check whether matrix rows equal a vector in R , vectorized

Question

I\'m very surprised this question has not been asked, maybe the answer will clear up why. I want to compare rows of a matrix to a vector and return whether the row == the v

Answer 1

One possibility is

rowSums(M == v[col(M)]) == ncol(M)
## [1] FALSE FALSE FALSE FALSE  TRUE

Or simlarly

rowSums(M == rep(v, each = nrow(M))) == ncol(M)
## [1] FALSE FALSE FALSE FALSE  TRUE

Or

colSums(t(M) == v) == ncol(M)
## [1] FALSE FALSE FALSE FALSE  TRUE

v[col(M)] is just a shorter version of rep(v, each = nrow(M)) which creates a vector the same size as M (matrix is just a vector, try c(M)) and then compares each element against its corresponding one using ==. Fortunately == is a generic function which has an array method (see methods("Ops") and is.array(M)) which allows us to run rowSums (or colSums) on it in order to makes sure we have the amount of matches as ncol(M)