Feal - 2 months ago 4x
R Question

# Why is this matrix not numeric? Then `as.numeric` destroys the matrix and return a vector

I have a data frame called

`input`
. The first column refers to an Article ID (
`ArtID`
), the subsequent columns will be used to create the matrix.

Based on the
`ArtID`
, I want R to generate a 2x2 matrix (more precise: It needs to be a numeric 2x2 matrix). Specifically, I want to create a matrix for the first row (
`ArtID == 1`
), the second row(
`ArtID == 2`
) and so on...

What I came up with so far is this:

``````for(i in 1:3) {stored.matrix = matrix(input[which(ArtID ==i),-1],nrow = 2)
``````

This gives me a 2x2 matrix, but it is not numeric (which it needs to be).

If I apply
`as.numeric`
, the matrix is no longer a 2x2 matrix.

How do I get a 2x2 numerical matrix?

Minimal reproducible example:

``````ArtID = c(1,2,3)
AC_AC = c(1,1,1)
MKT_AC = c(0.5,0.6,0.2)
AC_MKT = c(0.5,0.6,0.2)
MKT_MKT = c(1,1,1)
input = data.frame(ArtID, AC_AC, MKT_AC, AC_MKT, MKT_MKT)

stored.matrix = matrix(input[which(ArtID ==i),-1],nrow = 2)
#     [,1] [,2]
#[1,] 1    0.5
#[2,] 0.5  1

is.numeric(stored.matrix)
# [1] FALSE

as.numeric(stored.matrix)
## [1] 1.0 0.5 0.5 1.0
``````

As you can see after applying
`as.numeric()`
the matrix is no longer 2x2.

Can anyone help?

when you have only numerical values in your data frame, it is more appropriate to use a matrix. Convert your data frame to a matrix will solve all problem. Also,

``````input <- data.matrix(input)
``````

``````ArtID = c(1,2,3)
AC_AC = c(1,1,1)
MKT_AC = c(0.5,0.6,0.2)
AC_MKT = c(0.5,0.6,0.2)
MKT_MKT = c(1,1,1)
input = data.frame(ArtID, AC_AC, MKT_AC, AC_MKT, MKT_MKT)

input <- data.matrix(input)   ## <- this line

stored.matrix = matrix(input[which(ArtID ==i),-1], 2)
is.numeric(stored.matrix)
# [1] TRUE
``````

So what was the problem?

If `input` is a data frame, `input[which(ArtID == i),-1]` by row subsetting still returns a data frame. A data frame is a special type of list. When you feed a list to `matrix()`, you get into a situation of matrix list.

If you read `?matrix` for what data it can take, you will see:

``````data: an optional data vector (including a list or ‘expression’
vector).  Non-atomic classed R objects are coerced by
``````

Note that a list is also of vector data type (e.g., `is.vector(list(a = 1))` gives `TRUE`), so it is legitimate to feed a list to `matrix`. You can try

``````test <- matrix(list(a = 1, b = 2, c = 3, d = 4), 2)
#     [,1] [,2]
#[1,] 1    3
#[2,] 2    4
``````

This is indeed a matrix in the sense that `class(test)` give "matrix"), but

``````str(test)
#List of 4
# \$ : num 1
# \$ : num 2
# \$ : num 3
# \$ : num 4
# - attr(*, "dim")= int [1:2] 2 2

typeof(test)
# [1] "list"
``````

so it is not the usual numerical matrix we refer to.

The input list can be ragged, too.

``````test <- matrix(list(a = 1, b = 2:3, c = 4:6, d = 7:10), 2)
#     [,1]      [,2]
#[1,] 1         Integer,3
#[2,] Integer,2 Integer,4

str(test)
#List of 4
# \$ : num 1
# \$ : int [1:2] 2 3
# \$ : int [1:3] 4 5 6
# \$ : int [1:4] 7 8 9 10
# - attr(*, "dim")= int [1:2] 2 2
``````

And I was wondering why `typeof()` gives me list... :)

Yes, so had realized something unusual. The storage mode of a matrix is determined by that of its element. For a matrix list, elements are list, hence the matrix has "list" mode.