Force evaluation of a symbol when a function is created

-----Original Message-----
From: Bert Gunter [mailto:gunter.berton at gene.com]
Sent: Monday, August 06, 2012 3:03 PM
To: William Dunlap
Cc: Schoenfeld, David Alan,Ph.D.,Biostatistics; r-help at r-project.org
Subject: Re: [R] Force evaluation of a symbol when a function is created

Thanks to both: Cute question, clever, informative answer.

However, Bill, I don't think you **quite** answered him, although the
modification needed is completely trivial. Of course, I could never
have figured it out without your response.

Anyway, I interpret the question as asking for the function definition
to _implicitly_ pick up the value of Y at the time the function is
defined, rather than explicitly assigning it in local(). The following
are two essentially identical approaches: I prefer the second, because
it's more transparent to me, but that's just a matter of taste.

Y <- 3
F <-local({y <- Y;function(x)x*y})
G <- evalq(function(x)x*y,env=list(y=Y))

Yielding:

Y <- 3
F <-local({y <- Y;function(x)x*y})
G <- evalq(function(x)x*y,env=list(y=Y))
F(5)

[1] 15

G(5)

[1] 15

Y <- 2
F(5)

[1] 15

G(5)

[1] 15

F <-local({y <- Y;function(x)x*y})
G <- evalq(function(x)x*y,env=list(y=Y))
F(5)

[1] 10

G(5)

[1] 10

Cheers,
Bert

On Mon, Aug 6, 2012 at 2:24 PM, William Dunlap <wdunlap at tibco.com> wrote:

You could use local(), as in

   > F <- local({

   +        Y <- 3
   +        function(x) x * Y
   +    })

   >    F(7)

   [1] 21

   > Y <- 19
   > F(5)

  [1] 15

Look into 'environments' for more.

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On

Behalf

Of Schoenfeld, David Alan,Ph.D.,Biostatistics
Sent: Monday, August 06, 2012 2:08 PM
To: 'r-help at r-project.org'
Subject: [R] Force evaluation of a symbol when a function is created


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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Bert Gunter
Genentech Nonclinical Biostatistics

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