Small request of a feature improvement in the next version of R

Hi Martyn,

I understand now what is happening and thank you for the nice explanation.
I am wondering now, based on the following definition:

"When $<- is applied to a NULL x, it first coerces x to list(). This is what
also happens with [[<- if the replacement value value is of length greater
than one: if value has length 1 or 0, x is first coerced to a zero-length
vector of the type of value."

The following works as expected:

m <- list()
m$"A3V6HVSALQ835D"$'profiles' <- 3
m$"A3V6HVSALQ835D"$'stars' <- c(1, 23)

But if the object encapsulating them (m) is already a list, why would it not
be sensible for any [[<- assignments underneath it be automatically
converted to a list?  It would be nice to have it automatically have m
become a list if one performs m[["A3V6HVSALQ835D"]][['profiles']] <- 3 on
the first assignment - without having to instantiate m to a list (i.e. m <-
list()).  Since R is heavily influenced by SEXP this would be as natural as
a cons().

For instance, I can create a list of functions but not with any other type:

a <- function(something="Hi") { print( something ) }
b <- a
list( a, b )[[1]]()

[1] "Hi"

list( a, b )[[2]]("there :)")

[1] "there :)"

In fact vectors become lists:

c(a,b)

[[1]]
function (something = "Hi") 
{
    print(something)
}

[[2]]
function (something = "Hi") 
{
    print(something)
}

Even matrices become lists:

as.matrix(c(a,b))

[,1]
[1,] ?   
[2,] ?

as.matrix(c(a,b))[1]

[[1]]
function (something = "Hi") 
{
    print(something)
}

as.matrix(c(a,b))[2]

[[1]]
function (something = "Hi") 
{
    print(something)
}

Since all become lists and the assignments are list-like, then it would be
nice to have the children of a list to automatically become a list, even
when they are accessed like a list and the parent (m) is not instantiated as
a list, which should automatically become a list.

I know I wrote a lot, but let me know if I should expand on anything for
clarification purposes.

Thank you,
Paul

-----Original Message-----
From: Martyn Plummer [mailto:plummerm at iarc.fr] 
Sent: Tuesday, November 17, 2015 5:13 AM
To: pgrosu at gmail.com
Cc: r-devel at r-project.org
Subject: Re: [Rd] Small request of a feature improvement in the next version
of R

Hi Everyone,

Sorry to bother the list with this small request, but I've run into 
this issue and was wondering if it could be fixed in the next version of

R.

Sorry if it was raised in a previous thread:

So when I try the following I get an error:

m <- list()
m[["A3V6HVSALQ835D"]][['profiles']] <- 3 
m[["A3V6HVSALQ835D"]][['stars']] <- c(1, 23)

Error in m[["A3V6HVSALQ835D"]][["stars"]] <- c(1, 23) : 
  more elements supplied than there are to replace
  
As does the following:

m <- list()
m[["A3V6HVSALQ835D"]][['profiles']] <- c() 
m[["A3V6HVSALQ835D"]][['stars']] <- c() 
m[["A3V6HVSALQ835D"]][['profiles']] <- 3 
m[["A3V6HVSALQ835D"]][['stars']] <- c(1, 23)

Error in m[["A3V6HVSALQ835D"]][["stars"]] <- c(1, 23) : 
  more elements supplied than there are to replace

But when I reverse the order, I don't:

m <- list()
m[["A3V6HVSALQ835D"]][['stars']] <- c(1, 23) 
m[["A3V6HVSALQ835D"]][['profiles']] <- 3

As doesn't the following, with the order reversed for the assignment:

m <- list()
m[["A3V6HVSALQ835D"]][['profiles']] <- c() 
m[["A3V6HVSALQ835D"]][['stars']] <- c() 
m[["A3V6HVSALQ835D"]][['stars']] <- c(1, 23) 
m[["A3V6HVSALQ835D"]][['profiles']] <- 3

And when I instantiate it in this way, it does not with the original

order:

m <- list()
m[["A3V6HVSALQ835D"]][['profiles']] <- c() 
m[["A3V6HVSALQ835D"]][['stars']] <- list() 
m[["A3V6HVSALQ835D"]][['profiles']] <- 3 
m[["A3V6HVSALQ835D"]][['stars']] <- c(1, 23)

The request is so that order-specific assignments would not throw an 
error, and I am using version 3.2.2 of R.

Your example combines two nested calls to the replacement function "[[<-".
It is a lot easier to understand what is going wrong if you break this down
into two separate function calls.

First, the element of m that you want to modify is NULL:

m <- list()
m[["A3V"]]

NULL

So the expression

m[["A3V"]][['profile']] <- 3

is equivalent to:

tmp <- NULL
tmp[['profile']] <- 3
m[["A3V"]] <- tmp

Inspecting the result:

m

$A3V
profile 
      3

class(m$A3V)

[1] "numeric"

So m$A3V is a numeric vector and not, as you expected, a list. This
behaviour of "[[<-" when applied to NULL objects is documented on the help
page: See help("[[<-")

The solution is to create m[["A3V"]] as a list before modifying its
elements:

m <- list()
m[["A3V"]] <- list()

...

Martyn

Thank you,
Paul

______________________________________________
R-devel at r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-devel

-----------------------------------------------------------------------
This message and its attachments are strictly confidenti...{{dropped:8}}