[R] mapply & assign to generate functions
Brian Diggs
diggsb at ohsu.edu
Mon Mar 12 19:04:11 CET 2012
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
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