[R] variable (column) in a data frame

[Paulo Barata]

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