[R] variable (column) in a data frame

[Peter Ehlers]

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 <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
>>
>> --
>> This message has been scanned for viruses and
>> dangerous content by MailScanner, and is
>> believed to be clean.
> ------- End of Original Message -------
>
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