How do I use `[` correctly with (l|s)apply to select a specific column from a list of matrices?

Question

Consider the following situation where I have a list of n matrices (this is just dummy data in the example below) in the object myList

mat <- matrix(1:12, ncol = 3)
myList <- list(mat1 = mat, mat2 = mat, mat3 = mat, mat4 = mat)

I want to select a specific column from each of the matrices and do something with it. This will get me the first column of each matrix and return it as a matrix (lapply() would give me a list either is fine).

sapply(myList, function(x) x[, 1])

What I can't seem able to do is use [ directly as a function in my sapply() or lapply() incantations. ?'[' tells me that I need to supply argument j as the column identifier. So what am I doing wrong that this does't work?

> lapply(myList, `[`, j = 1)
$mat1
[1] 1

$mat2
[1] 1

$mat3
[1] 1

$mat4
[1] 1

Where I would expect this:

$mat1
[1] 1 2 3 4

$mat2
[1] 1 2 3 4

$mat3
[1] 1 2 3 4

$mat4
[1] 1 2 3 4

I suspect I am getting the wrong [ method but I can't work out why? Thoughts?

Answer 1

It's because [ is a .Primitive function. It has no j argument. And there is no [.matrix method.

> `[`
.Primitive("[")
> args(`[`)
NULL
> methods(`[`)
 [1] [.acf*            [.AsIs            [.bibentry*       [.data.frame     
 [5] [.Date            [.difftime        [.factor          [.formula*       
 [9] [.getAnywhere*    [.hexmode         [.listof          [.noquote        
[13] [.numeric_version [.octmode         [.person*         [.POSIXct        
[17] [.POSIXlt         [.raster*         [.roman*          [.SavedPlots*    
[21] [.simple.list     [.terms*          [.ts*             [.tskernel* 

Though this really just begs the question of how [ is being dispatched on matrix objects...

Answer 2

I think you are getting the 1 argument form of [. If you do lapply(myList, `[`, i =, j = 1) it works.

Answer 3

After two pints of Britain's finest ale and a bit of cogitation, I realise that this version will work:

lapply(myList, `[`, , 1)

i.e. don't name anything and treat it like I had done mat[ ,1]. Still don't grep why naming j doesn't work...

...actually, having read ?'[' more closely, I notice the following section:

Argument matching:

     Note that these operations do not match their index arguments in
     the standard way: argument names are ignored and positional
     matching only is used.  So ‘m[j=2,i=1]’ is equivalent to ‘m[2,1]’
     and *not* to ‘m[1,2]’.

And that explains my quandary above. Yeah for actually reading the documentation.