[R] variable (column) in a data frame

[Paulo Barata]

Dear Frans and Peter,

Yes, the notation df[,'var'] is able to catch a non-existent
variable var inside a data frame df. But the notation df$var
isn't. 

So we have this situation, where two different notations, which
(as far as I understand) perform the same action, have different
kinds of response.

Couldn't this situation be fixed? Isn't it possible to make the 
df$var notation to issue an error when referring to a non-existent
variable inside the data frame?

Thank you very much.

Paulo Barata

---------------------------------------------------------------------


---------- Original Message -----------
From: "Frans Marcelissen" <frans.marcelissen at digipsy.nl>
To: "'Paulo Barata'" <paulo.barata at ensp.fiocruz.br>, <r-help at r-project.org>
Sent: Mon, 16 Jul 2012 14:25:21 +0200
Subject: RE: [R] variable (column) in a data frame

> Hoi Pauli,
> There is a difference between two ways of accessing columns in a matrex:
> > df$aaa
> NULL
> > df["AAA"]
> Error in `[.data.frame`(df, "AAA") : undefined columns selected
> So df["AAA"] or df[,"AAA"] gives the error message you expect.
> -------------------
> Frans
> 
> -----Oorspronkelijk bericht-----
> Van: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
> Namens Paulo Barata
> Verzonden: zondag 15 juli 2012 16:31
> Aan: r-help at r-project.org
> Onderwerp: [R] variable (column) in a data frame
> 
> To the R help list,
> 
> When using a data frame, there is no warning or error message when I 
> refer to a non-existent variable inside the data frame.
> 
> Example:
> 
> ##----------------------------------------------
> 
> a <- c(1,2,3)
> b <- c(11,22,33)
> df <- data.frame(a,b)
> df
> 
> ## correct: there is a column in df named 'a'
> ## the sum is correctly performed
> sum(df$a==2)
> 
> ## incorrect: there is no column in df named 'aaa', ## but the sum is
> performed anyway without either warning or error
> sum(df$aaa==2)
> 
> ##----------------------------------------------
> 
> Is there some way to make R issue either a warning or an error 
> message in such a situation?
> 
> I am using R version 2.15.1 64-bit on Windows 7 Professional.
> 
> Thank you very much.
> 
> Paulo Barata
> 
> ---------------------------------------------------------------------
> Paulo Barata
> 
> ENSP - Fundação Oswaldo Cruz
> Rua Leopoldo Bulhões 1480 - 8A
> 21041-210  Rio de Janeiro - RJ
> Brazil
> E-mail: paulo.barata at ensp.fiocruz.br
> 
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------- End of Original Message -------