), 1, Heads->False]//Flatten

{202, 459, 721}

sel == m[[pos]]

True


Bob Hanlon


---- Gregory Lypny <gregory.lypny at videotron.ca> wrote: 
> Hello everyone,
> 
> I've discovered another use or need for the Select function, which I  
> suspect requires mapping of some sort.
> 
> In my previous posts, members of this MathGroup kindly showed me how  
> to apply Select to many columns of a matrix at once.  For example,
> 
> (Select[#1, #1 > K & ] & ) /@ Transpose[theMatrix]
> 
> will pull out all values greater than K, where K is a number such as  
> 100.
> 
> But suppose now that K is a list of numbers, such as K={34, 876,  
> 199}, and I simply want to extract or identify all of the rows in the  
> first column of theMatrix equal to any one of those numbers.  How  
> would I do that?  I started with
> 
> Select[theMatrix, #[[1]]==any element of list K]
> 
> and I imagine something similar could be applied to the Position  
> function.
> 
> Any hint would be much appreciated.
> 
> 	Gregory
> 


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