typeof ( substitute(expression(),..) ) -- musings
Peter Dalgaard BSA
p.dalgaard@biostat.ku.dk
29 Jun 1999 20:06:54 +0200
Martin Maechler <maechler@stat.math.ethz.ch> writes:
> [Relates to math text / normal text mixing in plots ..]
>
> Peter, I knew you'd know the answer quickly...
>
> Anyway, one needs to be somewhat careful here [R version pre 0.64.2]:
..and the first thing to be careful with is the version, expressions
behave slightly different in 0.65!
>
> typeof(print( e1 <- expression(a + b))) # expression
> typeof(print(se1 <- substitute(expression(a + b), list(a = 1)))) # language
>
> length(e1) # == 1
> length(se1)# == 2
>
> text.e <- function(x,y,e) {
> text(x,y,paste(deparse(substitute(e)),": "),adj=c(1,0))
> text(x,y,e, adj=0)
> }
> plot(1,type="n", axes= F)
> text.e(1,1.4, e1); text.e(1,1.3, se1); text.e(1,1.2, se1[[2]])
>
> Which shows that you can't use the result of `substitute' directly, but
> rather the 2nd element [[which is *still* not an expression !!]]
Yes. I forgot. se1 is not an expression object but a call to the
expression() - which evaluates to an expression object, i.e. one needs
se2 <- eval(substitute(expression(a + b), list(a = 1)))
or you can ditch the expression() bit and use
sc2 <- substitute(a + b, list(a = 1))
which is 1 + b, mode "call" and will work with *most* graphics
operations, but there's a caveat: Sometimes language objects get
evaluated when you don't want them to. The notorious example is the
do.call construction in boxplot/bxp.
> This is all +- S-plus (3.4) compatible
> {{some details are NOT compatible; e.g.
> as.expression(se1[[2]])
> gives `what you expect' in R, but not in S-plus 3.4 or 5.0r3
> }}
>
> e1 is an expression whereas
> se1 is a call (aka "language" in R).
>
> ----
> Is this documented anywhere? [blue book; language reference; ?? ]
It will be... (It may be in the blue book, but my fading memory of it
says that it is not all that clear on this particular subject.)
--
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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