r - Create a binary adjacency matrix from a vector of indices -

suppose have vector looks this:

x <- sample(5, 500, replace = true) 

so each element corresponds index 1 through 5.

what's efficient way create binary adjacency matrix vector? elaborate, matrix a should such a[i,j] = 1 if x[i] = x[j] , 0 otherwise.

in 1 line, do

outer(x, x, function(x, y) as.integer(x==y)) 

which returns

     [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]  [1,]    1    0    0    0    0    0    1    0    0     0  [2,]    0    1    1    1    0    1    0    0    1     0  [3,]    0    1    1    1    0    1    0    0    1     0  [4,]    0    1    1    1    0    1    0    0    1     0  [5,]    0    0    0    0    1    0    0    0    0     0  [6,]    0    1    1    1    0    1    0    0    1     0  [7,]    1    0    0    0    0    0    1    0    0     0  [8,]    0    0    0    0    0    0    0    1    0     0  [9,]    0    1    1    1    0    1    0    0    1     0 [10,]    0    0    0    0    0    0    0    0    0     1 

or, in 2 lines

mymat <- outer(x, x, "==") mymat[] <- as.integer(mymat) 

check they're same.

identical(mymat, outer(x, x, function(x, y) as.integer(x==y))) [1] true 

data

set.seed(1234) x <- sample(5, 10, replace = true)