[R] confusion with R syntax

[Giovanni Petris]

Mark,

That has to do with how operators bind and their precedence. 

> Date: Thu, 11 Oct 2007 15:34:22 -0400
> From: "Leeds, Mark (IED)" <Mark.Leeds at morganstanley.com>
> Sender: r-help-bounces at r-project.org
> Importance: normal
> Priority: normal
> Precedence: list
> Thread-topic: confusion with  R syntax
> Thread-index: AcgMPbmc7chsZtPAT+K0aJ+6a2FyxQ==
> 
> I just noticed something by accident with R syntax that I'm sure is
> correct but I don't understand it. If I have
> a simple numeric vector x and I subscript it, it seems that I can then
> subscript a second time with TRUE
> or FALSE, sort of like a 2 dimensional array in C. Does someone know if
> this is documented somewhere
> Because it's neat but I never knew it existed. To me it seems like a 1
> dimensional vector should
> have only one dimensional indexing ?
> 
> x <- seq(1,10)
> > x
>  [1]  1  2  3  4  5  6  7  8  9 10
> > x[2:4][c(TRUE,FALSE,TRUE)]
> [1] 2 4
> 

This is interpreted as 

(x[2:4])[c(TRUE,FALSE,TRUE)]


> But, it only works for TRUE or FALSE and not numbers so I think it's not
> really 2 dimensional indexing.
> 
> x[1][2]
> 
> [1] NA
> 

This doesn't work because you are asking for the second element of a
vector with only one element.

Best,
Giovanni


> If someone could explain this mechanism or tell me what I should look
> for in the archives, it would
> be appreciated. Thanks.
> --------------------------------------------------------
> 
> This is not an offer (or solicitation of an offer) to bu...{{dropped:22}}
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-- 

Giovanni Petris  <GPetris at uark.edu>
Associate Professor
Department of Mathematical Sciences
University of Arkansas - Fayetteville, AR 72701
Ph: (479) 575-6324, 575-8630 (fax)
http://definetti.uark.edu/~gpetris/