#
# Unit 9 - Data frames 
#
# Using data frames argumens in user-functions 
#

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


# Data frames may be passed to a user-function
foo <- function(df) {
  print(df)
}

foo(employees)

goo <- function(df) {
  return (dim(df))
}

print(goo(employees))

# Seems pretty obvious. 
# Yet there is one aspect 
# of this issue which is 
# not at all obvious. 
# Inspect the following: 
foo <- function(df, colName) {
  print(df$colName)
}

foo(employees, "GENDER")

# That is strange, because 
# given this specific call to foo, 
# the above function is 
# allegedly equivalent to the following, 
# which works: 
foo <- function(df) {
  print(df$"GENDER")
}

foo(employees)

# However, the codes in fact 
# are not equivalent. An equivalent 
# coude would be the follwing, 
# which does not work. 
foo <- function(df) {
  colName <- "Gender"
  print(df$colName)
}

foo(employees)

# To understand the problem we should 
# delve into the work of the dollar operator. 
# it passes the argument name as a character string,
# therefore employees$colName is interpreted as 
# employees$"colName". 
# Inspect the following code: 
foo <- function(df, colName) {
  print(df$colName)
}

employees$colName <- 5 
foo(employees, "GENDER")


# So how would we access, a certain 
# variable in a data frame argument, using 
# a parameter holding the column's name? 
# The [ ] comes to our rescue 
foo <- function(df, colName) {
  print(df[,colName])
}

foo(employees, "GENDER")