Today: insights about loops, if statement, ==, make-a-drawing, left_clear(), puzzles

Reminders

• Please turn off all video-watching, video-ads in class - we're trying to keep the network working inhere
• Putting out HW1a today, all parts due next tue 11:55pm
  LaIr office hours start Sun eve
• Section signup Thu 5pm (link on course page)
• Nick office hours today
  Stand around right after class
  Then walk up to Durand
  Exit this room stage right, go up hill a bit, 3rd floor
• In office hours: lecture examples are great to work on
  Like everyone present can watch, not staring at another student's hw code
• We have lots of lecture examples today
  There is a Show Solution button on each problem!
  So if stuck, you can compare your code to the solution yourself

Lego Bricks!

• We're looking at a series of little code techniques
• Each is like a little Lego brick, maybe not that useful on its own
• Once we have enough brick types..
• We can re-combine them to do anything
• Today we're adding a few more bricks + some coding techniques

Aside: Bit Controls

• Cmd-Return (Mac) or Ctrl-Return with cursor in code - just Run
  Very handy when pounding away on your code
• Diff marks - red marks on world vs. desired output
• Speed
• Auto Play - uncheck, Run button shows bit at start state, can play from there
• End State - check, instead of animating, just show end state

Recall: Go-Green

> go-green

Move-To Convention

• The go-green loop looks like this inside:
  bit.move()
bit.paint('green')
• This works fine, painting all the square except the first square

Other Convention: paint, then move

• Could write the code in the loop like this:
  bit.paint('green')
bit.move()
• This works fine too!
• It paints all the squares except missing the last square
• Demo: try swapping move/paint in go-green, turn off "diff" option

Loop = Always Misses One Square

• We tend to use the move-to convention
• The key thing: the loop always misses one square at the end
• Don't drive yourself crazy trying to use one loop to hit all the squares
• You need to add a little code before or after the loop for the missing square

Aside: Checkmark / Diff Features

• The system knows what the world is supposed to look like when the code works correctly
  If the output is correct at the end of the run, it gets a green checkmark
• "diff" Feature - diagonal red marks on incorrect squares
  The system knows what the world is supposed to look like, marking squares that are the wrong color with a red diagonal

Recall: All Blue Example

> All-Blue

Specification: Bit starts in the upper left square facing the right side of the world. Move bit forward until blocked. Paint every square bit occupies blue, including the first. When finished, turn bit to face downwards. (Demo - this code is complete. Shows the basic while-loop with front_clear() to move until blocked, and also taking an action before and after the loop.)

all-blue() Code

def all_blue(filename):
    bit = Bit(filename)
    bit.paint('blue')  # 1 paint before the loop
    while bit.front_clear():
        bit.move()
        bit.paint('blue')
    bit.right()  # Turn after the loop

1. Run All Blue Normally, test = go

Watch how it paints every square, turns down at the end. When a problem says "until blocked", you typically use front_clear() as the test. This code uses the move-to convention. In the loop, the code moves forward one square, paints it blue.

Notice that the test of the while is essentially the "go" condition - when that test is true, Bit should move forward. Conversely, when the test is False, don't move.

2. Lines Before/After Loop Different

• Lines before and after the loop to handle the first/last squares (see above)
  bit.paint('blue') - before loop
  bit.right() - after loop
• These lines are outside the loop, run once
• Point: lines inside vs. outside the loop is a big difference
• Indentation controls what's in the loop
• Demo: what if want to paint the last square red?
  Change the last line to bit.paint('red')
  Try indenting it wrong

3. One Code - Many Cases - Generality

• "Generality" - real Math Department word here
• There is one copy of your code algorithm
• There are many different cases it should work on
  Infinitely many cases actually
  Think of all the different sizes and shapes of Bit world
  The while construct is very powerful for generality
• General = it should work for all the different input worlds
• Click the menu to select different "cases", run against them
• Then click Run All
• Code needs to work for all of them
• Run All gives the Big Green Checkmark
  Final test for each HW problem
• I like how generality is kind of ethereal, but with 1-code many-cases .. it's very concrete

4. What Comes After Line 6?

• Run Bit on Case-1, stop it a couple squares from the end
• What comes after Line 6?
• Experiment: Use step button, predict what line comes next before clicking
• What comes after line 4? Varies
• Keep stepping to get to the end of the lop

5. Zero Loops is OK - Edge Cases

Look at Case-3 and then Case-4. Turn off the Auto-Play option, so when you click Run, the world stays at the start, waiting for you to click Step. See how the while test behaves on Case-3 and then Case-4. For Case-4, the while-test is False the very first time! The result is, the loop body lines run zero times. This is fine actually. There were zero squares to move to, so looping zero times is perfect. This is an example of an "edge" case, the smallest possible input. It's nice that the while loop handles this edge case without any extra code.

6. Bad Move Bug

Let's do something wrong. Change the while-test to just True which is valid Python, it just doesn't work right for this problem. With this code, bit keeps going forward no matter what. Having a True test like this is a legitimate technique we'll use later, but here it is a bug.

    ...
    while True:   # BUG
        bit.move()
        bit.paint('blue')

The role of the while-test is to stop bit once there is a wall in front (i.e. front is not clear). With this buggy code, the test is never False, so the loop never exits. Bit reaches the right side, but keeps trying to move(). Moving into a wall or black square (i.e. not a clear square) is an error, resulting in an error message like this:

Exception: Bad move, front is not clear, in:all_blue line 5 (Case 1)

If you see an exception like that, your code is doing a move() when the front is not clear You need to look at your code logic. Normally each move() has a front_clear() check before it, ensuring that the world has space for a move.

7. Infinite Loop Bugs

Infinite Loops

• Oddball bug case: infinite loop
• The body lines run forever
• The lines fail to make the while-test False
• May get error message: timed out - possible infinite loop
  The Bit system notices code running for a long time, terminates it
• Click the Bit "stop" button or click on the code to stop the line hilighting

Infinite Loop 1 - Waggle Dance

For a loop to work, each run of the body needs to get closer to being done. With these bugs, bit is not making progress and so it never ends.

Here is a buggy All Blue - instead of move() have bit do right() and the left() - a sort of "waggle dance" like bees do in the hive. Notice that .. bit never moves forward. Bit can do this for eternity, and never leave that first square! Everyone has days like that.

    while bit.front_clear():
        bit.right()    # waggle dance!
        bit.left()
        bit.paint('blue')

Run the code - you will see a message about a timeout, suggesting that this code does not seem to reach an end.

Infinite Loop 2 - move vs. move()

Python syntax requires the parenthesis () to make a function call. Unfortunately, if you omit them, easy enough to do, it does not make a function call. So the code below looks right, but actually it's an infinite loop. Bit never moves. Try it yourself and see, then fix it.

    while bit.front_clear():
        bit.move
        bit.paint('blue')

More Practice

If we have time in class we'll do this one:

> red2

After lecture, or if you get stuck on the homework, try some of the other problems in the "bit loop" section. These work similarly to the All Blue example.

> Bit Loop

If Statement - Logic

While loop is power. If-statement is control, controlling if lines are run or not

If-Statement Demo - Change Blue

We'll run this simple bit of code first, so you can see what it does. Then we'll look at the bits of code that go into it.

Problem statement: Move bit forward until blocked. For each moved-to square, if the square is blue, change it to green. (Demo: this code is complete.).

> Change-Blue Demo

For reference here is the code:

def change_blue(filename):
    bit = Bit(filename)
    while bit.front_clear():
        bit.move()
        if bit.get_color() == 'blue':
            bit.paint('green')

If Statement Syntax

Syntax 4 parts (similar to "while"): if, boolean test-expression, colon, indented body lines

if test-expression:
    body lines

If Statement Operation

• If statement evaluates the test expression, looking for True or False
• If test is True, runs the body top to bottom
• Otherwise skips the body, run continues after the body
• Basically an On/Off switch - runs the body lines or runs nothing
• There are more complex if forms for later, for now just this common/simple form

Look At Test Expression - Take Apart

Here are the key lines of code:

    ....
    if bit.get_color() == 'blue':
        bit.paint('green')
    ....

1. bit.get_color() Expression

• Code that runs and returns a value to use is an "expression"
  e.g. bit.front_clear() expression - returns True or False
  e.g. bit.get_color() expression - today's example
• bit.get_color() expression returns one of these four:
  'red', 'green', 'blue', None
None returned if the square is not painted
• None is the special Python value meaning "nothing"
  It's handy in computer code to a formal "nothing" value for situations like this

Expression Visualization

Suppose bit is on a squared painted 'blue', as shown below. Here is diagram - visualizing that bit.get_color() is called and it's like there's an arrow coming out of it with the returned 'blue' to be used by the calling code.

2. == Compares Two Values - Boolean

• The operator == compares two values
  (two equal signs next to each other)
• Returns True if two values are equal, False otherwise
• Very often used in an if-test or a while-test
• if x == 6:
  if-test is True if the variable x is 6

Look at Change-Blue Code Again

Look at the if-statement.

Q: What is the translation of that code into English?

A: For each moved-to square. If the square is blue, paint it green.

while bit.front_clear():
    bit.move()
    if bit.get_color() == 'blue':
        bit.paint('green')

!= Not Equal Form

• Not-equal comparison written with exclamation mark: !=
• if x != 6:
  above is True if x is not 6
• If you want to express "not-equal" in your code, use !=
• Examples below

More Examples == !=

if bit.get_color() == 'red':
    # run this if color is 'red'

if bit.get_color() == None:
    # run this if square is not painted

if bit.get_color() != 'red':
    # run this if square is anything
    # but 'red'
    # e.g. 'green' or 'blue' or None

The Fix Tree Problem

This is a challenging problem for where we are at this point. Just watch (and you can do it in parallel) as I work through the problem. I want to show the thought process and steps to build and debug some code.

Note: it is very common to move bit with an until-blocked loop. This problem is a little unusual, moving bit until hitting a particular color.

> Fix Tree

Before:

After:

Important Ideas - Thinking Drawing and Coding

We have art for these. No expense has been spared!

1. Don't do it in your head

Don't do it just in your head - too much detail. Need to be able to work gradually and carefully.

2. Make a Drawing / Sketch

Draw a typical "before" state. The drawings do not need to be fancy, but it's easier to work out the details on a sketch than in your head. Key lesson for today.

3. What is a next-step goal - what would that look like?

Look at the before state - what code can advance this?

4. Question: what code?

You have the current state. What code could advance things to the next goal?

Q: Look at those bit positions. When do you want bit to move, and when not to move?

A: Want bit to move when the square is not red. What is the code for that?

Aside: if you were talking to a human, you would just say "make it look like this" and point to the goal. But not with a computer! You need to spell out the concrete steps to make the goal for the computer.

In this, case we're thinking while-loop. Draw in the various spots bit will be in. What is a square where we do not want bit to move? When bit is on the red square. Otherwise bit should move. What does that look like in code?

Code looks like...

while bit.get_color() != 'red':
    bit.move()

5. Try The Code, See What It Does

Ok, run it and see. Sometimes it will not do what we thought.

6. Ok what is the next goal.

Ok what is the next goal? What would be code to advance to that?

I think a reasonable guess here is similar to the first loop, but painting red, like this

while bit.get_color() != 'green':
    bit.move()
    bit.paint('red')

7. OOPS Not What Expected!

OOPS, that code looks reasonable, but does not do what we wanted. The question is: what sequence of actions did the code take to get this output? That's a better question than "why doesn't this work".

Go back to our drawing. Think about the paint/red and the while loop, how they interact.

8. Solution

The problem is that we paint the square red, and then go back to the loop test that is looking for green, which we just obliterated with red. One solution: add an if-statement, only paint blank squares red, otherwise leave the square alone.

That gives us this solution, which works perfectly..

def fix_tree(filename):
    bit = Bit(filename)
    while bit.get_color() != 'red':
        bit.move()
    bit.left()
    while bit.get_color() != 'green':
        bit.move()
        if bit.get_color() == None:
            bit.paint('red')

Other Solution

Another solution is to change the loop body to do the paint and then the move - that works perfectly too and is fine. I slightly prefer the == None solution as it seems more obvious. Both are fine. Very often there are a few different ways to solve something. If we have a strong preference for one form, we will say so, but in this case both are fine.

9. Thinking - Drawing - Coding

Use a drawing to think through the details of what the code is doing. It's tempting to just stare at the code and hit the Run button a lot! That doesn't work! Why is the code doing that?

Or put another way, in office hours, a very common question from the TA would be: can you make a little drawing of the input here, and then look at your line 6, and think about what it's going to do. All the TA does is prompt you to make a drawing and then use that to think about your code.

More Bit Tests: bit.right_clear()

• bit.right_clear() - another Bit function
• Returns True if the square to bit's right is clear, False otherwise
• Same as bit.front_clear() but 90 degrees to the side
• Also have bit.left_clear()

Using not

• Putting not to the left of a True/False value inverts it
• Use not with an if-test or while-test to check for the False condition
  Normally they look for the True condition, but we use not to invert this

if bit.right_clear():
    # run here if right is clear

if not bit.right_clear():
    # run here if right is blocked,
    # aka not-clear

Server: Bit Puzzles

On the experimental server, the Bit Puzzles section has many problems where bit is in some situation and needs to move around using Python code. We'll look at the Coyote problems, and there are many more problems for practice.

Reverse Coyote, remember Test = Go

> reverse-coyote (while + right_clear)

Based on the RoadRunner cartoons, which have a real lightness to them. In the cartoons, the coyote is always running off the side of the cliff and hanging in space for a moment. For this problem, the coyote tries to run back onto the cliff before falling.

Before:

After:

Q: What is the while test for this? Remember that Test = Go. What is the test that is True when we want to move, and False when do not want to move?

A: bit.right_clear()

Then students can try the standard_coyote(), or climb() which is a little more difficult.