Hello Yannick,
That behavior is documented in the help for subsetting ( ?'$' ):
Both ‘[[’ and ‘$’ select a single element of the list. The main
difference is that ‘$’ does not allow computed indices, whereas
‘[[’ does. ‘x$name’ is equivalent to ‘x[["name", exact =
FALSE]]’. Also, the partial matching behavior of ‘[[’ can be
controlled using the ‘exact’ argument.
You can avoid it by using [[]] instead:
> swiss[['Ex']]
NULL
> head(swiss[['Examination']])
[1] 15 6 5 12 17 9
That's one of the major reasons using $ is sometimes discouraged.
Sarah
On Thu, Jul 18, 2019 at 11:38 AM <yannick.su...@coop.ch> wrote:
>
> Hello all
> I noticed today that you can access dataframe columns by using incomplete
> names. This is a really unexpected behavior which led to some unexpected
> errors and I was wondering whether it's a bug or not and whether it should be
> changed in the future.
> Here's a working example using the preinstalled "swiss" dataset:
>
> > head(swiss)
> Fertility Agriculture Examination Education Catholic
> Courtelary 80.2 17.0 15 12 9.96
> Delemont 83.1 45.1 6 9 84.84
> Franches-Mnt 92.5 39.7 5 5 93.40
> Moutier 85.8 36.5 12 7 33.77
> Neuveville 76.9 43.5 17 15 5.16
> Porrentruy 76.1 35.3 9 7 90.57
> Infant.Mortality
> Courtelary 22.2
> Delemont 22.2
> Franches-Mnt 20.2
> Moutier 20.3
> Neuveville 20.6
> Porrentruy 26.6
> > swiss$E
> NULL
> > swiss$Ex
> [1] 15 6 5 12 17 9 16 14 12 16 14 21 14 19 22 18 17 26 31 19 22 14 22 20 12
> [26] 14 6 16 25 15 3 7 5 12 7 9 3 13 26 29 22 35 15 25 37 16 22
> > swiss$Ed
> [1] 12 9 5 7 15 7 7 8 7 13 6 12 7 12 5 2 8 28 20 9 10 3 12 6 1
> [26] 8 3 10 19 8 2 6 2 6 3 9 3 13 12 11 13 32 7 7 53 29 29
>
> So in order to access the column "Examination", I can type any substring from
> "Ex" to "Examination" and will always get the column swiss$Examination.
>
> Thanks for reading and Greetings
> Yannick Suter
>
--
Sarah Goslee (she/her)
http://www.numberwright.com
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