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.
• 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 conditions like beeperPresent(), frontisClear().
• Discuss how other conditions can be implemented using these conditions. e.g., leftIsClear() involves turnLeft(); ret = frontIsClear(); turnRight(); return ret;.
• Introduce the notion of a variable. ret is a variable in the example above.
  • Code Variables: A "variable" is like a box that holds a value

    x = 7;
    

    This stores the value 7 into the variable (i.e. box) x.
  • Later x in the code retrieve the value from the box
  • x becomes a shorthand for 7 (whatever is in the box)
  • Using = in this way is called "variable assignment"
  • Example usage
    • Assign a value into a variable once
    • Then use that variable many times below
    • A convenient shorthand in the code
  • Programming with variables:

    name = "ABCDEFGH"
    print "happy birthday to " + name
    print "happy birthday to " + name
    

• Similarly introduce constants TRUE and FALSE. Implement noBeeperPresent() using beeperPresent().
• Introduce if-then-else 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:

  if (cond) {
    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:

  for (i = 0; i < _count_; i++)
  {
    S1;
    S2; 
  }
  

• Only the count can be changed by the user in this loop. This loop says that make i count from 0 to count-1.
• Can write the code as:

  numPotholes = 5
  for (i = 0; i < numPotholes; i++) {
    fillNextPothole();
  }
  

• Graphically show the state after every loop iteration for this example.
• Show the semantics of this loop for general instructions S1 and S2 in the body.
• 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:

  define moveTillClear()
  {
    while (frontIsClear())
    {
      move()
    }
  }