Convert numerical value into function which returns numerical value

Bert Gunter <gunter.berton at gene.com> writes:

1. An important point that Bill uses but did not explicitly state is
that R is (essentially) a functional programming language, which means
among other things that functions can return functions as values.

Yup - that is essential here. A function is also only an object in R,
like characters or numeric values.

2. As a perhaps slightly amusing variant of Bill's construct that
illustrates this is the function below whose value is a function that
either returns a constant determined when it is defined or its
argument when called if no constant was given to it on definition:

fconv <- function(arg=NULL){
  function(z)if(is.null(arg))z else arg
}

You know, this is exactly what I want to do: I have a function, which
takes either a numerical value or a function (from PAI) as the argument
dep. So I have to check if the dep is a function or a value. At the
moment, I am using is.function(), and when dep is not a function,
convert it to a function which returns dep. If it is a function, I can
leave it as it is. I could also replace, in your code, the is.null()
with is.function() and do effectively the same here (some edits
required).

Very neat indeed.

And a perfect way of making functions very versatile. IU really have to
look closer into these things.

Is there a good resource for these advanced programming techniques in R?

Thanks,

Rainer

x <- 5
g1 <- fconv(x) ## g1 will always return 5
g1()

[1] 5

g1(1)

[1] 5

x <- 2
g1(x) ## Still uses the "x" in its defining environment

[1] 5

## But ...

g2 <- fconv() ## No constant given to it in its definition
g2(x)

[1] 2

g2(1)

[1] 1

g2()

Error in g2() : argument "z" is missing, with no default

Cheers,
Bert

Bert Gunter
Genentech Nonclinical Biostatistics
(650) 467-7374

"Data is not information. Information is not knowledge. And knowledge
is certainly not wisdom."
Clifford Stoll




On Thu, Apr 9, 2015 at 7:57 AM, William Dunlap <wdunlap at tibco.com> wrote:

You can make such functions by using the fact that a function
(really, a 'closure') always has access to the environment in
which the function was created.  E.g.
  makeConstantFunction <- function(constant) {
      force(constant) # evaluate the argument now
      function(PAI) {
          constant
      }
  }
  f17 <- makeConstantFunction(17)
  flog17 <- makeConstantFunction(log(17))
  f17(111)
  # [1] 17
  flog17(111)
  # [1] 2.833213

If you print f17 and flog17 they will look the same, except for
their environments and you have to inspect those to see why
they act differently.

  ls.str(environment(f17))
  # constant :  num 17
  ls.str(environment(flog17))
  # constant :  num 2.83

If you really want the functions to look different you can use
substittute or bquote, but that is also a bit mysterious (you need the
eval()
their outputs):
  g17 <- eval(substitute(function(PAI)x, list(x=17)))
  h17 <- eval(bquote(function(PAI).(x), list(x=17)))
  g17(10)
  [1] 17
  h17(10:1)
  [1] 17




Bill Dunlap
TIBCO Software
wdunlap tibco.com

On Thu, Apr 9, 2015 at 5:39 AM, Rainer M Krug <Rainer at krugs.de> wrote:

Hi

I want convert, in a function, an argument from a numerical value to a
function which returns this value.:

My Code:

--8<---------------cut here---------------start------------->8---
dep <- 13
dep <- function() {dep}
dep
--8<---------------cut here---------------end--------------->8---

This is what I get:
#+RESULTS:
,----
| function(PAI) { dep }
`----

This is what I want
,----
| function(PAI) { 13 }
`----

I thought about using eval(dep), but this gives me the effectively the
same.

Is it possible to achieve what I want? I somehow have the feeling this
is not that easily possible, as the code in the function definition is
only evaluated when the function is evaluated.

I could obviously do something like

--8<---------------cut here---------------start------------->8---
dep <- 13
depVal <- dep
dep <- function() {depVal}
dep()
--8<---------------cut here---------------end--------------->8---

But is there a better solution?

Thanks,

Rainer

--
Rainer M. Krug, PhD (Conservation Ecology, SUN), MSc (Conservation
Biology, UCT), Dipl. Phys. (Germany)

Centre of Excellence for Invasion Biology
Stellenbosch University
South Africa

Tel :       +33 - (0)9 53 10 27 44
Cell:       +33 - (0)6 85 62 59 98
Fax :       +33 - (0)9 58 10 27 44

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email:      Rainer at krugs.de

Skype:      RMkrug

PGP: 0x0F52F982

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--
Rainer M. Krug, PhD (Conservation Ecology, SUN), MSc (Conservation Biology, UCT), Dipl. Phys. (Germany)

Centre of Excellence for Invasion Biology
Stellenbosch University
South Africa

Tel :       +33 - (0)9 53 10 27 44
Cell:       +33 - (0)6 85 62 59 98
Fax :       +33 - (0)9 58 10 27 44

Fax (D):    +49 - (0)3 21 21 25 22 44

email:      Rainer at krugs.de

Skype:      RMkrug

PGP: 0x0F52F982