row {base}  R Documentation 
Returns a matrix of integers indicating their row number in a matrixlike object, or a factor indicating the row labels.
row(x, as.factor = FALSE)
x 
a matrixlike object, that is one with a twodimensional

as.factor 
a logical value indicating whether the value should be returned as a factor of row labels (created if necessary) rather than as numbers. 
An integer (or factor) matrix with the same dimensions as x
and whose
ij
th element is equal to i
(or the i
th row label).
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.
col
to get columns.
x < matrix(1:12, 3, 4) # extract the diagonal of a matrix dx < x[row(x) == col(x)] dx # create an identity 5by5 matrix x < matrix(0, nrow = 5, ncol = 5) x[row(x) == col(x)] < 1 x