On 2012-07-15 08:41, 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$var notation often in a program.
Is there some way for R to behave, in relation to a variable
inside a data frame, the same way it behaves for a variable
not in a data frame? For example:
##----------------------------------------
a <- c(1,2,3)
## the variable exists, we get a correct answer
a==1
## the variable does not exist, R rightly points this out
aaa==1
##----------------------------------------
My point is, if we make a spelling mistake in a program when referring
to a variable inside a data frame, using the df$var notation,
there seems to be no way of getting warned about that.
You could wean yourself from the $-habit. It's convenient but can
lead to the problems you're experiencing (and this has been
discussed before). For programming, if you're prone to make
spelling errors, you should prefer df[, "aaa"]. See ?Extract.
Peter Ehlers
Thank you once again.
Paulo Barata
---------------------------------------------------------------------
---------- Original Message -----------
From: peter dalgaard <pda...@gmail.com>
To: "Paulo Barata" <paulo.bar...@ensp.fiocruz.br>
Sent: Sun, 15 Jul 2012 16:47:35 +0200
Subject: Re: [R] variable (column) in a data frame
On Jul 15, 2012, at 16:30 , Paulo Barata wrote:
To the R help list,
When using a data frame, there is no warning or error message
when I refer to a non-existent variable inside the data frame.
Example:
##----------------------------------------------
a <- c(1,2,3)
b <- c(11,22,33)
df <- data.frame(a,b)
df
## correct: there is a column in df named 'a'
## the sum is correctly performed
sum(df$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....@cbs.dk Priv: pda...@gmail.com
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