R devel - Nov 1997 - R-alpha: expression(..) objects -- c(ex1, ex2) `fails'

When investigating  the  
	legend(x,y, expression ( <math>, <math> ), ....)
			 [as suggested by Jim Lindsey],
I once again stumbled over the fact that expression objects behave somewhat
``wierdly'', IMHO.

We can have expressions of length(.) > 1, and access elements of these like
list elements.
However, other natural things don't work with expressions; some example
code:


##-- `Simple'  length--1  expressions:
ex1 <- expression(hat(beta) == (X^t * X)^{-1} * X^t * y)
ex2 <- expression(bar(x) == sum(over(x[i], n), i==1, n))
ex3 <- expression(paste(over(1, sigma*sqrt(2*pi)), " ",
    plain(e)^{over(-(x-mu)^2, 2*sigma^2)}))

##-- A length--2 expression:
ex12 <- expression(bar(x) == sum(over(x[i], n), i==1, n),
    		   hat(x) == median(x[i], i==1,n))
mode(ex12) # expression
length(ex12)# 2

mode(ex12 [1]) # expression
mode(ex12[[1]]) # call 
length(ex12[1])# 1
ex12 == ex12[1:2] #> TRUE TRUE


mode(c(ex1,ex2))#> "list" --->> This should be
"expression" I think ...
##				---------------------------
mode(ex.12 <- as.expression(c(ex1,ex2))) # "expression"
## but the wrong one:
length(ex.12)# [1] 1
ex.12 #-- still has 'list(..)' in it...

###-> How could I take ex1, ex2 and "concat" them to an expression
###  of length 2 ?





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Martin Maechler <maechler@stat.math.ethz.ch> writes:
[I've taken out the ">" citation symbols to allow cut and
paste]

##-- `Simple'  length--1  expressions:
ex1 <- expression(hat(beta) == (X^t * X)^{-1} * X^t * y)
ex2 <- expression(bar(x) == sum(over(x[i], n), i==1, n))

##-- A length--2 expression:
ex12 <- expression(bar(x) == sum(over(x[i], n), i==1, n),
  hat(x) == median(x[i], i==1,n))

##please avoid TABS, Martin. It confuses readline...
> ###-> How could I take ex1, ex2 and "concat" them to an
expression
> ###  of length 2 ?
#Heh. One would at least expect
l<-NULL
l[[1]]<-ex1[[1]]
l[[2]]<-ex2[[1]]
#to be a list of calls, which by definition is what an expression is,
#so you'd only need to change the mode, but:

 
mode(e)<-"expression" 
e
# expression(list(hat(beta) == (X^t * X)^{
#         -1
# } * X^t * y, bar(x) == sum(over(x[i], n), i == 1, n)))

#(Why does it deparse in that strange way, BTW?)

#Also, textual representations of length-2 expressions is acting up:
deparse(ex12)
#[1] "bar(x) == sum(over(x[i], n), i == 1, n)"
str(ex12)
# bar(x) == sum(over(x[i], n), i == 1, n)
dput(ex12)
# bar(x) == sum(over(x[i], n), i == 1, n)
deparse(ex12[[2]])
# [1] "hat(x) == median(x[i], i == 1, n)"

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard@biostat.ku.dk)             FAX: (+45) 35327907
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