[R] Compare rows of two matrices

[TEMPL Matthias]

Hello,

#I have two matrices, eg.:

y <-  matrix( c(20,  NA,  NA,  45,  50,  19,  32, 101,  10,  22,  NA,  NA,  80,  49,  61, 190), ncol=4 )
x <-  matrix( c(20,  NA,  NA,  NA,  50,  19,  32, 101,  10,  22,  NA,  NA,  80,  49,  61, 190), ncol=4 )

#Whereas x contains all NA´s from y plus some additional NA´s.
#I want to find the index of these additional NA´s. I think, there must be a very easy way to do this.

#Here are the indices of NA´s in x and y:
l1 <- which(is.na(x), arr.ind=TRUE)
l2 <- which(is.na(y), arr.ind=TRUE)

#> l1
#     [,1] [,2]
#[1,]    2    1
#[2,]    3    1
#[3,]    4    1
#[4,]    3    3
#[5,]    4    3

#> l2
#     row col
#[1,]   2   1
#[2,]   3   1
#[3,]   3   3
#[4,]   4   3

#Now I want to find a matrix, which includes the values of l1, without the rows of l2, 
#which has equal entities (the index of the additional NA´S).
#In this example the result should be row 3 of l1 with the values 4 and 1.
#The following code works, but I think there must be a much more elegant way to do this.

l3 <- l1
l3 <- cbind( l1, rep(0, nrow(l1)) )
num <- 1
   
for( i in 1:nrow(l1) ){
  for( j in 1:nrow(l2) ){
    if( l1[i,1] == l2[j,1] & l1[i,2] == l2[j,2]){
      l3[i,3] <- 1
    }
  }
}

l4 <- l3[l3[,3]==0, c(1,2)]

#> l4
#row col 
#  4   1  

I have often such problems like this and I assume, that other people have similar tasks.
My question is: Does anybody know a function in one package, which compares rows of two matrices like this or have anybody an idea to do this in a much more elegant way"? 

Thank you very much,
Matthias