Karel

• Introduce Karel's world as on Page 2 of the document.
• Discuss the example on page 8 of the document.
• Discuss how the code can become large and hard-to-read. How abstractions through function definitions can help --- turnRight and turnAround. Why these abstractions? Why not moveAndTurnLeft? Intuitiveness, generality.

def turnRight():
  turnLeft()
  turnLeft()
  turnLeft()

def turnAround():
  turnLeft()
  turnLeft()

• Notice the indentation: indicates that we are still within the body of the procedure. If we indent unnecessarily, that is an error.
• Discuss pothole example on page 14 where a beeper needs to be placed to fill the pothole.
• Discuss the case where we do not know if a beeper is already present or not. Introduce in-built conditions like beeperPresent(), frontisClear().
• Discuss how other conditions can be implemented using these conditions. e.g., leftIsClear() involves

  def leftIsClear():
    turnLeft()
    ret = frontIsClear()
    turnRight()
    return ret

• We need to remember the result of frontIsClear(). Introduce the notion of a variable. ret is a variable in the example above.
  • Code Variables: A "variable" is associated with a value

    x = 7
    

    This associates the object 7 into the variable x, i.e., x -> 7.
  • Later x in the code will evaluate to the associated object
  • x becomes a shorthand for 7
  • Using = in this way is called "variable assignment"
  • Example usage
    • Assign an object into a variable once
    • Then use that variable many times below
    • Can potentially delete a variable using del x. Deleting does not necessarily remove the object.
    • A special object called None represents a null value. None is not the same as 0, False, or an empty string. None is a data type of its own (NoneType) and only None can be None.
    • A convenient shorthand in the code
  • Programming with variables:

    >>> name = "ABCDEFGH"
    >>> print("happy birthday to " + name)
    happy birthday to ABCDEFGH
    >>> print("happy birthday to " + name)
    happy birthday to ABCDEFGH
    >>> print("happy birthday to " + x)
    NameError: name 'x' is not defined
    >>> print("happy birthday to " + x)
    NameError: name 'x' is not defined
    >>> y = None
    >>> print("happy birthday to " + y)
    TypeError: can only concatenate str (not "NoneType") to str
    

• Similarly introduce constants True and False.
• Introduce if-then-else. Implement noBeeperPresent() using beeperPresent(). Construct and show its example usage to implement the case where you fill the pothole only if a beeper is not already present.
• Formalize the meaning of if-then-else a bit more using arbitrary statements as follows: S1 S2 else: S3 S4 S5 S6 If cond is True, then S1;S2;S5;S6; get executed.

  If cond is False, then S3;S4;S5;S6; get executed.

• if can appear without else:

  if (cond):
    S1
    S2
  S5
  S6
  

  What happens if cond is True? What happens if cond is False?
• Now consider the case where we need to repeatedly fill potholes in the road; All potholes are equidistant at distance 1 from each other.
  • One option is to write the code multiple times. e.g., if there are 5 potholes, write the code 5 times. What if there are one million potholes?
  • Motivate the need for a construct that allows repitition of a given program/instructions: for loop.
• For loop: allows repeating up to "count" number of times, i.e., count iterations:

  S0
  count = 1000
  for i in range(count):   #0..999
    S1
    S2
  S3
  

  The range function returns a sequence of numbers.
• Can write the code as:

  numPotholes = 5
  for i in range(numPotholes):
    fillNextPothole()
  

• Graphically show the state after every loop iteration for this example.

S0
count = 1000
for i in range(count):   #0..999
  S1
  S2
S3

• Show the semantics of this loop for general instructions S1 and S2 in the body.
• What if S1 is count = 0? Answer: The loop still executes 1000 times; the range construct has already constructed a range.
• What if S1 is i = i + 3? Answer: The loop still executes 1000 times; i gets re-assigned each time control transfers to the for loop. At the end of the program (after S3), the value of i will be the 999+3=1002.

• Can generalize the range construct to start from a particular index (instead of the default index of 0) using range(begin, end). This leads to begin, begin+1, ..., end-1.
• Can further generalize the range construct to skip by a particular step (instead of the default step of 1) using range(begin, end, step). This leads to begin, begin+step, ..., endi (where endi is the largest number of the form begin+step*n that is smaller than end.

• Discuss another program where Karel wants to keep walking till it hits the wall. Can this be done with the for loop? One option is: if we know the maximum length of a street or an avenue, we can use that count, and use an if condition in the body. But this is unsatisfactory because how will we know the maximum length? Overestimation = bad because inefficient. Underestimation = very bad because incorrect.
• Discuss while loop:

  def moveTillClear():
    while (frontIsClear()):
      move()