Converting array to matrix in R

Question

I have an array, including two proficiency variables (theta0, theta1) over an item (Yes, No) called \"comp\". This needs to be converted to one matrix. Is there any way that

Answer 1

You can easily get columns of values by flattening the matrix on the 3rd margin:

z1 <- apply(priCPT.i6, 3L, c)
## we can also simply use `matrix`; but remember to set `dimnames`
## otherwise we lose dimnames
## z1 <- matrix(priCPT.i6, ncol = 2L,
##              dimnames = list(NULL, dimnames(priCPT.i6)[[3]]))

What you need for the rest is to append the "dimnames" columns:

z2 <- expand.grid(dimnames(priCPT.i6)[1:2])

Now you can merge them into a data frame (you definitely need a data frame than a matrix, because columns of z1 are numeric while columns of z2 are character) via:

data.frame(z2, z1)

---

Reproducible example

x <- array(1:18, dim = c(3L, 3L, 2L), dimnames = list(
           c("Low", "Medium", "High"), c("Low", "Medium", "High"), c("Yes", "No")))

#, , Yes
#
#       Low Medium High
#Low      1      4    7
#Medium   2      5    8
#High     3      6    9
#
#, , No
#
#       Low Medium High
#Low     10     13   16
#Medium  11     14   17
#High    12     15   18

z1 <- apply(x, 3L, c)
## z1 <- matrix(x, ncol = 2L, dimnames = list(NULL, dimnames(x)[[3]]))
z2 <- expand.grid(dimnames(x)[1:2])
data.frame(z2, z1)

#    Var1   Var2 Yes No
#1    Low    Low   1 10
#2 Medium    Low   2 11
#3   High    Low   3 12
#4    Low Medium   4 13
#5 Medium Medium   5 14
#6   High Medium   6 15
#7    Low   High   7 16
#8 Medium   High   8 17
#9   High   High   9 18

Answer 2

An alternative using reshape2 would be

x <- array(1:18, dim = c(3L, 3L, 2L), dimnames = list(
           c("Low", "Medium", "High"), 
           c("Low", "Medium", "High"), 
           c("Yes", "No")))

library(reshape2)
df <- dcast(melt(x), Var1+Var2~Var3) 
df

     Var1   Var2 Yes No
1    Low    Low   1 10
2    Low Medium   4 13
3    Low   High   7 16
4 Medium    Low   2 11
5 Medium Medium   5 14
6 Medium   High   8 17
7   High    Low   3 12
8   High Medium   6 15
9   High   High   9 18