R devel - Jan 2010 - function curve() (PR#14191)

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

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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