Help with functions as arguments

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

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]]

-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.11 (GNU/Linux)
Comment: Using GnuPG with Thunderbird - http://www.enigmail.net/

iQEcBAEBAgAGBQJRGf97AAoJENvXNx4PUvmCn0EIALjKZK4XsBkELCKJtWU7AXPd
7Qs5Sk2gQF64Jc1uHYm17hwgonZIFIj9IHE05lRP4dl2F6onMACfIEpzz+GF9cO5
qS2Kq3Oe/+bglBZIJ1oBGSxs2YEh5gGIykP+PZcWr4xP8QPQ23wnnTbafxTa7PaU
BkkNrmNBbre5f+wYBbFaxSlSdlslDFg9c6b5OgLzwBLB0o9tr7KA6POCa8HrX7H4
UBsPfwSEkOfyIEwq5drKjXF853nUNRVtd0cPA+mpo+5y/qIkGTiehMRlEGwcBBg7
6uNA8wpTuJI49tdY7rkVEIEGH34atwvBA1kwFYh1UfBzzIg+oRikHm1ZJ4UeJCM=
=h9ju
-----END PGP SIGNATURE-----

______________________________________________
R-help at r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.

+- Jan T. Kim -------------------------------------------------------+
 |             email: jttkim at gmail.com                                |
 |             WWW:   http://www.jtkim.dreamhosters.com/              |
 *-----=<  hierarchical systems are for files, not for humans  >=-----*

Help with functions as arguments

Thread (15 messages)