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:

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

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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This message has been scanned for viruses and
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believed to be clean.

------- End of Original Message -------

On Jul 15, 2012, at 17:41 , Paulo Barata wrote:

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:

You could try reading the 2nd half of my one-line reply....

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

-- 
This message has been scanned for viruses and
dangerous content by MailScanner, and is
believed to be clean.

Dr. Dalgaard,

Thank you. You are right, with() is able to catch
spelling errors in the name of variables inside a data frame.

But couldn't some error or warning be included in R when referring
to a non-existent variable inside a data frame with the df$var
notation, without the use of with()? 

Is there any reason why R does not have such a kind of error
message?