Matching rows?

At 01:15 PM 03/07/00 +0200, gb wrote:

I have a matrix X with many rows and a vector y, which I want
to match against the rows of the matrix. I especially want
to know if there is a match or not. I can do this with 'all'
and a 'while' construct:

row.match <- function(y, X)
{
 found <- FALSE
 j <- 0
 while ( (!found) && (j < nrow(X)) )
   {
     j <- j + 1
     found <- all(y == X[j, ])
   }
 return ( found )
}

Two alternatives:

any( apply(X, 1, all.equal, y) == "TRUE")

any(apply(apply(X, 1, "==", y), 1, all))

Here is a third alternative.  It sets out to find which rows match the
vector rather than merely if there is a match.  It returns a vector of row
indices of the matching rows, which is empty if there are no matches, of
course:

row.matches <- function(y, X) {

	i <- seq(nrow(X))
	j <- 0
	while(length(i) && (j <- j + 1) < ncol(X)) 
		i <- i[X[i, j] == y[j]]
	i
}

Here is a quick checkout

X <- matrix(sample(0:1, 53000*11, rep=T), 53000, 11)
y <- X[53000, ]
dos.time(row.match(y, X))

[1] 1.48

dos.time(row.matches(y, X))

[1] 0.16003