How to remove a diagonal square element from a matrix?

Question

We can use diag () to remove the diagonal elements of the matrix, but what if we want to remove the diagonal square of the elements? As in the 6x6 matrix, I want to remove 2x2 squares diagonally. It looks very simple, but how to do it in r?

     [,1] [,2] [,3] [,4] [,5] [,6]
[1,]   23   98   12   98   32   99
[2,]   54   11   13   02   31   78
[3,]   25   85   15   09   46   87
[4,]   98   98   16   17   45   48
[5,]   88   00   68   99   89   89
[6,]   05   33   66   12   14   78

and I want to set the diagonal square to NA

     [,1] [,2] [,3] [,4] [,5] [,6]
[1,]   NA   NA   12   98   32   99
[2,]   NA   NA   13   02   31   78
[3,]   25   85   NA   NA   46   87
[4,]   98   98   NA   NA   45   48
[5,]   88   00   68   99   NA   NA
[6,]   05   33   66   12   NA   NA

delete all NA and then combine the columns

     [,1] [,2] [,3] [,4]
 [1,]  12   98   32   99
 [2,]  13   02   31   78
 [3,]  25   85   46   87
 [4,]  98   98   45   48
 [5,]  88   00   68   99
 [6,]  05   33   66   12

Answer 1

I expect there is a more elegant way to do this, but here is one way:

# Create a matrix
mat = matrix(1:36, nrow=6)

# Set block diagonal elements to NA
for (i in seq(1, nrow(mat), 2)) {
  mat[i:(i+1),i:(i+1)] = NA
}

# Reform the matrix with the NA values excluded
matrix(mat[!is.na(mat)], nrow=nrow(mat)-2)

Answer 2

You can use the Kronecker product to quickly build a block diagonal matrix indicating which items should be saved and which should be deleted.

## An example matrix
set.seed(1)
m <- matrix(sample(1:100, 36), ncol=6)

## Construct a logical block diagonal matrix, then use it to remove blocks along diagonal
ii <- !kronecker(diag(1, nrow(m)/2), matrix(1, ncol=2, nrow=2))
matrix(m[ii], ncol = ncol(m)-2)
##      [,1] [,2] [,3] [,4]
## [1,]   57   19   32    1
## [2,]   89   16   63   28
## [3,]   20   61   51   87
## [4,]   86   34   10   42
## [5,]   58   88   21   70
## [6,]    6   83   29   13

Answer 3

We create 3 matrixof 1 in list, construct a block diagonal matrix with bdiagdouble negation to force binary to logical, replace TRUE elements of NA, multiply by 'm1' to get 'm2'.

library(Matrix)
m2 <- as.matrix((NA^!!bdiag(lapply(1:3, matrix, data=1,ncol=2, nrow=2)))*m1)
m2
#      [,1] [,2] [,3] [,4] [,5] [,6]
#[1,]   NA   NA   12   98   32   99
#[2,]   NA   NA   13    2   31   78
#[3,]   25   85   NA   NA   46   87
#[4,]   98   98   NA   NA   45   48
#[5,]   88    0   68   99   NA   NA
#[6,]    5   33   66   12   NA   NA

We remove the elements NAin each row using applyc MARGIN=1to get the expected result.

t(apply(m2, 1, function(x) x[!is.na(x)]))
 #      [,1] [,2] [,3] [,4]
#[1,]   12   98   32   99
#[2,]   13    2   31   78
#[3,]   25   85   46   87
#[4,]   98   98   45   48
#[5,]   88    0   68   99
#[6,]    5   33   66   12

data

m1 <- structure(c(23L, 54L, 25L, 98L, 88L, 5L, 98L, 11L, 85L, 98L, 
0L, 33L, 12L, 13L, 15L, 16L, 68L, 66L, 98L, 2L, 9L, 17L, 99L, 
12L, 32L, 31L, 46L, 45L, 89L, 14L, 99L, 78L, 87L, 48L, 89L, 78L
), .Dim = c(6L, 6L))