Matrix-shaped list

Question

Occasionally, I've written buggy R code that produced an object that printed like a matrix, but where the elements were vectors. It might look something like this:

 [,1] [,2] [,3] [1,] character,2 character,2 character,2 [2,] character,2 character,2 character,2 [3,] character,2 character,2 character,2

I've never intended to create one before, but now I'm working on a project where this data structure could be useful.

What is it? How do I make one? What are its properties? For iterating over rows and columns, how does its efficiency compare to a nested list, an array, or a list of matrices?

It's a matrix of lists. To make one, you can do matrix(list(c("a", "b"))) for example — Rich Scriven
– Rich Scriven, Commented Mar 18, 2015 at 19:24
@RichardScriven so it's just a list with a .Dim attribute? That's my impression based on dput(matrix(list(c("a", "b")))). This is really cool and I wonder why nobody does it. At any rate you should post that as an answer. — shadowtalker
– shadowtalker, Commented Mar 18, 2015 at 19:32
It's actually a matrix of objects, which as you say is just a list with a dimension attribute. You can put anything you want in it, including functions. It works basically like a list, except that you can access it with 2-dimensional indices. — mrip
– mrip, Commented Mar 18, 2015 at 19:44
You can also access items in a non-dimensioned lists with 2 or greater length vectors. — IRTFM
– IRTFM, Commented Mar 18, 2015 at 23:21

IRTFM · Accepted Answer · 2015-03-19 01:30:48Z

Making one is fairly trivial:

mtx <- matrix( list(letters[1:2]), 4,4) mtx #---------- [,1] [,2] [,3] [,4] [1,] Character,2 Character,2 Character,2 Character,2 [2,] Character,2 Character,2 Character,2 Character,2 [3,] Character,2 Character,2 Character,2 Character,2 [4,] Character,2 Character,2 Character,2 Character,2

The attribute (and the function that retrieves it) is named "dim". I would not expect it to be very much different in terms of access efficiency. Matrices in R are really just folded vectors.

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