I have a dataframe df with two columns x and y. I want to count the number
of times a unique x, y combination occurs.

For example

x<- c(1,2,3,4,5,1,2,3,4)
y<- c(1,2,3,4,5,1,2,4,1)

df<-as.data.frame(cbind(x, y))

#what is the correct way to use ddply for this example?
ddply(df, c('x','y', summarize, ??)

#desired output -- format and order doesn't matter
# (x, y) count
#--------------------
# (1, 1) 2
# (2, 2) 2
# (3, 3) 1
# (4, 4) 1
# (5, 5) 1
# (2, 3) 1
# (3, 4) 1
# (4, 1) 1

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