Learn R – How to Get Random Training and Test Data Set

# Read the data from a file; The command below assumes that the working 
# directory has already been set. One could set working directory using 
# setwd() command.
sample_df <- read.csv("glass.data", header=TRUE, stringsAsFactors=FALSE)

# get a vector comprising of all indices starting from 1 and ending with row number
index <- 1:nrow(sample_df)

# Get random indices of size n from index vector; In command below, the 
# size n is determined using trunc(length(index))/3
randindex <- sample(index, trunc(length(index))/3)

# Get the training set consisting of all the items except one represented using 
# randindex
trainset <- sample_df[-randindex,]
# Get the test set represented using random index
testset <- sample_df[randindex,]