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How to set all rows of a list of matrices to zero using if condition statement in R

July 20, 2022

Suppose I have a matrix, mat. Suppose further that the sum of one row of this matrix is equal to zero. Then, I need to set all the coming rows (the rows after the zero row) to zero. For example,

     mat <- c(1,2,0,0,0,
         3,4,0,2,1,
         0,0,0,1,0,
         1,2,0,0,0,
         0,1,0,1,0)
mat <- matrix(mat,5,5)
mat


      [,1] [,2] [,3] [,4] [,5]
[1,]    1    3    0    1    0
[2,]    2    4    0    2    1
[3,]    0    0    0    0    0
[4,]    0    2    1    0    1
[5,]    0    1    0    0    0

All the entries of row 3 are zero. Hence, I want rows 4, and 5 to become zeros as well. I have a list of matrices and would like to apply the same to all the matrices using the lapply function. For simplicity, I make a list of 3 matrices similar to the mat.

mat <- c(1,2,0,0,0,
         3,3,0,2,1,
         0,0,0,4,0,
         1,3,0,0,0,
         0,1,0,1,0)
mat <- matrix(mat,5,5)
mat1 <- c(1,2,0,0,0,
         3,4,0,2,1,
         0,0,0,1,0,
         1,2,0,0,0,
         0,1,0,1,0)
mat1 <- matrix(mat1,5,5)
mat2 <- c(1,2,0,0,0,
         3,4,0,2,1,
         0,0,0,2,0,
         1,2,0,0,0,
         0,2,0,3,0)
mat2 <- matrix(mat2,5,5)
Mat <- list(mat1, mat2, mat3)

>Solution :

You did not actually post mat3 in your data so I just used mat3 <- matrix(1, 5, 5), i.e. a 5×5 matrix of ones. This was to ensure it could handle cases where there is no row where all values are zero.

This will return a list of matrices where all rows are zero after the first row of zeroes:

lapply(Mat, \(mat) {
    first_zero_row  <- which(rowSums(mat)==0)[1]
    if(!is.na(first_zero_row)) {
        mat[first_zero_row:nrow(mat),]  <- 0    
    }
    mat
})

Output:

[[1]]
     [,1] [,2] [,3] [,4] [,5]
[1,]    1    3    0    1    0
[2,]    2    4    0    2    1
[3,]    0    0    0    0    0
[4,]    0    0    0    0    0
[5,]    0    0    0    0    0

[[2]]
     [,1] [,2] [,3] [,4] [,5]
[1,]    1    3    0    1    0
[2,]    2    4    0    2    2
[3,]    0    0    0    0    0
[4,]    0    0    0    0    0
[5,]    0    0    0    0    0

[[3]]
     [,1] [,2] [,3] [,4] [,5]
[1,]    1    1    1    1    1
[2,]    1    1    1    1    1
[3,]    1    1    1    1    1
[4,]    1    1    1    1    1
[5,]    1    1    1    1    1