Re: [R] unexpected behavior when creating a list of functions

From: Thomas Lumley <tlumley_at_u.washington.edu>
Date: Wed 04 Apr 2007 - 22:49:45 GMT

On Wed, 4 Apr 2007, Matthew Suderman wrote:

> I wanted to create a list of functions whose output differs depending
> the value of a variable when the function was created. Generally this
> did not work. Each function was exactly the same, as in the simple
> example below:
>
> get_data_function <- function(v) {
> function() {
> print(v)
> }
> }
> data_functions <- sapply(1:10,function(v) get_data_function(v))
> (data_functions[[1]])() # prints 10!
>
> However, if I insert a statement in get_data_function to print argument
> v, then I get the different functions that I wanted:
>
> get_data_function <- function(v) {
> print(v)
> function() {
> print(v)
> }
> }
> data_functions <- sapply(1:10,function(v) get_data_function(v))
> (data_functions[[1]])() # prints 1, as expected!
>
> I have two questions about this:
> * Is this a bug in R?

No, it's lazy evaluation at work.

> * Is there a more direct way to get the effect of printing v?

The function force() is designed for this -- it doesn't do anything special that any other evaluation wouldn't do, but it makes clear that all you are doing is forcing evaluation.

         -thomas



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