[Rd] function curve() (PR#14191)

[Tony Plate]

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.boshnakov at 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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