I have a R x C matrix filled to the k-th row and empty below this row. What i need to do is to fill the remaining rows. In order to do this, i have a function that takes 2 entire rows as arguments, do some calculations and output 2 fresh rows (these outputs will fill the matrix). I have a list of all 'pairs' of rows to be processed, but my for loop is not helping performance:

# M is the matrix
# nrow(M) and k are even, so nLeft is even

M = matrix(1:48, ncol = 3)
# half to fill
k = nrow(M)/2

# simulate empty rows to be filled
M[-(1:k), ] = 0

 cat('before fill')
 print(M)

# number of empty rows to fill
nLeft  = nrow(M) - k
nextRow = k + 1

# list of rows to process (could be any order of non-empty rows)
idxList = matrix(1:k, ncol = 2)

for ( i in 1 : (nLeft / 2))
{
    row1 = M[idxList[i, 1],]
    row2 = M[idxList[i, 2],]
    # the two columns in 'results' will become 2 rows in M

    # fake result, return 2*row1 and 3*row2
    results = matrix(c(2*row1, 3*row2), ncol = 2)

    # fill the matrix
    M[nextRow, ] = results[, 1]
    nextRow = nextRow + 1
    M[nextRow, ] = results[, 2]
    nextRow = nextRow + 1
}

 cat('after fill')
 print(M)

I tried to vectorize this, but failed... appreciate any help on improving this code, thanks!