And, of course, there's transform, as David knows very well:

d <- transform(d, c = (a + b) > 25)
> head(d)
  a    b    z    c
1 1 2001 5001 TRUE
2 2 2002 5002 TRUE
3 3 2003 5003 TRUE
4 4 2004 5004 TRUE
5 5 2005 5005 TRUE
6 6 2006 5006 TRUE

As far as David's 'wrong way' is concerned, I think he may have meant to use
within() instead of with():

d0 <- d[, -4]
> head(d0, 2)
  a    b    z
1 1 2001 5001
2 2 2002 5002

d2 <- within(d0, c <- (a + b) > 25)

> identical(d, d2)
[1] TRUE


HTH,
Dennis

On Mon, Aug 23, 2010 at 5:36 PM, David Winsemius <dwinsemius@comcast.net>wrote:

>
> On Aug 23, 2010, at 6:28 PM, David Winsemius wrote:
>
>
>> On Aug 23, 2010, at 5:51 PM, ivo welch wrote:
>>
>>  quizz---what does this produce?
>>>
>>>  d=data.frame( a=1:1000, b=2001:3000, z= 5001:6000 )
>>>  attach(d); c <- (a+b)>25; detach(d)
>>>  d= subset(d, TRUE, select=c( a, b, c ))
>>>
>>> yes, I know I have made a mistake, in that the code does not do what I
>>> presumably would have wanted.  it does seem like unexpected behavior,
>>> though, without an error.  there probably is some reason why this does
>>> not ring an alarm bell...
>>>
>>
>> You have created a perfect example for why it is a bad idea to attach
>> data.frames.
>>
>> ?attach     # yes, I am yet again saying: "read the help page..."
>>
>> ... especially the 4th paragraph of the Details section.
>>
>
> I think it might helpful to consider the right way and the wrong way to do
> the same assignment using with(), which is my choice as an alternative to
> attache
>
> Right;
>
> d$c <- with(d, a+b >25) # note:  using "c" as an object name is a really
> confusing strategy
>
> Wrong:
> with(d, c <- a+b <25)
>
> The wrong way is similar to what you might have thought would be happening.
> The attach() operation created its own environment, but that did not
> necessarily mean that all assignments would be creating new columns inside
> "d".
>
>
>
>> --
>> David.
>>
>>
>>
>> David Winsemius, MD
>> West Hartford, CT
>>
>> ______________________________________________
>> R-help@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.
>>
>
> David Winsemius, MD
> West Hartford, CT
>
> ______________________________________________
> R-help@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.
>

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