[Rd] [R] converting result of substitute to 'ordidnary'expression

[Matthew Dowle]

> ## 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()?

Whats wrong with needing to call eval ?  If I'm reading that code, and 
someone else wrote it, or I wrote it a long time
ago, then I immediately know to look above that line for where subsetexp 
gets its expression. I like seeing eval() as
it makes it clearer what the programmer intended.

Matthew


"Vadim Ogranovich" <vogranovich at jumptrading.com> wrote in message 
news:22D850BC39A25742977325ADDE208E7702CB5CC1A0 at chiexchange02.w2k.jumptrading.com...
>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
>>
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>
> 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) 
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