puzzling behavior of within
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
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-- 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
______________________________________________ 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.