#
# Unit 8 - Data frames 
#
# Reading a data frame from a .csv file, 
# and the $ operator 
#

# Our goal: to read data from a file
#           into the present session. 

# Specifically: a .csv file 
# More specifically: employees. csv 


# Note: the extension ".csv" is necessary. 
employees <- read.csv("employees.csv")

# Setting the working directory 
# Rules: 
# - Name: only English letters and digits 
# - No spaces 
# - No Hebrew 
# - Not on the cloud
# - Not in Documents 
# - Not in Desktop
setwd("c:/R")
print(getwd())

# Possible errors 
setwd("c:/AR")

setwd("c:\AR")

# Now it should work 
# Inspect the Environment. 
employees <- read.csv("employees.csv")
print(employees)
View(employees)


# Now that we have the data frame 
# in hand, we may access each of its 
# columns using the $ operator 
gender <- employees$GENDER
print(gender)
print(class(gender))

print(employees$GENDER)
print(class(employees$GENDER))


# Note, if you do not store 
# a column/variable in a specific 
# variable, you must indicate 
# the data frame to which the column
# belongs.
print(employees$SALARY)
print(employees$GENDER[employees$GENDER == 1])


# Also, the indicated column 
# of course must exist in the data frame...
print(employees$SALARY)
print(employees$SALARY1)
print(employees$salary)

print(employees$SALARY)

# With the a column in hand
# as a vector, we may carry out 
# operations in the vector level. 
print(length(employees$GENDER[employees$GENDER == 1]))