[R] generate simple function with pre-defined constants

[William Dunlap]

Try the following:
   generateABFunction <- function(a, b) {
      force(a)
      force(b)
      function(x) a*x + b
   }
   f12 <- generateABFunction(1, 2)
   f53 <- generateABFunction(5,6)
   f12(10:12) # get 12, 13, 14
   f53(10:12) # get 56, 61, 66

See, e.g., yesterday's discussion under the subject
"Trying to build up functions with its names by means of lapply"
on why the force() calls are required.  Read up on R's environments
to see why f12 and f53 look the same but act differently (hint:
look at ls.str(environment(f12))).

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com


> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf
> Of Liviu Andronic
> Sent: Thursday, June 06, 2013 8:00 AM
> To: r-help at r-project.org Help
> Subject: Re: [R] generate simple function with pre-defined constants
> 
> On Thu, Jun 6, 2013 at 4:48 PM, Liviu Andronic <landronimirc at gmail.com> wrote:
> > Dear all,
> > Given:
> > a <- 2
> > b <- 3
> >
> > I'd like to obtain the following function:
> > f <- function(x) 2 + 3*x
> >
> > but when I do this:
> > f <- function(x) a + b*x
> > ##f
> > ##function(x) a + b*x
> >
> > the 'a' and 'b' objects do not get evaluated to their constants. How
> > could I do that?
> >
> I found one solution:
> a <- 2
> b <- 3
> f <- eval(parse(text=paste("function(z)", a, "+ z * ", b)))
> f
> ##function(z) 2 + z *  3
> 
> but I still have nightmares from:
> > fortune("parse")
> 
> If the answer is parse() you should usually rethink the question.
>    -- Thomas Lumley
>       R-help (February 2005)
> 
> Is there a nicer way to approach this? Thanks,
> Liviu
> 
> 
> > Thanks,
> > Liviu
> >
> >
> > --
> > Do you know how to read?
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> > http://garbl.home.comcast.net/~garbl/stylemanual/e.htm#e-mail
> 
> 
> 
> --
> Do you know how to read?
> http://www.alienetworks.com/srtest.cfm
> http://goodies.xfce.org/projects/applications/xfce4-dict#speed-reader
> Do you know how to write?
> http://garbl.home.comcast.net/~garbl/stylemanual/e.htm#e-mail
> 
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