[R] Putting an index explicitly into function code --- a curiosity.

[jim holtman]

Here is yet another way of doing it using 'local':

> junk<- vector("list",4)
> for(i in 1:4) {
+     junk[[i]] <- local({
+         local_i <- i
+         function(x) 42 + local_i * x
+         })
+ }
> for (i in 1:4) cat(i, junk[[i]](1), '\n')
1 43
2 44
3 45
4 46


On Sat, Jan 7, 2012 at 10:38 AM, Gabor Grothendieck
<ggrothendieck at gmail.com> wrote:
> On Sat, Jan 7, 2012 at 10:23 AM, Duncan Murdoch
> <murdoch.duncan at gmail.com> wrote:
>> On 12-01-06 10:21 PM, Rolf Turner wrote:
>>>
>>> On 07/01/12 15:51, R. Michael Weylandt<michael.weylandt at gmail.com>  wrote:
>>>>
>>>> I imagine the answer will involve lazy evaluation and require you use
>>>> force() but I'm not quite qualified to pronounce and not at a computer to
>>>> test.
>>>
>>>
>>> I think you've got it;  I tried
>>>
>>> junk<- vector("list",4)
>>> for(i in 1:4) {
>>>     junk[[i]]<- eval(bquote(function(x){42 + .(force(i))*x}))
>>> }
>>>
>>> and got the result that I wanted.  Still don't completely understand, but
>>> it at least makes vague sense and makes me a bit more comfy.
>>
>>
>> I'm not so sure.  The index in a for loop isn't supposed to be a promise.
>>  To me, it looks like a bug, maybe in bquote()...
>>
>
> These two variations without bquote and the third which just replaces
> for with while all (that I had previously posted) do work:
>
> # 1
> junk <- vector("list",4)
> for(i in 1:4) {
>        junk[[i]] <- eval(substitute(function(x) { 42 + i * x }, list(i = i)))
> }
> junk
>
> # 2
> junk <- vector("list",4)
> for(i in 1:4) {
>   junk[[i]] <- eval(parse(text = paste("function(x) { 42 +", i, "*x }")))
> }
> junk
>
> # 3 - from my prior post
> junk <- vector("list",4)
> i <- 1
> while(i <= 4) {
>  junk[[i]] <- eval(bquote(function(x){42 + .(i)*x}))
>  i <- i + 1
> }
> junk
>
> --
> Statistics & Software Consulting
> GKX Group, GKX Associates Inc.
> tel: 1-877-GKX-GROUP
> email: ggrothendieck at gmail.com
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.



-- 
Jim Holtman
Data Munger Guru

What is the problem that you are trying to solve?
Tell me what you want to do, not how you want to do it.