[Rd] several bugs (PR#918) lists and matrices

[rmh@surfer.sbm.temple.edu]

### I got bit again by the same bugs I wrote about a year ago.
### The bugs are related to matrices and arrays of lists.

### 1. There is a clear inconsistency in how R handles two
### functionally equivalent statements.
### array() is able to take a list and create a matrix.
### matrix() is unable to create that matrix.


> vector("list", 2)
[[1]]
NULL

[[2]]
NULL

> array(vector("list", 2), dim=c(2,1))
     [,1]  
[1,] "NULL"
[2,] "NULL"
> tmpa <- array(vector("list", 2), dim=c(2,1))
> class(tmpa)
NULL
> is.array(tmpa)
[1] TRUE
> is.matrix(tmpa)
[1] TRUE
> tmpm <- matrix(vector("list", 2), nrow=2)
Error in matrix(vector("list", 2), nrow = 2) : 
	Unimplemented feature in copyVector
> 




### 2. Here the matrix single-[ subscript is doing the wrong thing,
### without giving a warning.

> tmp <- array(vector("list", 2), 1:2)
> tmp
     [,1]   [,2]  
[1,] "NULL" "NULL"
> tmp[1]
[[1]]
NULL

> tmp[[1]]
NULL
> tmp[1,1]
[[1]]
NULL

> tmp[[1,1]]
NULL
> list("abc")
[[1]]
[1] "abc"

> tmp[1] <- list("abc")  ## assignment
> tmp[1]
[[1]]
[1] "abc"

> tmp[[1]]
[1] "abc"
> tmp[1,1]     ## incorrect retrieval
[[1]]
NULL

> tmp[[1,1]]
[1] "abc"

> tmp[1,2] <- list("def")  ## refuse to do similar assignment
Error: incompatible types in subset assignment
> 



### 3. subsetting a vector of lists doesn't behave the way I anticipate.
### It looks like NULL is not handled consistently inside a vector.
### Sometimes it can be an element, sometimes not.

> tmpv <- vector("list", 2)
> tmpv
[[1]]
NULL

[[2]]
NULL

> tmpv[1] <- "abcd"  ## I assigned to the first element.
> tmpv               ## Both elements are still here.
[[1]]
[1] "abcd"

[[2]]
NULL

> tmpv[1] <- NULL    ## I assigned to the first element.
> tmpv               ## The second element is gone.
[[1]]
NULL

> 



### This time the issue came up in an attempt to make abind work with R.
### abind() is a generalization of rbind and cbind to arrays.
### Tony Plate and I posted it to StatLib in 1996
### (http://lib.stat.cmu.edu/S/abind).

### I would like you to consider adding abind to R.
### The posted abind doesn't work in R, for the reasons above and due to
### a difference in behavior of match.call between S -Plusand R that I am
### reporting in a separate bug report.
### There are two revised files, currently available at
### http://surfer.sbm.temple.edu/~rmh/abind.r
### http://surfer.sbm.temple.edu/~rmh/abind.test.in.r
### The documentation in statlib applies to these files.
### I will update the statlib version after I understand why R differs.

> version
         _              
platform i386-pc-mingw32
arch     x86            
os       Win32          
system   x86, Win32     
status                  
major    1              
minor    4.1            
year     2002           
month    01             
day      30             
language R              
>

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