[R] variable (column) in a data frame

[Frans Marcelissen]

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

______________________________________________
R-help at r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.