Feal - 5 months ago 13

R Question

I have a data frame called

`input`

`ArtID`

Based on the

`ArtID`

`ArtID == 1`

`ArtID == 2`

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`

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()`

Can anyone help?

Answer

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.