You may have figured this out already, but you can get a "call" object
from parse() by extracting its first element, as in
mycurveC <-
function(x,from=1,to=10){
xloc <- parse(text=x)[[1]]
do.call("curve",list(expr=xloc,from=from,to=to))
}
mycurveC("(x-4) ** 2")
This doesn't address the confusion you note, but it is an easy way of
getting your code to work without needing any functions to be redefined.
-- Tony Plate
georgi.boshna...@manchester.ac.uk wrote:
Full_Name: Georgi Boshnakov
Version: 2.10.1pat
OS: Windows XP
Submission from: (NULL) (130.88.123.205)
When calling programmatically function curve() from package:graphics I
experienced some trouble since it reports
stop("'expr' must be a function or an expression containing 'x'")
even if expr is "expression". Naturally, the user message uses "expression" in
its usual generic meaning but it is somewhat confusing that an object of type
"expression" is rejected.
The message is produced in the following piece in the source of function
curve()
else {
if (!(is.call(sexpr) && match("x", all.vars(sexpr), nomatch = 0L)))
stop("'expr' must be a function or an expression containing 'x'")
expr <- sexpr
if (is.null(ylab))
ylab <- deparse(sexpr)
}
The "if" statement only checks with is.call, and not with is.expression.
Maybe it is worth including the check with is.expression, e.g. as in
else {
if (!((is.call(sexpr) || is.expression(sexpr)) &&
match("x", all.vars(sexpr), nomatch = 0L)
))
stop("'expr' must be a function or an expression containing 'x'")
expr <- sexpr
if (is.null(ylab))
ylab <- deparse(sexpr)
}
Example:
# use curve()
mycurveA <-
function(x,from=1,to=10){
xloc <- parse(text=x)
# call("mycurve0",expr=xloc,from=from,to=to)
do.call("curve",list(expr=xloc,from=from,to=to))
}
# use curve modified as suggested above.
mycurveB <-
function(x,from=1,to=10){
xloc <- parse(text=x)
# call("mycurve0",expr=xloc,from=from,to=to)
do.call("mycurve0",list(expr=xloc,from=from,to=to))
}
mycurveA("x^2")
Error in curve(expr = expression(x^2), from = 1, to = 10) :
'expr' must be a function or an expression containing 'x'
mycurveB("x^2")
# (no error, plots the graph)
Best regards,
Georgi Boshnakov
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