From georgi.boshnakov at manchester.ac.uk  Wed Jan 20 11:35:12 2010
From: georgi.boshnakov at manchester.ac.uk (georgi.boshnakov at manchester.ac.uk)
Date: Wed, 20 Jan 2010 11:35:12 +0100 (CET)
Subject: [Rd] function curve() (PR#14191)
Message-ID: <20100120103512.46C5D282EF29@mail.pubhealth.ku.dk>

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



[Rd] function curve() (PR#14191)
georgi.boshnakov at manchester.ac.uk