converting result of substitute to 'ordidnary'expression

## this does work (thanks to the help page), but one needs to remember to 
call eval
subset(dat, eval(subsetexp))
Is there a way to create subsetexp that needs no eval inside the call to 
subset()?

I switched the thread to r-devel because here I am proposing a patch for 
subset.data.frame().


Thank you Chuck, it was inspiring. It turns out that a simple modification 
to subset.data.frame makes my example work:

subset.data.frame <-
 function (x, subset, select, drop = FALSE, ...)
{
 if (missing(subset))
   r <- TRUE
 else {
   r <- eval(substitute(subset), x, parent.frame())

   if (!is.logical(r)) {
     ## try w/o substitute
     r <- eval(subset, x, parent.frame())
   }

   if (!is.logical(r))
     stop("'subset' must evaluate to logical")

   r <- r & !is.na(r)
 }
 if (missing(select))
   vars <- TRUE
 else {
   nl <- as.list(1L:ncol(x))
   names(nl) <- names(x)
   vars <- eval(substitute(select), nl, parent.frame())
 }
 x[r, vars, drop = drop]
}


And now:

dat <- data.frame(x=1:10, y=1:10)

subset(dat, 5<x)

   x  y
6   6  6
7   7  7
8   8  8
9   9  9
10 10 10

subsetexp <- expression(5<x)

subset(dat, subsetexp)

   x  y
6   6  6
7   7  7
8   8  8
9   9  9
10 10 10

do.call(subset, list(dat, subsetexp))

   x  y
6   6  6
7   7  7
8   8  8
9   9  9
10 10 10

version

              _
platform       i386-pc-mingw32
arch           i386
os             mingw32
system         i386, mingw32
status
major          2
minor          9.1
year           2009
month          06
day            26
svn rev        48839
language       R
version.string R version 2.9.1 (2009-06-26)


Thank you,
Vadim



-----Original Message-----
From: Charles C. Berry [mailto:cberry at tajo.ucsd.edu]
Sent: Friday, June 25, 2010 9:16 PM
To: Vadim Ogranovich
Cc: 'r-help at r-project.org'
Subject: Re: [R] converting result of substitute to 'ordidnary' expression

On Fri, 25 Jun 2010, Vadim Ogranovich wrote:

Dear R users,


As substitute() help page points out:
    Substituting and quoting often causes confusion when the argument
    is 'expression(...)'. The result is a call to the 'expression'
    constructor function and needs to be evaluated with 'eval' to give
    the actual expression object.

And indeed I am confused. Consider:

dat <- data.frame(x=1:10, y=1:10)

subsetexp <- substitute(a<x, list(a=5))

## this doesn't work

subset(dat, subsetexp)

Error in subset.data.frame(dat, subsetexp) :
 'subset' must evaluate to logical

## this does work (thanks to the help page), but one needs to remember to 
call eval

subset(dat, eval(subsetexp))

Is there a way to create subsetexp that needs no eval inside the call to 
subset()?

I do not think so. See

       page(subset.data.frame,'print')

Then think about this:

eval(substitute(subsetexp))

5 < x

eval(substitute(subsetexp),list(x=2))

5 < x

eval(substitute(eval(subsetexp)),list(x=2))

[1] FALSE

The added layer of substitution is making things a bit tricky.

One alternative is to build up your own call like this:

sss <- expression(subset(dat,sbst))
sss[[1]][[3]] <- subsetexp
sss

expression(subset(dat, 5 < x))

eval(sss)

    x  y
6   6  6
7   7  7
8   8  8
9   9  9
10 10 10

HTH,

Chuck

I experimented with the following, but it didn't work:

subsetexp <- eval(substitute(a<x, list(a=5)))

Error in eval(expr, envir, enclos) : object 'x' not found

Thank you very much for your help,
Vadim

Note: This email is for the confidential use of the named addressee(s) 
only and may contain proprietary, confidential or privileged information. 
If you are not the intended recipient, you are hereby notified that any 
review, dissemination or copying of this email is strictly prohibited, 
and to please notify the sender immediately and destroy this email and 
any attachments.  Email transmission cannot be guaranteed to be secure or 
error-free.  Jump Trading, therefore, does not make any guarantees as to 
the completeness or accuracy of this email or any attachments.  This 
email is for informational purposes only and does not constitute a 
recommendation, offer, request or solicitation of any kind to buy, sell, 
subscribe, redeem or perform any type of transaction of a financial 
product.

Charles C. Berry                            (858) 534-2098
                                            Dept of Family/Preventive 
Medicine
E mailto:cberry at tajo.ucsd.edu               UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/  La Jolla, San Diego 92093-0901



Note: This email is for the confidential use of the named addressee(s) 
only and may contain proprietary, confidential or privileged information. 
If you are not the intended recipient, you are hereby notified that any 
review, dissemination or copying of this email is strictly prohibited, and 
to please notify the sender immediately and destroy this email and any 
attachments.  Email transmission cannot be guaranteed to be secure or 
error-free.  Jump Trading, therefore, does not make any guarantees as to 
the completeness or accuracy of this email or any attachments.  This email 
is for informational purposes only and does not constitute a 
recommendation, offer, request or solicitation of any kind to buy, sell, 
subscribe, redeem or perform any type of transaction of a financial 
product.