CODEWITHSUNDEEP
rmatrix
user3816346
user3816346

Reputation:

Selecting rows conditional on column value from multiple matrices

I have

mat1 = matrix(c(2, 4, 3, 6, 7, 8), nrow=2, ncol=3) 
mat2 = matrix(c(5, 6, 7, 1, 2, 3), nrow=2, ncol=3) 
mat3 = matrix(c(8, 5, 8, 6, 7, 9), nrow=2, ncol=3)

which gives me 3 matrices:

      [,1] [,2] [,3]
[1,]    2    3    7
[2,]    4    6    8

      [,1] [,2] [,3]
[1,]    5    7    2
[2,]    6    1    3

     [,1] [,2] [,3]
[1,]    8    8    7
[2,]    5    6    9

What I would like to do is compare the three matrices per row per first column, and select the row of the matrix that has the highest value on the first column.

For example: in row 1 column 1, matrix3 has the highest value (8) compared to matrix1 (2) and matrix2 (5). In row 2 column 1, matrix2 has the highest value (6). I would like to create a new matrix that copies the row of the matrix that has that highest value, resulting in:

     [,1] [,2] [,3]
[1,]    8    8    7      <- From mat3
[2,]    6    1    3      <- From mat2

I know how to get a vector with the highest values from column 1, but I cannot get the whole row of the matrix copied into a new matrix. I have:

mat <- (mat1[1,])

which just copies the first row of the first matrix

[1] 2 3 7

I can select which number is the maximum number:

max(mat1[,1],mat2[,1],mat3[,1])

[1] 8

But I cannot seem to combine the two to return a matrix with the whole row. Getting the code to loop for each row will be no problem, but I cannot seem to get it to work for the first row and as such, I am missing the essential code. Any help would be greatly appreciated. Thank you.

Upvotes: 1

Views: 537

Answers (3)

akrun
akrun

Reputation: 887058

You may also try:

 a2 <- aperm(simplify2array( mget(ls(pattern="mat"))),c(3,2,1)) #gets all matrices with name `mat`
 t(sapply(1:(dim(a2)[3]), function(i) {x1 <- a2[,,i]; x1[which.max(x1[,1]),]}))
 #    [,1] [,2] [,3]
 #[1,]    8    8    7
 #[2,]    6    1    3

Upvotes: 1

alexis_laz
alexis_laz

Reputation: 13122

Are you working interactively? Do you manipulate multiple matrices spread in your workspace? A straightforward answer to your problem could be:

#which matrices have the largest element of column 1 in each row?
max.col(cbind(mat1[, 1], mat2[, 1], mat3[, 1])) 
#[1] 3 2

rbind(mat3[1, ], mat2[2, ])  #use the above information to get your matrix
#     [,1] [,2] [,3]
#[1,]    8    8    7
#[2,]    6    1    3

On a more ganeral use-case, a way could be:

mat_ls = list(mat1, mat2, mat3) #put your matrices in a "list"
which_col = 1  #compare column 1
which_mats = max.col(do.call(cbind, lapply(mat_ls, function(x) x[, which_col])))
do.call(rbind, lapply(seq_along(which_mats), 
                      function(i) mat_ls[[which_mats[i]]][i, ]))
#     [,1] [,2] [,3]
#[1,]    8    8    7
#[2,]    6    1    3

Upvotes: 1

David Arenburg
David Arenburg

Reputation: 92282

Probably not the prettiest solution

temp <- rbind(mat1, mat2, mat3)
rbind(temp[c(T,F),][which.max(temp[c(T,F),][, 1]),],
temp[c(F,T),][which.max(temp[c(F,T),][, 1]),])

##      [,1] [,2] [,3]
## [1,]    8    8    7
## [2,]    6    1    3

Upvotes: 1

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