Force evaluation of a symbol when a function is created

Duncan, et.al:

Many thanks: let the closure do the work automatically rather than
manually manipulating it.

However, in the spirit of the OP's original request, I believe the
call would be:

Y <- 3  ## That is, Y gets a value at some prior point, perhaps programmatically.
   F <- multiply_by_Y(Y) # ... F picks up this value "implicitly" -- no need for explicit assignment.

But then there is no need for force(), is there?

fy <- function(Y)function(x) x*Y
Y <- 2
F <- fy(Y)
F(5)

[1] 10

Y <- 3
F(5)

[1] 10

G <- fy(Y)
G(5)

[1] 15

That is, one simply relies on lexical scoping/closures to "retain" the
value of  Y used as a free variable in  function(x)x*Y when it is
defined. No need to explicitly force() it. If wrong, I would be
grateful for correction. This appears to me to duplicate the Matlab
behavior rather closely.


-- Bert

On Tue, Aug 7, 2012 at 3:48 AM, Duncan Murdoch <murdoch.duncan at gmail.com> wrote:

Here's one more way.  It seems to me this is the most R-like way to do what
you want:

   multiply_by_Y <- function(Y) {
     force(Y)
     function(x) x*Y
   }

   F <- multiply_by_Y(3)

The "force" call forces Y to be evaluated at that point, so its value is
fixed from that point forward.

Duncan Murdoch

On 12-08-06 5:07 PM, Schoenfeld, David Alan,Ph.D.,Biostatistics wrote:

I am porting a program in matlab to R,
The problem is that Matlab has a feature where symbols that aren't
arguments are evaluated immediately.
That is:
Y=3
F=@(x) x*Y

Will yield a function such that F(2)=6.
If later say. Y=4 then F(2) will still equal 6.

R on the other hand has lazy evaluation.
F<-function(x){x*Y}
Will do the following
Y=3
F(2)=6
Y=4
F(2)=8.
Does anyone know of away to defeat lazy evaluation in R so that I can
easily simulate the Matlab behavior.  I know that I can live without this in
ordinary programming but it would make my port much easier.

Thanks.




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