# This program bootstraps the ratio of means for the 
# city data set in the `boot' library.

library(boot)
set.seed(12345)
# Now we define the function which calculates the statistic
# ie the ratio of means. The function has two inputs - the dataframe
# and a vector of indices defining the bootstrap sample (generated automatically by 
# the `boot' command). The first command sets up the bootstrapped dataframe.  The
# second calculates and returns the ratio.

city.fun<- function(data,i){
	bsample<-data[i,]
	mean(bsample$x)/mean(bsample$u)
	}
	
# Now we create bootstrap object using predefined boot command.

city.boot<-boot(data=city, statistic=city.fun, R=250)

# We can obtain information on the bootstrap object city.boot by typing:
#
# 1. `city.boot' which prints the original statistic, its estimated se and bias
# 2. `plot(city.boot)' which gives summary plots
# 3. `names(city.boot)' which lists components of boot.

city.boot
plot(city.boot)
names(city.boot)