R help - Apr 2003 - A function as argument of another function

Dear all,
I would like to write a function like:
myfun<-function(x,fn) {xx<-exp(x); x*fn(xx)}
where fn is a symbolic description of any function with its argument to be
specified. Therefore
myfun(5,"2+0.3*y^2")
should return 5*(2+0.3*exp(5)^2),
myfun(5,"log(y)") should return 5*log(exp(5)) and so on.

I tried with "expression" and others, but without success. How can I
solve
my problem?

Many thanks,
vito

Pronto,

It is very easy.  You have to define your fn as a function:

myfun <- function(x,fn) {xx<-exp(x); x*fn(xx)}
testf <- function(y) 2 + 0.3*y^2
myfun(5, testf)

or you may write

myfun(5, function(y) 2 + 0.3*y^2)

In your example you passed a character string into the function, I am
pretty sure it can be transformed into function but you don't need to.

Cheers,

Ott

 | From: "vito muggeo" <vito.muggeo at giustizia.it>
 | Date: Thu, 17 Apr 2003 09:45:41 +0200
 | 
 | Dear all,
 | I would like to write a function like:
 | myfun<-function(x,fn) {xx<-exp(x); x*fn(xx)}
 | where fn is a symbolic description of any function with its argument to be
 | specified. Therefore
 | myfun(5,"2+0.3*y^2")
 | should return 5*(2+0.3*exp(5)^2),
 | myfun(5,"log(y)") should return 5*log(exp(5)) and so on.
 | 
 | I tried with "expression" and others, but without success. How can
I solve
 | my problem?

>>>>> "vito" == vito muggeo <vito.muggeo at
giustizia.it>
>>>>>     on Thu, 17 Apr 2003 09:45:41 +0200 writes:
(MM: I've added " " around "<-" and empty lines:)

    vito> Dear all,
    vito> I would like to write a function like:

    vito> myfun <- function(x,fn) {xx<-exp(x); x*fn(xx)}

    vito> where fn is a symbolic description of any function
    vito> with its argument to be specified. Therefore

    vito> myfun(5,"2+0.3*y^2")
    vito> should return 5*(2+0.3*exp(5)^2),

    vito> myfun(5,"log(y)") should return 5*log(exp(5)) and so on.

    vito> I tried with "expression" and others, but without
    vito> success. How can I solve my problem?

As you say yourself in the subject, the argument must be a
*function*, not a string or an expression, e.g.,  
   function(y) y^2  

is a function.  So,

  myfun <- function(x,fn) { xx <- exp(x); x*fn(xx) }
  myfun(5, function(y) 2+0.3*y^2)

gives 33049.7, as desired.

Regards,
Martin Maechler <maechler at stat.math.ethz.ch>
http://stat.ethz.ch/~maechler/

Dear all,
I would like to write a function like:
myfun<-function(x,fn) {xx<-exp(x); x*fn(xx)}
where fn is a symbolic description of any function with its argument to be
specified. Therefore
myfun(5,"2+0.3*y^2")
should return 5*(2+0.3*exp(5)^2),
myfun(5,"log(y)") should return 5*log(exp(5)) and so on.

I tried with "expression" and others, but without success. How can I
solve
my problem?

Many thanks,
vito

hello Vito,

how about:


myfun <- function(x,fn)
  {
    xx <- exp(x)
    aux <- function(y)
      {
        eval(parse(text=fn))
      }
    x*aux(xx)
  }

myfun(5,"2+0.3*y^2")

as an alternative to Ott's answer.

HTH,
Bernhard

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What you wrote works if "fn" is a function.  If it is a character 
string, I would look at Venables and Ripley (2000) S Programming;  I 
believe they discuss several ways of doing that.

For certain applications of this nature, I have used constructs like 
"eval(pars(text= paste('...')))".

hth,
Spencer Graves

vito muggeo wrote:
> Dear all,
> I would like to write a function like:
> myfun<-function(x,fn) {xx<-exp(x); x*fn(xx)}
> where fn is a symbolic description of any function with its argument to be
> specified. Therefore
> myfun(5,"2+0.3*y^2")
> should return 5*(2+0.3*exp(5)^2),
> myfun(5,"log(y)") should return 5*log(exp(5)) and so on.
> 
> I tried with "expression" and others, but without success. How
can I solve
> my problem?
> 
> Many thanks,
> vito
> 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://www.stat.math.ethz.ch/mailman/listinfo/r-help