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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?

Zheyuan Li
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Feal
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2 Answers2

5

You could use unlist():

matrix(unlist(input[ArtID ==i,-1]),2)

or use

storage.mode(m) <- "numeric"
Ben Bolker
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2

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
      ‘as.vector’ and all attributes discarded.

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.

Zheyuan Li
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