Is there an R function for finding the rows that contains a specific element in a matrix?

x[-which(x == 2 | x == 7, arr.ind = TRUE)[,1],] 

is the simplest, most efficient way I can think of.

the single '|' checks if each element is 2 or 7 (which '||' won't do). arr.ind gives each position as a pair of coordinates, rather than the default single number. [,1] selects each row which has a 2 or 7.

Hope that helps :)

As @Dirk said, which is the right function, here is my answer:

index <- apply(x, 1, function(a) 2 %in% a || 7 %in% a)
> index
[1] FALSE  TRUE  TRUE FALSE
x[index, ]

x[-which(...), ] is not the right approach... Why? See what happens in case which finds no match:

x <- matrix(8, nrow = 4, ncol = 2)
x[-which(x == 2 | x == 7, arr.ind = TRUE)[,1],] 
#      [,1] [,2]

(it returns nothing, while it should return the whole x.)

Instead, indexing with logicals is the safer approach: you can negate the vector of matches without risking the odd behavior shown above. Here is an example:

x[!(rowSums(x == 2 | x == 7) > 0), , drop = FALSE]

which can also be written in the shorter form:

x[!rowSums(x == 2 | x == 7), , drop = FALSE]