[R] puzzling behavior of within

[Ista Zahn]

On Thu, Jun 13, 2013 at 12:17 PM, Ista Zahn <istazahn at gmail.com> wrote:
> On Thu, Jun 13, 2013 at 12:11 PM, Bert Gunter <gunter.berton at gene.com> wrote:
>> Why are you surprised?
>
> Because I missed the significance of "examines the environment after
> the evaluation of 'expr' and makes the corresponding modifications to
> 'data'" in ?within, and thought my example should be equivalent to
>
> mtcars <- within(mtcars, {
>     x <- 0
>     x[gear==4] <- 1
> })

I meant, "thought my example should be equivalent to

mtcars$x <- 0
mtcars$x[mtcars$gear==4] <- 1
"

Thanks again,
Ista

>
>
> It has nothing to do with within() . ?"[<-"
>>
>>> x <- 0
>>> g <- 1:2
>>> x[g==1]<- 5
>>> x
>> [1]  5 NA
>
> Thanks Bert. Makes me feel stupid, but I guess that's the price I pay
> for enlightenment.
>
> Best,
> Ista
>
>>
>> -- Bert
>>
>> On Thu, Jun 13, 2013 at 9:00 AM, Ista Zahn <istazahn at gmail.com> wrote:
>>> Hi all,
>>>
>>> In answering a question yesterday about avoiding repeatedly typing the
>>> name of a data.frame when making modifications to it I discovered the
>>> following unexpected behavior of within:
>>>
>>> mtcars <- within(mtcars, {
>>>     x <- 0
>>>     x[gear==4] <- 1
>>> })
>>>
>>> generates a column named x in mtcars equal to 1 if gear equals 4, and
>>> NA otherwise. What happend to my zeros? I thought maybe you just can't
>>> modify an object more than once, but that is not true:
>>>
>>> mtcars <- within(mtcars, {
>>>     x <- 0
>>>     x[gear==4] <- 1
>>>     x[gear==3] <- 2
>>> })
>>>
>>> returns a data.frame in with the x column is 1 if gear is 4, 2 if gear
>>> is 3, and NA otherwise. What is going on here?
>>>
>>> Surprised by these results I tried a few other things, and found
>>> another surprising behavior:
>>>
>>> mtcars <- within(mtcars, {
>>>     x <- 0
>>>     x[1] <- 1
>>> })
>>>
>>> generates a column named x in mtcars equal to 1. But I thought I said
>>> only change the first value to one! What happend?
>>>
>>> I've been reading and re-reading the documentation, but I can't see
>>> anything that explains these results.
>>>
>>> Thanks for any insight,
>>> Ista
>>>
>>> ______________________________________________
>>> R-help at r-project.org mailing list
>>> https://stat.ethz.ch/mailman/listinfo/r-help
>>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>>> and provide commented, minimal, self-contained, reproducible code.
>>
>>
>>
>> --
>>
>> Bert Gunter
>> Genentech Nonclinical Biostatistics
>>
>> Internal Contact Info:
>> Phone: 467-7374
>> Website:
>> http://pharmadevelopment.roche.com/index/pdb/pdb-functional-groups/pdb-biostatistics/pdb-ncb-home.htm