On Sat, Feb 20, 2010 at 2:40 AM, Romain Francois <romain.franc...@dbmail.com> wrote: > On 02/19/2010 10:31 PM, William Dunlap wrote: >> >>> -----Original Message----- >>> From: r-help-boun...@r-project.org >>> [mailto:r-help-boun...@r-project.org] On Behalf Of blue sky >>> Sent: Friday, February 19, 2010 12:11 PM >>> To: Peter Dalgaard >>> Cc: r-h...@stat.math.ethz.ch >>> Subject: Re: [R] What is the difference between expression >>> and quote whenused with eval()? >>> >>> On Fri, Feb 19, 2010 at 12:39 PM, Peter Dalgaard >>> <p.dalga...@biostat.ku.dk> wrote: >>>> >>>> blue sky wrote: >>>>> >>>>> I made the following example to see what are the difference between >>>>> expression and quote. But I don't see any difference when they are >>>>> used with eval()? Could somebody let me know what the difference is >>>>> between expression and quote? >>>> >>>> Expressions are vectors of unevaluated expressions, so one >>> >>> difference is >>>> >>>> that expressions can have more than one element. >>>> >>>> Another difference is more subtle: objects of mode >>> >>> "expression" are better >>>> >>>> at retaining their identity as an unevaluated expression >>>> >>>>> eval(substitute(2+x,list(x=expression(pi)))) >>>> >>>> Error in 2 + expression(pi) : non-numeric argument to >>> >>> binary operator >>>>> >>>>> eval(substitute(2+x,list(x=quote(pi)))) >>>> >>>> [1] 5.141593 >>>> >>>> The really convincing application of this escapes me for >>> >>> the moment, but the >>>> >>>> gist of it is that there are cases where a quoted >>> >>> expression may blend in a >>>> >>>> bit too seemlessly when using computing on the language. >>>> >>>> Also, expression objects are more easy to recognize >>> >>> programmeatically, >>>> >>>> quote() may result in objects of mode "call", "name", or >>> >>> one of the base >>>> >>>> classes. >>> >>> I want to see how expression(something) and quote(something) are >>> represented in R internally. But it seems that str() doesn't go to >>> that low level. Is there a way to show the internal representation? > > There is also the internal inspect function : > >> inspect <- function(x, ...) .Internal(inspect(x,...)) > >> inspect( expression(log(1), sqrt(2), trunc(pi)) ) > @9657560 20 EXPRSXP g0c2 [NAM(2)] (len=3, tl=153865256) > �...@97ab5e8 06 LANGSXP g0c0 [] > �...@92cf3fc 01 SYMSXP g0c0 [MARK,gp=0x4000] "log" > �...@9709a28 14 REALSXP g0c1 [] (len=1, tl=0) 1 > �...@97aa750 06 LANGSXP g0c0 [] > �...@92cf204 01 SYMSXP g0c0 [MARK,gp=0x4000] "sqrt" > �...@97099e8 14 REALSXP g0c1 [] (len=1, tl=0) 2 > �...@97aa84c 06 LANGSXP g0c0 [] > �...@92cf15c 01 SYMSXP g0c0 [MARK,gp=0x4000] "trunc" > �...@9347c38 01 SYMSXP g0c0 [MARK,gp=0x4000] "pi"
Where is the internal inspect documented? Would you please help explain what does '@9657560 20', 'g0c2', 'NAM(2)', 'MARK', 'tl' and 'gp' stand for? ______________________________________________ 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.
