R devel - Aug 2010 - Assignment of individual values to data frame columns: intentional or unintentional behavior?

Dear developeRs,

I have just discovered a strange feature when assigning some values to 
columns of a data frame: The column is matched by partial matching (as 
documented), but when assigning a value, a new column with the partial 
name is added to the data frame that is identical to the original column 
except for the changed value. Is that intentional ? An example:

sw <- swiss[1:5, 1:4]
sw
sw$Fert[1] <- 10
sw

Best, Ulrike

-- 
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* Ulrike Groemping                            *
* BHT Berlin - University of Applied Sciences *
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* +49 (30) 39404863 (Home Office)             *
* +49 (30) 4504 5127 (BHT)                    *
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* http://prof.beuth-hochschule.de/groemping         *
* groemping at bht-berlin.de                     *

On Thu, Aug 5, 2010 at 12:24 PM, Ulrike Gr?mping
<groemping at bht-berlin.de> wrote:
> Dear developeRs,
>
> I have just discovered a strange feature when assigning some values to
> columns of a data frame: The column is matched by partial matching (as
> documented), but when assigning a value, a new column with the partial name
> is added to the data frame that is identical to the original column except
> for the changed value. Is that intentional ? An example:
Note that the lack of partial matching when performing assignment is
also documented.

See second last paragraph in Details section of ?Extract

Gabor Grothendieck schrieb:
> On Thu, Aug 5, 2010 at 12:24 PM, Ulrike Gr?mping
> <groemping at bht-berlin.de> wrote:
>   
>> Dear developeRs,
>>
>> I have just discovered a strange feature when assigning some values to
>> columns of a data frame: The column is matched by partial matching (as
>> documented), but when assigning a value, a new column with the partial
name
>> is added to the data frame that is identical to the original column
except
>> for the changed value. Is that intentional ? An example:
>>     
>
> Note that the lack of partial matching when performing assignment is
> also documented.
>
> See second last paragraph in Details section of ?Extract
>   
Yes, I see, thanks. I looked at ?"[.data.frame", where this is not 
documented.

However, given the documentation that partial matching is not used on 
the left-hand side, I would have expected even more that the assignment

sw$Fert[1] <- 10

works differently, because I am using it on the left-hand side. 
Probably, extraction ([1]) is done first here, so that the right-hand 
side won. At least, this is very confusing.

Best, Ulrike

Ulrike Gr?mping wrote:
> 
> 
> However, given the documentation that partial matching is not used on 
> the left-hand side, I would have expected even more that the assignment
> 
> sw$Fert[1] <- 10
> 
> works differently, because I am using it on the left-hand side. 
> Probably, extraction ([1]) is done first here, so that the right-hand 
> side won. At least, this is very confusing.
> 
> 
I totally agree! I think that 
sw <- data.frame(Fertility=1:5)
sw$Fert[1] <- 10

should work either like

sw$Fert2[1] <- 10

i.e. create new column, containing just 10.
or like

sw$Fertility[1] <- 10

i.e. replace the 1st item in sw$Fertility by 10.

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