variable (column) in a data frame

Dr. Dalgaard,

Thank you. But pre-checking with is.null() or using with()
doesn't solve the problem of catching spelling mistakes
in the name of a variable inside a data frame, when using 
the df$var notation often in a program.

Is there some way for R to behave, in relation to a variable 
inside a data frame, the same way it behaves for a variable 
not in a data frame? For example:

##----------------------------------------
a <- c(1,2,3)

## the variable exists, we get a correct answer
a==1

## the variable does not exist, R rightly points this out
aaa==1
##----------------------------------------

My point is, if we make a spelling mistake in a program when referring
to a variable inside a data frame, using the df$var notation, 
there seems to be no way of getting warned about that. 

Thank you once again.

Paulo Barata

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


---------- Original Message -----------
From: peter dalgaard <pdalgd at gmail.com>
To: "Paulo Barata" <paulo.barata at ensp.fiocruz.br>
Sent: Sun, 15 Jul 2012 16:47:35 +0200
Subject: Re: [R] variable (column) in a data frame

On Jul 15, 2012, at 16:30 , Paulo Barata wrote:

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?

You can pre-check for is.null(df$aaa) or use with(df, sum(aaa==2)).

-- 
Peter Dalgaard, Professor,
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
Phone: (+45)38153501
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com

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