R help - May 2010 - scope of a function + lazy evaluation

Hey guys,

I have a doubt here , It is something simple I guess, what am I missing out
here ??


f <- function(y) function() y
tmp <- vector("list", 5)
for (i in 1:5) tmp[[i]] <- f(i)
tmp[[1]]() # returns 5;

z <- f(6)
tmp[[1]]() # still returns 5; it should return 6 "ideally" right ???

Even if  I dont evaluate the function tmp[[1]] before i.e I do
rm(list=ls())
f <- function(y) function() y
tmp <- vector("list", 5)
for (i in 1:5) tmp[[i]] <- f(i)
 z <- f(6)
tmp[[1]]() # it still returns 5; it should return 6 "ideally" right
???

	[[alternative HTML version deleted]]

You are missing 'force'.

See 'The R Inferno' page 90.

In this case you can define:

f <- function(y) { force(y); function() y}


On 10/05/2010 11:06, sayan dasgupta wrote:
> Hey guys,
>
> I have a doubt here , It is something simple I guess, what am I missing out
> here ??
>
>
> f<- function(y) function() y
> tmp<- vector("list", 5)
> for (i in 1:5) tmp[[i]]<- f(i)
> tmp[[1]]() # returns 5;
>
> z<- f(6)
> tmp[[1]]() # still returns 5; it should return 6 "ideally" right
???
>
> Even if  I dont evaluate the function tmp[[1]] before i.e I do
> rm(list=ls())
> f<- function(y) function() y
> tmp<- vector("list", 5)
> for (i in 1:5) tmp[[i]]<- f(i)
>   z<- f(6)
> tmp[[1]]() # it still returns 5; it should return 6 "ideally"
right ???
>
> 	[[alternative HTML version deleted]]
>
> ______________________________________________
> 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.
>
-- 
Patrick Burns
pburns at pburns.seanet.com
http://www.burns-stat.com
(home of 'Some hints for the R beginner'
and 'The R Inferno')

sayan dasgupta wrote:
> Hey guys,
>
> I have a doubt here , It is something simple I guess, what am I missing out
> here ??
>
>
> f <- function(y) function() y
> tmp <- vector("list", 5)
> for (i in 1:5) tmp[[i]] <- f(i)
> tmp[[1]]() # returns 5;
>
> z <- f(6)
> tmp[[1]]() # still returns 5; it should return 6 "ideally" right
???
>   
No, each time you call f you create a new y variable in its local 
evaluation frame.  So all 6 of your y variables are different.  However, 
the first 5 of them are all defined by the same expression, i.e. "i".
Thus the first time they are evaluated they will each get the current 
value of that variable.  After the first evaluation, the value will be 
fixed, because that is when the value of y is forced.  So for example,

 > f <- function(y) function() y
 > tmp <- vector("list", 5)
 > for (i in 1:5) tmp[[i]] <- f(i)
 > tmp[[1]]() # returns 5;
[1] 5
 >
 > i <- 10
 > tmp[[2]]()
[1] 10
 > tmp[[1]]()
[1] 5
> Even if  I dont evaluate the function tmp[[1]] before i.e I do
> rm(list=ls())
> f <- function(y) function() y
> tmp <- vector("list", 5)
> for (i in 1:5) tmp[[i]] <- f(i)
>  z <- f(6)
> tmp[[1]]() # it still returns 5; it should return 6 "ideally"
right ???
>   
See above.

Duncan Murdoch
> 	[[alternative HTML version deleted]]
>
> ______________________________________________
> 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.
>

When you call a function R passes a "promise" to it for each argument.
 A promise consists of the unevaluated variable together with the
environment in which it should evaluate the variable when time comes
to evaluate it.  Thus tmp[[1]] contains function(y) y and in the
environment of function(y) y there is a promise y to evaluate i the
first time that y is actually used.  When you write tmp[[1]]() the
promise is evaluated and since i is 5 at the point that is what you
get.  If f had actually used y when it was called then you would have
gotten 1.

On Mon, May 10, 2010 at 6:06 AM, sayan dasgupta <kittudg at gmail.com>
wrote:
> Hey guys,
>
> I have a doubt here , It is something simple I guess, what am I missing out
> here ??
>
>
> f <- function(y) function() y
> tmp <- vector("list", 5)
> for (i in 1:5) tmp[[i]] <- f(i)
> tmp[[1]]() # returns 5;
>
> z <- f(6)
> tmp[[1]]() # still returns 5; it should return 6 "ideally" right
???
>
> Even if ?I dont evaluate the function tmp[[1]] before i.e I do
> rm(list=ls())
> f <- function(y) function() y
> tmp <- vector("list", 5)
> for (i in 1:5) tmp[[i]] <- f(i)
> ?z <- f(6)
> tmp[[1]]() # it still returns 5; it should return 6 "ideally"
right ???
>
> ? ? ? ?[[alternative HTML version deleted]]
>
> ______________________________________________
> 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.
>