[R] extract index during execution of sapply

[Thomas Lumley]

On Fri, 22 Jun 2007, Ben Bolker wrote:

> Christian Bieli <christian.bieli <at> unibas.ch> writes:
>
>>
>> Hi there
>> During execution of sapply I want to extract the number of times the
>> function given to supply has been executed. I came up with:
>>
>> mylist <- list(a=3,b=6,c=9)
>> sapply(mylist,function(x)as.numeric(gsub("[^0-9]","",deparse(substitute(x)))))
>>
>> This works fine, but looks quite ugly. I'm sure that there's a more
>> elegant way to do this.
>>
>> Any suggestion?
>>
>> Christian
>>
>
>   I would love to have an answer to this -- when I run
> into this kind of problem I usually end up using mapply:
> e.g., suppose I have
>
> mylist <- replicate(5,list(x=runif(10),y=runif(10)),simplify=FALSE)
>
> and I want to plot each element in a different color.  I'd like
> to be able to do
>
> plot(0:1,0:1,type="n")
> lapply(mylist,plot,col=i)
>
> but instead I do
>
> mapply(function(x,i) points(x,col=i),mylist,1:5)
>
> would it be too ugly to have a special variable called INDEX
> that could be used within an sapply/lapply statement?
>

There are two distinct suggestions here: a variable that says *how many* 
times the function has been called, and a variable that say *which 
element* is currently being operated on.   The first seems undesirable as 
order of evaluation really should not matter in the apply functions.

The second makes more sense but is still a little tricky. AFAICS there is 
no way for lapply() to find out whether FUN will accept an argument INDEX 
without an "unused argument(s)" error, so it can't just be passed as an 
argument.  This suggests having yet another apply function, that would 
assume an INDEX argument and might be written
   yapply<-function(X,FUN, ...) {
 	index<-seq(length.out=length(X))
         mapply(FUN,X,INDEX=index,MoreArgs=list(...))
        }

However, I think it would be preferable in many cases for INDEX to be 
names(X) if it exists, rather than 1:n.  In any case, it is easy  to write 
the function.

 	-thomas