#
# Unit 6 - User-functions
#
# Additions: default arguments, lazy evaluation, and more 
# 

# Default arguments 
# If we do not pass arguments to 
# function's parameters, it may 
# use predefined default arguments. 

# Must we pass arguments to all parameters? 
print(rep(x = 3, times = 1))
print(rep(x = 3))

?rep
?sample

# User-functions 
foo <- function(x = 5, y = 2) {
  return (x + y)
}

print(foo(1, 1))
print(foo(1))
print(foo())


# Not all parameters must have default arguments
foo <- function(z, y = 2) {
  return (z + y)
}

print(foo(1, 1))
print(foo(1))
print(foo())
print(foo(y = 1))


foo <- function(a = 1, b) {
  return (a + b)
}

print(foo(2, 1))
print(foo(2))
print(foo())
print(foo(b = 2))


# Lazy evaluation

# Let us take another look at this example 
foo <- function(c = 1, d) {
  print(c)
  return (c + d)
}

print(foo(1))


# Now look at the output of executing this: 
foo <- function(x = 1, y) {
  print(x)
  return (x + y)
}

print(foo(1))


# Two "See also's" 

# (I) Take another look at the c function. 
# The number of arguments passed to it 
# may vary. 
# This is done using a special operator, 
# The ... operator. 
c(3, 5, 4, 2, 8)
c(3, 2, 8)
c(3, 5, 4, 2, 8, 9)
?c

# (1I) Functions are objects and as arguments 
goo <- function(x = 5, y = 2) {
  return (x + y)
}

print(class(goo))


goo <- function(x) {
  use x as a function 
}

goo(unique)

tapply