R devel - Feb 2015 - iterated lapply

Hi everybody,

with the following code I generate a list of functions. Each function
reflects a "condition". When I evaluate this list of functions by
another lapply/sapply, I get an unexpected result: all values coincide.
However, when I uncomment the print(), it works as expected. Is this a
bug or a feature?

conditions <- 1:4
test <- lapply(conditions, function(mycondition){
  #print(mycondition)
  myfn <- function(i) mycondition*i
  return(myfn)
})

sapply(test, function(myfn) myfn(2))



Cheers,
Daniel

-- 
Daniel Kaschek
Institute of Physics
Freiburg University

Room 210
Phone: +49 761 2038531

On Mon, Feb 23, 2015 at 12:57 PM, Daniel Kaschek
<daniel.kaschek at physik.uni-freiburg.de> wrote:
> Is this a bug or a feature?
I think it is a bug. If we use substitute to inspect the promise, it
appears the index number is always equal to its last value:

vec <- c("foo", "bar", "baz")
test <- lapply(vec, function(x){
  function(){x}
})
substitute(x, environment(test[[1]]))
substitute(x, environment(test[[2]]))

Use force() (or anything that evaluates mycondition, e.g. your print):

function(mycondition) {
  force(mycondition)
  function(i) mycondition * i
}

within the lapply() loop. 

Not a bug, but does surprise people. It is lazy evaluation.

  D.

On 2/23/15 12:57 PM, Daniel Kaschek wrote:
> Hi everybody,
> 
> with the following code I generate a list of functions. Each function
> reflects a "condition". When I evaluate this list of functions by
> another lapply/sapply, I get an unexpected result: all values coincide.
> However, when I uncomment the print(), it works as expected. Is this a
> bug or a feature?
> 
> conditions <- 1:4
> test <- lapply(conditions, function(mycondition){
>   #print(mycondition)
>   myfn <- function(i) mycondition*i
>   return(myfn)
> })
> 
> sapply(test, function(myfn) myfn(2))
> 
> 
> 
> Cheers,
> Daniel
> 
-- 
Director, Data Sciences Initiative, UC Davis
Professor, Dept. of Statistics, UC Davis

http://datascience.ucdavis.edu
http://www.stat.ucdavis.edu/~duncan

Greetings!

I thought it was a lazy evaluation thing. I added "force" around
mycondition and everything worked as expected.

Cheers!

On Mon Feb 23 2015 at 1:47:20 PM Jeroen Ooms <jeroenooms at gmail.com>
wrote:
> On Mon, Feb 23, 2015 at 12:57 PM, Daniel Kaschek
> <daniel.kaschek at physik.uni-freiburg.de> wrote:
> > Is this a bug or a feature?
>
> I think it is a bug. If we use substitute to inspect the promise, it
> appears the index number is always equal to its last value:
>
> vec <- c("foo", "bar", "baz")
> test <- lapply(vec, function(x){
>   function(){x}
> })
> substitute(x, environment(test[[1]]))
> substitute(x, environment(test[[2]]))
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>
	[[alternative HTML version deleted]]

On 23/02/2015 3:57 PM, Daniel Kaschek wrote:
> Hi everybody,
> 
> with the following code I generate a list of functions. Each function
> reflects a "condition". When I evaluate this list of functions by
> another lapply/sapply, I get an unexpected result: all values coincide.
> However, when I uncomment the print(), it works as expected. Is this a
> bug or a feature?
> 
Arguments aren't evaluated until they are used.  The force() function
can be used to force evaluation when you want it.

This is a feature:  it allows you to have arguments that are never
evaluated, because they are never used, or defaults that depend on
things that are calculated within the function.

Duncan Murdoch

> conditions <- 1:4
> test <- lapply(conditions, function(mycondition){
>   #print(mycondition)
>   myfn <- function(i) mycondition*i
>   return(myfn)
> })
> 
> sapply(test, function(myfn) myfn(2))
> 
> 
> 
> Cheers,
> Daniel
>

On Mo, 2015-02-23 at 16:54 -0500, Duncan Murdoch wrote:
> This is a feature:  it allows you to have arguments that are never
> evaluated, because they are never used, or defaults that depend on
> things that are calculated within the function.
I haven't thought about the thing with the default arguments. That's
really a feature.

Thanks,
Daniel
> 
> Duncan Murdoch
> 
> 
> > conditions <- 1:4
> > test <- lapply(conditions, function(mycondition){
> >   #print(mycondition)
> >   myfn <- function(i) mycondition*i
> >   return(myfn)
> > })
> > 
> > sapply(test, function(myfn) myfn(2))
> > 
> > 
> > 
> > Cheers,
> > Daniel
> > 
> 
-- 
Daniel Kaschek
Institute of Physics
Freiburg University

Room 210
Phone: +49 761 2038531