R help - Mar 2012 - mapply & assign to generate functions

Hi,

I have a problem that I'm finding a bit tricky.  I'm trying to use
mapply and assign to generate curried functions.  For example, if I
have the function divide

divide <- function(x, y) {
  x / y
}

And I want the end result to be functionally equivalent to:
half <- function(x) divide(x, 2)
third <- function(x) divide(x, 3)
quarter <- function(x) divide(x, 4)

But I want to do it using mapply and assign:
mapply(assign,
       c("half", "third", "quarter"),
       lapply(2:4, function(i) {function(x) divide(x, i)}),
       pos = 1)

The problem is in the third line.  I end up with 3 functions that are
all functionally equivalent to quarter().  Any suggestions on how to
get this to work properly.

Thanks.


James

On 3/12/2012 10:47 AM, J Toll wrote:
> Hi,
>
> I have a problem that I'm finding a bit tricky.  I'm trying to use
> mapply and assign to generate curried functions.  For example, if I
> have the function divide
>
> divide<- function(x, y) {
>    x / y
> }
>
> And I want the end result to be functionally equivalent to:
> half<- function(x) divide(x, 2)
> third<- function(x) divide(x, 3)
> quarter<- function(x) divide(x, 4)
>
> But I want to do it using mapply and assign:
> mapply(assign,
>         c("half", "third", "quarter"),
>         lapply(2:4, function(i) {function(x) divide(x, i)}),
>         pos = 1)
The problem is with lazy argument evaluation/promises. i will be the 
last value of i (in this case 4) for all instances.  You can change that.

mapply(assign,
        c("half", "third", "quarter"),
        lapply(2:4, function(i) {force(i); function(x) divide(x, i)}),
        pos = 1)

Note the inclusion of the force function.  This gives expected results.

 > half(12)
[1] 6
 > third(12)
[1] 4
 > quarter(12)
[1] 3

> The problem is in the third line.  I end up with 3 functions that are
> all functionally equivalent to quarter().  Any suggestions on how to
> get this to work properly.
>
> Thanks.
>
>
> James
>

-- 
Brian S. Diggs, PhD
Senior Research Associate, Department of Surgery
Oregon Health & Science University

I think that adding a force(i) to your inner function should fix things up.
  > z <- mapply(assign,
  +        c("half", "third", "quarter"),
  +        lapply(2:4, function(i) {
  +             force(i)
  +             function(x) divide(x, i)}),
  +        pos = 1)
  > half(10)
  [1] 5
  > third(10)
  [1] 3.333333
  > quarter(10)
  [1] 2.5

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 J Toll
> Sent: Monday, March 12, 2012 10:47 AM
> To: r-help
> Subject: [R] mapply & assign to generate functions
> 
> Hi,
> 
> I have a problem that I'm finding a bit tricky.  I'm trying to use
> mapply and assign to generate curried functions.  For example, if I
> have the function divide
> 
> divide <- function(x, y) {
>   x / y
> }
> 
> And I want the end result to be functionally equivalent to:
> half <- function(x) divide(x, 2)
> third <- function(x) divide(x, 3)
> quarter <- function(x) divide(x, 4)
> 
> But I want to do it using mapply and assign:
> mapply(assign,
>        c("half", "third", "quarter"),
>        lapply(2:4, function(i) {function(x) divide(x, i)}),
>        pos = 1)
> 
> The problem is in the third line.  I end up with 3 functions that are
> all functionally equivalent to quarter().  Any suggestions on how to
> get this to work properly.
> 
> Thanks.
> 
> 
> James
> 
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