[R] puzzling behavior of within

[arun]

within(mtcars,{ x<-rep(0,nrow(mtcars));x[gear==4]<-1;x[gear==3]<-2}) #should fix this;
A.K.



----- Original Message -----
From: Ista Zahn <istazahn at gmail.com>
To: Bert Gunter <gunter.berton at gene.com>
Cc: r-help at r-project.org
Sent: Thursday, June 13, 2013 12:17 PM
Subject: Re: [R] puzzling behavior of within

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
})


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

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