[R] What is the difference between expression and quote when used with eval()?

[blue sky]

On Fri, Feb 19, 2010 at 12:39 PM, Peter Dalgaard
<p.dalgaard at 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?


>> expr=expression(2*3)
>> quo=quote(2*3)
>>
>> eval(expr)
>> str(expr)
>> class(expr)
>> typeof(expr)
>> mode(expr)
>> attributes(expr)
>>
>> eval(quo)
>> str(quo)
>> class(quo)
>> typeof(quo)
>> mode(quo)
>> attributes(quo)
>>
>> ______________________________________________
>> 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.
>
>
> --
>   O__  ---- Peter Dalgaard             Øster Farimagsgade 5, Entr.B
>  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
>  (*) \(*) -- University of Copenhagen   Denmark      Ph:  (+45) 35327918
> ~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)              FAX: (+45) 35327907
>