[R] Help with functions as arguments

[Jan T Kim]

On Tue, Feb 12, 2013 at 09:38:19AM +0100, Rainer M Krug wrote:
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> On 12/02/13 08:30, Ian Renner wrote:
> > Hi,
> > 
> > I am trying to write a function which defines some arguments, then uses those arguments as 
> > arguments of other function calls. It's a bit tricky to explain, so a simple example will have
> > to suffice. I imagine this has a simple solution, but perusing through environments and other
> > help lists has not helped. Suppose I have two functions:
> > 
> > 
> > f1 = function(a) { b = a + 1 b } f2 = function(x, z) { y = x*z -2 y }
> > 
> > Where I am running into trouble is when I want to call function f1 within function f2:
> > 
> > f2(x = 3, z = f1(x))
> > 
> > This returns the error:
> > 
> > "Error in f1(x) : object 'x' not found"
> 
> Obviously easiest:
> 
> X <- 3
> f2(X=x, Z=f1(X))
> 
> Your solution does not work, as the f1(x) is evaluated and the value is passed on to f1, and your
> x is an argument and *only in the function f2* available (= in this context *no* assignment).
> 
> I remember something similar, and the solution had to do with eval() and quote() and friends -
> i.e. you have to only evaluate f(x) *inside* the function f2 - but unfortunately I do not remember
> details.

If you really want to force it, you could use

    f2(x = X <- 3, z = f1(X))

I'd always recommend Rainer's two-line version (first assign X, then
invoke f2), for reasons of legibility / code maintainability and others.

If you want to evaluate f1 inside f2, then you can rewrite f2 to take
f1 as an argument:

    f2 <- function(x, f) { z <- f(x); return(x * z - 2); }

and invoke that as follows:

    f2(x = 3, f = f1)

If the value of z always depends in some way on x, passing in the
function to compute z this way is much better than to write f2 as
though x and z are independent variables.

Best regards, Jan

> Cheers,
> 
> Rainer
> 
> 
> 
> > 
> > I'm not sure how to define environments within the functions so that the just-defined 'x' may
> > be passed as an argument to f1 within f2.
> > 
> > Any help would be greatly appreciated!
> > 
> > Ian Renner [[alternative HTML version deleted]]
> > 
> > 
> > 
> 
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-- 
 +- Jan T. Kim -------------------------------------------------------+
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