# Simulation of computers failing and heading to the repair shop.


# FUNCTION TO SIMULATE N COMPUTERS.  Each computer might have a memory failure
# or a processor failure.  Failures of memory and processor are independent,
# and occur with probabilities specified by the arguments.  The computer will 
# be in the repair shop if it has either sort of failure.  The result returned
# is a list with elements called mem.fail, proc.fail, and in.shop, which are
# TRUE/FALSE vectors with one element for each computer.  

simulate.computers = function (n, pr.mem.fail, pr.proc.fail)
{
  mem.fail = runif(n) < pr.mem.fail
  proc.fail = runif(n) < pr.proc.fail
  in.shop = mem.fail | proc.fail

  list (mem.fail=mem.fail, proc.fail=proc.fail, in.shop=in.shop)
}


# FUNCTION TO FIND CONDITIONAL PROBABILITIES.  Given information on simulated 
# computers, this function find the conditional probability that a computer has
# a memory failure given that it is in the repair shop, and the conditional 
# probability that it has a memory failure given that it is in the repair shop 
# and has a processor failure.  The argument is the matrix of data on simulated
# computers from sim.computers.  Value returned is a list containing the two 
# conditional probabilities.

find.cond.prob = function (sim)
{
  list ( mem.fail.given.in.shop 
           = mean (sim$mem.fail [sim$in.shop]),
         mem.fail.given.in.ship.and.proc.fail 
           = mean (sim$mem.fail [sim$in.shop & sim$proc.fail]) )
}