Building a contingency table - r

Question

I have a data like this:
A B
1 10
1 20
1 30
2 10
2 30
2 40
3 20
3 10
3 30
4 20
4 10
5 10
5 10
and I want to build a contingency table like this:
10 20 30 40
10 1 3 2 0
20 3 0 2 0
30 2 2 0 0
40 0 0 0 0
Meaning: According to column A, for each two values of column B mark + 1 in the specific Contingency table.
Can you help me do this?

Answer 1

Here is a very ugly answer, using the data from the image, because I already spent too much time on your problem. In general, it's not practical to have your result depend on the order of variables.
A <- rep(c(1:4),c(3,2,3,3))
B <- c(10,10,30,10,20,30,20,10,10,20,30)
data <- data.frame(cbind(A,B))
#split by A
library(plyr)
data2 <- ddply(data,.(A),function(x){
combined_pairs <- cbind(x$B[-nrow(x)],
x$B[-1])
#return data where first is always lowest
smallest <- apply(combined_pairs,MARGIN=1,
FUN=min)
largest <- apply(combined_pairs,MARGIN=1,
FUN=max)
return(data.frame(small=smallest,large=largest))
})
library(reshape2)
result <- dcast(small~large,data=data2,
fun.aggregate=length)
> result
small 10 20 30
1 10 1 3 1
2 20 0 0 2
I think you can add the empty rows yourself if you still need them.