Convert a dataframe to a vector (by rows)

You can try as.vector(t(test)). Please note that, if you want to do it by columns you should use unlist(test).

You can try this to get your combination:

as.numeric(rbind(test$x, test$y))

which will return:

26, 34, 21, 29, 20, 28

c(df$x, df$y)
# returns: 26 21 20 34 29 28

if the particular order is important then:

M = as.matrix(df)
c(m[1,], c[2,], c[3,])
# returns 26 34 21 29 20 28 

Or more generally:

m = as.matrix(df)
q = c()
for (i in seq(1:nrow(m))){
  q = c(q, m[i,])
}

# returns 26 34 21 29 20 28