I see. I will be careful of this in the future. Thank you very much
for your feedback.
Elliott Forney
On Tue, Oct 13, 2009 at 5:59 AM, Duncan Murdoch <murd...@stats.uwo.ca> wrote:
> On 13/10/2009 1:50 AM, elliott.for...@gmail.com wrote:
>>
>> Full_Name: Elliott Forney
>> Version: 2.9.2
>> OS: Linux, Fedora 10
>> Submission from: (NULL) (129.82.47.235)
>
> This is not a bug, just a consequence of lazy evaluation. Arguments are not
> evaluated when passed, but when first used. Since the "input" argument is
> never evaluated in the commented version of "funk" below, it isn't evaluated
> until one of the output functions is evaluated.
>
> It's a good practice to force the evaluation of arguments to function
> builders like this, for exactly the reason you saw. (The force() function
> is one way to do it, but any expression evaluating the argument will work.
>
> Duncan Murdoch
>
>
>>
>>
>> The following code creates a list of functions that are lexically closed
>> over a
>> single argument. If a print statement is included then each function in
>> the
>> list evaluates to a different value. If the print statement is not
>> included
>> then each function evaluates to something different, as expected. I am
>> convinced that this behavior is not correct as one would not expect the
>> presence
>> of a lone print statement to alter the behavior of a program.
>>
>> Please let me know if you have any questions and thank you for your time.
>> The
>> sample code follows:
>>
>> ## returns a function that sums input with 5
>> funk <- function(input)
>> {
>> ## expected behavior if either of
>> ## the following lines is uncommented
>> # print(input)
>> # input <- input
>>
>> ## function to sum input with 5
>> ## lexical closure over input
>> function()
>> {
>> input + 5
>> }
>> }
>>
>> ## create a list different funk's
>> funk.list <- list()
>> for (i in 1:5)
>> {
>> ## just some values to sum over
>> test.vector <- 1:i
>>
>> ## add funk that sums test.vector with 5
>> funk.list[[i]] <- funk(sum(test.vector))
>> }
>>
>> ## print result of evaluating each funk
>> ## They are all the same unless print or
>> ## reflexive assignment is uncommented!!
>> for (i in 1:5)
>> print(funk.list[[i]]())
>>
>> ______________________________________________
>> R-devel@r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-devel
>
>
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