library(boot)

y <- c(25,30,18,28,23,30,26,35,34,33,38,27,30,44,33,36,41,53,47,46)
stratum <- c(rep(1,5), rep(2,5), rep(3,5), rep(4,5))

Brep = 20000
N=400

# Bootstrap the sample mean from a stratified SRS
sampmean <- function(y,i) mean(y[i])

bootmean <- boot(data=y,statistic=sampmean,strata=stratum,R=Brep)
bootmean
boot.ci(bootmean,conf=.95)
par(mfrow=c(2,1))
hist(bootmean$t,main="Bootstrap Sample Means")
plot(ecdf(bootmean$t),main="Empirical CDF of Bootstrap Means")

# Bootstrap the estimates of the population total
samptotal <- function(y,i) N*mean(y[i])

boottotal <- boot(data=y,statistic=samptotal,strata=stratum,R=Brep)
boottotal
boot.ci(boottotal,conf=.95)
par(mfrow=c(2,1))
hist(boottotal$t,main="Bootstrap Population Total Estimates")
plot(ecdf(boottotal$t),main="Empirical CDF of Bootstrap Total Estimates")