How to concatenate two matrix column by column?

Question

Let's say I have two matrices A and B with the same dimensions like follow:

A = matrix(1:30, nrow=3, ncol=10)
B = matrix(30:1, nrow=3, ncol=10)
#dim(A) = dim(B)
#[1]  3 10
A
     [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
[1,]    1    4    7   10   13   16   19   22   25    28
[2,]    2    5    8   11   14   17   20   23   26    29
[3,]    3    6    9   12   15   18   21   24   27    30
B
     [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
[1,]   30   27   24   21   18   15   12    9    6     3
[2,]   29   26   23   20   17   14   11    8    5     2
[3,]   28   25   22   19   16   13   10    7    4     1

Is there an R function to concatenate these two matrices column by column to get a single matrix as follows?

     [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10] [,11] [,12] [,13] [,14] [,15] [,16] [,17] [,18] [,19] [,20]
[1,]    1   30    4   27    7   24   10   21   13    18    16    15    19    12    22     9    25     6    28     3
[2,]    2   29    5   26    8   23   11   20   14    17    17    14    20    11    23     8    26     5    29     2
[3,]    3   28    6   25    9   22   12   19   15    16    18    13    21    10    24     7    27     4    30     1

here is my code, is there any way to optimize it?

C = matrix(NA, nrow=3, ncol=20)
j=1
for(i in 1:10){
  C[,j] = A[,i]
  C[,j+1] = B[,i]
  j=j+2
}

Thank you

matrix(rbind(A,B), nrow=3)

jogo
– jogo

2020-06-24 13:34:30 +00:00
Commented Jun 24, 2020 at 13:34 — jogo
– jogo, Commented Jun 24, 2020 at 13:34

Stéphane Laurent · Accepted Answer · 2020-06-24 13:37:23Z

2

You can do:

matrix(rbind(A,B), nrow=3, ncol=20)

answered Jun 24, 2020 at 13:37

Stéphane Laurent

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Ronak Shah · Accepted Answer · 2020-06-24 13:36:32Z

1

You can cbind the two matrices and then select alternating columns from first half and second half of combined matrix :

mat <- cbind(A, B)
inds <- (ncol(mat)/2)
mat[, c(rbind(1:inds, (inds + 1):ncol(mat)))]


#     [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10] [,11]
#[1,]    1   30    4   27    7   24   10   21   13    18    16
#[2,]    2   29    5   26    8   23   11   20   14    17    17
#[3,]    3   28    6   25    9   22   12   19   15    16    18

#     [,12] [,13] [,14] [,15] [,16] [,17] [,18] [,19] [,20]
#[1,]    15    19    12    22     9    25     6    28     3
#[2,]    14    20    11    23     8    26     5    29     2
#[3,]    13    21    10    24     7    27     4    30     1

answered Jun 24, 2020 at 13:36

Ronak Shah

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Comments

akrun · Accepted Answer · 2020-06-24 19:36:20Z

1

We can use asplit the split the matrices into a list and then with Map, we can rbind

t(do.call(rbind, Map(rbind, asplit(A, 2), asplit(B, 2))))
#.    [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10] [,11] [,12] [,13] [,14] [,15] [,16] [,17] [,18] [,19] [,20]
#[1,]    1   30    4   27    7   24   10   21   13    18    16    15    19    12    22     9    25     6    28     3
#[2,]    2   29    5   26    8   23   11   20   14    17    17    14    20    11    23     8    26     5    29     2
#[3,]    3   28    6   25    9   22   12   19   15    16    18    13    21    10    24     7    27     4    30     1

answered Jun 24, 2020 at 19:36

akrun

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