#
# Unit 8 - Data frames 
#
# Producing a sample of observations in a data frame 
#

# Loading data 
setwd("c:/R")
employees <- read.csv("employees.csv")
print(employees)


# We have seen this: 
print(employees[c(3, 5, 8),])

# The order does not matter
print(employees[c(8, 3, 5),])

# Also, repetition is permitted. 
print(employees[c(8, 8, 1, 5, 1),])


# What is this, in essence 
print(employees[c(10, 9, 8, 7, 7, 5, 4, 4, 2, 1),])


# How would we produce a sample of observations 
# a given data frame? 
# Phase 1 - Producing a sample of indices 
#           of size equal to number of rows 
# Phase II - use [] with the sample of indices

# Phase I 
# Producing a sample of indices 
# The long way
print(employees)
result <- c() 
for (iteration in 1:nrow(employees)) {
  ind <- sample(1:nrow(employees), size = 1)
  result <- c(result, ind)
}
print(result)

# Thw short way 
result <- sample(1:nrow(employees), 
                 size = nrow(employees), 
                 replace = TRUE)
print(result)

mySample <- sample(5:10, size = 3, replace = TRUE)
print(mySample)

# Phase II
print(employees[result,])

print(employees[result,c("SALARY")])