Force evaluation of a symbol when a function is created

On 12-08-07 10:46 AM, Bert Gunter wrote:

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?

You still need force:

fy <- function(Y)function(x) x*Y
Y <- 2
F <- fy(Y)
Y <- 3
F(5)

This will print 15, because F only contains a promise to evaluate Y, it
hasn't been evaluated until the very last line, and by that time Y has been
changed to 3.

If you are going to construct functions in functions, and their results
depend on the arguments to the constructor, then it's almost always a good
idea to force the arguments.  Sometimes it isn't necessary (the value will
be forced implicitly), and in some rare circumstances you might want to
capture the promise instead of its value, but it's generally a good idea.
It is a fairly cheap operation.

Duncan Murdoch

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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