[R] A contingency table of counts by case

[Serguei Kaniovski]

Here is an example of the data.frame that I have,

df<-data.frame("case"=rep(1:5,each=9),"id"=rep(1:9,times=5),"x"=round(runif(length(rep(1:5,each=9)))))

"case" represents the cases,
"id" the persons, and
"x" is the binary state.

I would like to know in how many cases any two persons

a. both have "1",
b. the first has "0" - the second has "1",
c. the first has "0" - the second has "0",
d. both have "0".

There will be choose(9,2) sums, denoted s_ij for 1<=i<j<=9,
where i and j are running indices for an "id"-pair.

Please help, this is way beyond my knowledge of R!

Thank you,
Serguei Kaniovski
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