R help - Feb 2010 - What is the difference between expression and quote when used with eval()?

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?


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)

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.

-pd

> 
> 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             ?ster 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

> as.list(expression( 2*3))
[[1]]
2 * 3
> as.list(quote( 2*3))
[[1]]
`*`

[[2]]
[1] 2

[[3]]
[1] 3
> identical(as.list(expression( 2*3))[[1]],quote(2*3))
[1] TRUE

expression() wraps the call into an expression object (as pointed out to me
by Gabor Grothendieck).

Bert Gunter
Genentech Nonclinical Statistics

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On
Behalf Of blue sky
Sent: Friday, February 19, 2010 9:43 AM
To: r-help at stat.math.ethz.ch
Subject: [R] What is the difference between expression and quote when
usedwith eval()?

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?


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.