[Rcpp-devel] Help with accessing and manipulating List objects

[Romain Francois]

Hi Tal, 

I'm travelling right now, i'll give some additional pointers later. 

This is a nice non trivial example, and i hope that the code îve put in recently (with is<>) can help you so that you dont have to use TYPEOF for dispatching. 

Our exchange at useR is part of my motivation for adding this. 

Romain

Le 20 juil. 2013 à 18:44, Tal Galili <tal.galili at gmail.com> a écrit :

> Hello dear Rcpp users,
> 
> First - I'd like to say that I took Hadley and Romain's workshop at the useR conference this year, and I am very excited about trying out Rcpp - this project looks AMAZING.
> 
> Second - this is my first post, and I apologize if my question is too basic. To my defense, I tried looking through this mailing list / googled before asking, and read through Hadley's github chapter on Rcpp.
> 
> The questions:
> 
> I am looking to understand better how List objects can be navigated, their elements accessed, and manipulated - using Rcpp.
> 
> For example, here is an R list object:
> 
> x <- list(a = 1, b = 2, c = list(ca = 3, cb = 4, 5), 6)
> attr(x[[1]], "type") = "fun"
> attr(x[[3]][[1]], "type") = "fun"
> x
> 
> I would like to create two types of functions:
> 1) A function that will go through "x" and will RETURN all of the elements within it that are of type "fun".
> In R I would do it like this:
> 
> return_fun <- function(x) {
>    fun_nubmers <- numeric()
>    for(i in seq_along(x)) {      
>       if(class(x[[i]]) == "list") {
>          fun_nubmers <- c(fun_nubmers, return_fun(x[[i]]))
>       } else {
>          if(!is.null(attr(x[[i]], "type"))) {
>             if(attr(x[[i]], "type") == "fun") fun_nubmers <- c(fun_nubmers, x[[i]])  
>          }         
>       }
>    }
>    return fun_nubmers
> }
> return_fun(x) # output: 1 3
> 
> But in Rcpp there are many parts to this R function that I don't know how to do. I don't know how to access the attributes of a sub-element within a List, I don't know how to check if that attribute is null or not, etc. So any suggestions on either reading material (or better yet - an example of how this function might be created in Rcpp would be great.
> 
> 
> 2) A function that will go through "x" and will CHANGE all of the elements within it that are of type "fun". For example, adding 1 to them.
> In R I would do it like this:
> 
> add1_fun <- function(x) {
>    for(i in seq_along(x)) {      
>       if(class(x[[i]]) == "list") {
>          x[[i]] <- add1_fun(x[[i]])
>       } else {
>          if(!is.null(attr(x[[i]], "type"))) {
>             if(attr(x[[i]], "type") == "fun") x[[i]] <- x[[i]] + 1
>          }         
>       }
>    }
>    x
> }
> add1_fun(x) 
> return_fun(x) # output: 1 3 
> return_fun(add1_fun(x) ) # output: 2 4 
> 
> 
> 3) Is it possible to work with some "global" variable from within a recursive Rcpp function?
> For example:
> 
> 
> count_till_5 <- function() {
>    if(!exists("global_var")) global_var = 0 
> 
>    if(global_var <5) {
>       global_var <<- global_var+1
>       count_till_5()
>    }   
> }
> count_till_5()
> global_var
> 
> Is there a way to create something like this with Rcpp?
> 
> 
> Thanks in advance for any help.
> 
> With regards,
> Tal
> 
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> 
> ----------------Contact Details:-------------------------------------------------------
> Contact me: Tal.Galili at gmail.com |  
> Read me: www.talgalili.com (Hebrew) | www.biostatistics.co.il (Hebrew) | www.r-statistics.com (English)
> ----------------------------------------------------------------------------------------------
> 
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