The Birthday Problem

birthday.r

#The birthday Problem.

#Simple computation:
N <- 1:50
pshare <- 1- exp(-(N*(N-1)/2)/365)
cbind(N, pshare)

#just want one value:
bday <- function(X){
N <- X
pshare <- 1- exp(-(N*(N-1)/2)/365)
print(pshare)
      }
      
      bday(44)


#A nice plot of the results...
      library(ggplot2)
      
qplot(N, pshare, geom="point", colour=pshare, xlab="\n Number of People",
 ylab="Probability that two people share a birthday \n", main="The Birthday Problem \n") + geom_hline(y=.5, lty=2)   +
geom_vline(x=23, lty=2) + scale_colour_gradient(limits=c(0, 1), low="lightblue", high="red") +
scale_x_continuous(limits=c(0, 50),breaks=c(0, 10, 20, 23, 30, 40, 50)
) + theme_bw()   + opts(legend.position="none")