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

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