FAQ
Strident assertions?

My (non strident) assertion was about
matrices, not about matrix lists (which nobody said that would not exist).
In your example you omit the fact that
is.list(m) also results in TRUE:
m <- list(a1=1:3, a2=4:6, a3=pi, a4=c("a","b","c"))
dim(m) <- c(2,2)
is.matrix(m) [1] TRUE
is.list(m) [1] TRUE
m[1,1]
[[1]]
NULL

So it's a matrix and it's a list.
The NULL result of m[1,1] (that I pointed to you), might be, as you say, a
bug. But in any case, objects such as m in the example
are not what most people would understand as a matrix.
It's a different type of object. And the original message said
" ...I tried to create a matrix from a list..."
The "normal" matrix would yield FALSE for is.list():
m <- list(a1=1:2, a2=4:5)
m <- unlist(m)
dim(m) <- c(2,2)
is.matrix(m) [1] TRUE
is.list(m) [1] FALSE
m[1,1]
[1] 1


On Tue, 26 Feb 2002, Prof Brian D Ripley wrote:

Perhaps the following example will help demonstrate that matrix lists do
exist and do work up to a point, despite strident assertions on R-help to
the contrary:
m <- list(a1=1:3, a2=4:6, a3=pi, a4=c("a","b","c"))
dim(m) <- c(2,2)
m
[,1] [,2]
[1,] "Numeric,3" "Numeric,1"
[2,] "Numeric,3" "Character,3"
is.matrix(m) [1] TRUE
m[[1,2]]
[1] 3.141593
m[[2,2]]
[1] "a" "b" "c"

In S m[2,2] extracts a sub-list, but that seems not to work correctly in R:
neither does m[,2]. I am reporting that as a bug.

--
Brian D. Ripley, ripley at stats.ox.ac.uk
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272860 (secr)
Oxford OX1 3TG, UK Fax: +44 1865 272595

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