[R] Counterpart for Matlab's 'feval'?

[Barry Rowlingson]

Robin Hankin wrote:

>>I've just begun writing a program that searches for the minimum of a
>>function with golden section search. In order to do this in a nice way  
>>I
>>need a function that takes a function name and an argument and returns  
>>the
>>function value for that argument, i .e just like Matlab's 'feval'. Is
>>there any?

It might be better to write a function that takes a function as an 
argument, instead of the function name:

  > doit = function(f,...){f(...)}
  > doit(sqrt,2)
  [1] 1.414214
  > doit(sum,1,2,3)
  [1] 6

This way you can write little inline anonymous functions:

  > doit(function(a,b,c){a^3+b^2+c},1,4,3)
  [1] 20

The equivalent way with do.call, and function names is:

  > docall=function(fc,...){do.call(fc,list(...))}
  > docall("sqrt",2)
  [1] 1.414214
  > docall("sum",1,2,3)
  [1] 6

But:

  > docall("function(a,b,c){a^3+b^2+c}",1,4,3)
  Error in do.call(fc, list(...)) : couldn't find function 
"function(a,b,c){a^3+b^2+c}"

This can be fixed by using eval and parse instead:

  > doe=function(fc,...){eval(parse(text=fc))(...)}
  > doe("sqrt",2)
  [1] 1.414214
  > doe("sum",1,2,3)
  [1] 6
  > doe("function(a,b,c){a^3+b^2+c}",1,4,3)
  [1] 20

I see other solutions have been posted already. They're probably better.

Baz