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

R. Michael Weylandt <michael.weylandt@gmail.com>
Sat, 07 Jan 2012 02:51:23 -0000

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.

Michael

On Jan 6, 2012, at 8:43 PM, Rolf Turner <rolf.turner at xtra.co.nz> wrote:

>
> I want to create a list of functions in a for loop, with the index
> of the loop appearing explicitly in the function code.
>
> After quite a bit of thrashing around I figured out how to do it.
>
> Here is a toy example:
>
> junk <- vector("list",4)
> for(i in 1:4) {
>    itmp <- i
>    junk[[i]] <- eval(bquote(function(x){42 + .(itmp)*x}))
> }
>
> So I'm *basically* happy, but there's something I don't
understand:
> Why do I need "itmp"?
>
> That is, if I do
>
> junk <- vector("list",4)
> for(i in 1:4) {
>    junk[[i]] <- eval(bquote(function(x){42 + .(i)*x}))
> }
>
> then every entry of "junk" is equal to
>
> function (x)
> {
>    42 + 4L * x
> }
>
> i.e. I seem to get the *last* value of the index always substituted, rather
than
> the "current" value.  Something (subtle?) is going on that I
don't understand.
> And than makes me feel not quite comfy.  Can anyone enlighten me?
>
> Ta.
>
>    cheers,
>
>        Rolf Turner
>
> P. S.  Also:  Is there a *better* way of accomplishing my objective than
what I came up with?
>
>        R. T.
>
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