f <- function(toss=1){
  x <- sample(1:2, size=toss, replace=TRUE)
  y <- sample(1:2, size=toss, replace=TRUE)
  return(cbind(x,y))
}
set.seed(2500)
toss_times <- as.data.frame(f(2000))

library(plyr)

freq <- ddply(toss_times, ~x, summarize,
                    y1=sum(y==1), y2=sum(y==2))
row.names(freq) <- paste0('x',1:2)
prob_table1 <- freq_table[,-1]/2000
prob_table1


prob_x <- table(toss_times$x)/2000
prob_y <- table(toss_times$y)/2000
prob_table2 <- outer(prob_x,prob_y,'*')
row.names(prob_table2) <- paste0('x',1:2)
colnames(prob_table2) <- paste0('y',1:2)
prob_table2