R help - May 2018 - Calling the curve function with a character object converted into an expression

Hi,

Down a cascade of function calls, I want to use the curve function with an
expression that is a variable. For various reason, this variable must be a
character object and cannot be an expression as required by the curve function.
How do I convert my variable into a expression that is accepted by curve?

Thanks in advance for your help.

## The following attempts do not work

myf <- '1+x^2'
curve(myf, from = 0, to = 10) # obviously !
curve(parse(text=myf), from = 0, to = 10) # not sure why this does not work

## This works but does not feel elegant:
eval(parse(text=sprintf('curve(%s, from = 0, to = 10)', myf)))

Sebastian:

This is somewhat arcane, perhaps even a bug (correction on this
welcomed). The problem is that the "expr" argument to curve() must be
an actual expression, not a call to parse that evaluates to an
expression. If you look at the code of curve() you'll see why
(substitute() does not evaluate  expr in the code). Another simple
workaround other than sticking in the eval() call is this:

myf <- function(x)NUL
body(myf)<- parse(text = "{1+x^2}")   ## note the additional
"{   }"
for safety, though not necessary here
## this idiom will continue to work for any text.

curve(myf, from = 0, to = 10)  ## myf is now the name of a function
that executes the parsed text.

*** I would appreciate any wiser R programmers correcting any
misunderstanding or error in my explanation ***

Cheers,
Bert

Bert Gunter

"The trouble with having an open mind is that people keep coming along
and sticking things into it."
-- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )


On Wed, May 2, 2018 at 8:11 PM, Sebastien Bihorel
<sebastien.bihorel at cognigencorp.com> wrote:
>
> Hi,
>
> Down a cascade of function calls, I want to use the curve function with an
expression that is a variable. For various reason, this variable must be a
character object and cannot be an expression as required by the curve function.
How do I convert my variable into a expression that is accepted by curve?
>
> Thanks in advance for your help.
>
> ## The following attempts do not work
>
> myf <- '1+x^2'
> curve(myf, from = 0, to = 10) # obviously !
> curve(parse(text=myf), from = 0, to = 10) # not sure why this does not work
>
> ## This works but does not feel elegant:
> eval(parse(text=sprintf('curve(%s, from = 0, to = 10)', myf)))
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

Typo: should be NULL not NUL of course

An alternative approach closer to your original attempt is to use
do.call() to explicitly evaluate the expr argument:

w <- "1 + x^2"
do.call(curve, list(expr = parse(text = w), ylab ="y"))

Cheers,
Bert

Bert Gunter

"The trouble with having an open mind is that people keep coming along
and sticking things into it."
-- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )


On Wed, May 2, 2018 at 10:35 PM, Bert Gunter <bgunter.4567 at gmail.com>
wrote:
> Sebastian:
>
> This is somewhat arcane, perhaps even a bug (correction on this
> welcomed). The problem is that the "expr" argument to curve()
must be
> an actual expression, not a call to parse that evaluates to an
> expression. If you look at the code of curve() you'll see why
> (substitute() does not evaluate  expr in the code). Another simple
> workaround other than sticking in the eval() call is this:
>
> myf <- function(x)NUL
> body(myf)<- parse(text = "{1+x^2}")   ## note the additional
"{   }"
> for safety, though not necessary here
> ## this idiom will continue to work for any text.
>
> curve(myf, from = 0, to = 10)  ## myf is now the name of a function
> that executes the parsed text.
>
> *** I would appreciate any wiser R programmers correcting any
> misunderstanding or error in my explanation ***
>
> Cheers,
> Bert
>
> Bert Gunter
>
> "The trouble with having an open mind is that people keep coming along
> and sticking things into it."
> -- Opus (aka Berkeley Breathed in his "Bloom County" comic strip
)
>
>
> On Wed, May 2, 2018 at 8:11 PM, Sebastien Bihorel
> <sebastien.bihorel at cognigencorp.com> wrote:
>>
>> Hi,
>>
>> Down a cascade of function calls, I want to use the curve function with
an expression that is a variable. For various reason, this variable must be a
character object and cannot be an expression as required by the curve function.
How do I convert my variable into a expression that is accepted by curve?
>>
>> Thanks in advance for your help.
>>
>> ## The following attempts do not work
>>
>> myf <- '1+x^2'
>> curve(myf, from = 0, to = 10) # obviously !
>> curve(parse(text=myf), from = 0, to = 10) # not sure why this does not
work
>>
>> ## This works but does not feel elegant:
>> eval(parse(text=sprintf('curve(%s, from = 0, to = 10)', myf)))
>>
>> ______________________________________________
>> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.