How can I separate a matrix into smaller ones in R?

Question

I have the following matrix

2    4    1
6    32   1
4    2    1
5    3    2
4    2    2

I want to make the following two matrices based on 3rd column

first

2    4
6    32
4    2

second

5    3
4    2

Best I can come up with, but I get an error

x <- cbind(mat[,1], mat[,2]) if mat[,3]=1

y <- cbind(mat[,1], mat[,2]) if mat[,3]=2

Please post [reproducible examples](http://stackoverflow.com/questions/5963269/how-to-make-a-great-r-reproducible-example) in the future. You'll get better responses that way. — Ari B. Friedman, Oct 25 '12 at 00:49

score 13 · Answer 1 · answered Oct 25 '12 at 00:45

13

If mat is your matrix:

mat <- matrix(1:15,ncol=3)
mat[,3] <- c(1,1,1,2,2)
> mat
     [,1] [,2] [,3]
[1,]    1    6    1
[2,]    2    7    1
[3,]    3    8    1
[4,]    4    9    2
[5,]    5   10    2

Then you can use split:

> lapply( split( mat[,1:2], mat[,3] ), matrix, ncol=2)
$`1`
     [,1] [,2]
[1,]    1    6
[2,]    2    7
[3,]    3    8

$`2`
     [,1] [,2]
[1,]    4    9
[2,]    5   10

The lapply of matrix is necessary because split drops the attributes that make a vector a matrix, so you need to add them back in.

answered Oct 25 '12 at 00:45

Ari B. Friedman

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+1 I never use split, but everytime I see it I think I ought to start. – Brandon Bertelsen Oct 25 '12 at 01:37
@BrandonBertelsen Thanks. I've started using `split` and `stack` pretty regularly--they complement each other nicely. – Ari B. Friedman Oct 25 '12 at 01:39
@AriB.Friedman - see my answer above using `by` which I think will retain names and dimensions. – thelatemail Oct 25 '12 at 02:17
1

Coming back to this many years later, I realise that `split.data.frame(mat, mat[,3])` will work just fine. – thelatemail Aug 13 '17 at 23:30

score 6 · Answer 2 · answered Oct 25 '12 at 02:14

6

Yet another example:

#test data
mat <- matrix(1:15,ncol=3)
mat[,3] <- c(1,1,1,2,2)

#make a list storing a matrix for each id as components
result <- lapply(by(mat,mat[,3],identity),as.matrix)

Final product:

answered Oct 25 '12 at 02:14

thelatemail

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This is a great answer, as it doesn't drop the dimensions, unlike `split`. – Ari B. Friedman Oct 28 '12 at 23:08

pedrosorio · Accepted Answer · 2012-10-25T01:01:04.617

If you have a matrix A, this will get the first two columns when the third column is 1:

A[A[,3] == 1,c(1,2)]

You can use this to obtain matrices for any value in the third column.

Explanation: A[,3] == 1 returns a vector of booleans, where the i-th position is TRUE if A[i,3] is 1. This vector of booleans can be used to index into a matrix to extract the rows we want.

Disclaimer: I have very little experience with R, this is the MATLAB-ish way to do it.

score 1 · Answer 4 · answered Oct 25 '12 at 01:03

This is a functional version of pedrosorio's idea:

 getthird <- function(mat, idx) mat[mat[,3]==idx, 1:2]
 sapply(unique(mat[,3]), getthird, mat=mat)  #idx gets sent the unique values
#-----------
[[1]]
     [,1] [,2]
[1,]    1    6
[2,]    2    7
[3,]    3    8

[[2]]
     [,1] [,2]
[1,]    4    9
[2,]    5   10

How can I separate a matrix into smaller ones in R?

4 Answers4

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