install.packages("rbenchmark")
library (MASS)
library(tidyverse)
library (rbenchmark)


#An example of tapply and summarize & group 
tapply (Cars93$Price, Cars93$Manufacturer , mean)
summarize(group_by(Cars93,Manufacturer),mean=mean(Price))



#Examples of using snow library
install.packages("snow")
library(snow)

#First example
z = list(1,1,1,1,1,1,1)
system.time(sapply (z, Sys.sleep))


cl <-makeCluster (7)
system.time(parSapply (cl, z, Sys.sleep))
stopCluster (cl)


#Second example
z = rexp(6000000, 6)
z = list(z, z, z, z, z, z, z, z)
cl <-makeCluster (8)
benchmark(parSapply (cl, z, function(val){2^val + 2^val + 2^val}), sapply (z, function(val){2^val + 2^val + 2^val}),replications = 5)
stopCluster (cl)



#Third example
z = rexp(1000000, 8)
k =rexp(6000000, 8)
z = list(z, z, z, k, z, z, z, z)

cl <-makeCluster (8)
benchmark(clusterApplyLB(cl,c(z,z,z,z), function(val){(2^val + 2^val + 2^val)}),lapply(c(z,z,z,z), function(val){2^val + 2^val + 2^val}),
          parLapply (cl, c(z,z,z,z), function(val){(2^val + 2^val + 2^val)}),replications = 5)
stopCluster (cl)



#do.call command
A <- c(2,2,2,2)
f=c("mean","sum")                                
do.call(f[1], list(A)) 
do.call(f[2], list(A))