Control Structures - while/if

Today: loops, if-statement, maybe variables

See links to Python Guide front page: loops, if-logic, booleans

Big Picture: Patience + Legos

• Early on, learning many little pieces
  bit.move()
  while
  if
• Each piece is simple and mechanical
• Be patient
• Each lecture adds 5 more pieces
• With simple pieces .. put them together to make something great
• CS nutshell: each line is trivial, but programmer can make a masterpiece with them
• Next week we'll be there, but this week's output is limited

Hey Up Front: bit.front_clear()

• bit.move() function modifies bit+world
• Other functions look at bit+world, no modify
• Test if way-forward is clear before a move:
  bit.front_clear()
• Calling bit.front_clear() returns a boolean value True or False
  -True and False are Python's "boolean" values
  -Boolean values fit into "while" below
  -True here means the square available, move() is ok
  -False here means the square is blocked, move() would be an error
  -Regular squares are clear (even if painted)
  -Black blocks and the world-edge are not clear

While Loop - Power!

• Loop - huge jump up in power
• "Do these lines again and again"
• Every algorithm has key lines to run thousands or millions of times
• Loops are the way to do that
• Several types of loop - today while

While Loop Demo

> Loop Demo

• This code is complete
• Read problem statement
• Run Case-1, play with it to see how it works
• Like Jurassic Park - while loop frozen in amber!
• Slide the Steps back to start, use Step button to go one at a time
• See the order of test / body-lines
  -test, body-lines, test, body-lines, ...
• Question: Suppose world is width 1. What is the result?
  -Case 3, turn off auto-play. Predict what this will do, then step through it

For reference, here is the code:

def loop_demo(filename):
    bit = Bit(filename)
    bit.paint('green')
    while bit.front_clear():
        bit.move()
        bit.paint('blue')
    bit.paint('red')

While Loop Syntax

Syntax 4 parts: while, test, colon, indented body lines

while boolean-test:
  body lines inside the while
  indented

While Loop Operation

• First check the test at the top, True or False?
• If True, run all the lines of the body, top to bottom
  -then loop back to top, check test again, .. repeat
• When the loop-test is False, loop is done, running continue after the loop body
• If the test is False initially, the body runs zero times (see Case-3)

While Loop Observations

• Lines before and after loop run once, as usual
• Body lines in the loop are different, running many times
• The test is checked, then run all the body lines top to bottom
• It's possible for the body to run zero times
• Big picture: code generality
  -There is one copy of your code algorithm
  -"General" - it should work for all the different input worlds
  -Try Run All option

If Statement - Logic

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

Problem statement: for each moved-to square, if it is blue, change it to green. Run it, then we'll talk about how the if-statement works.

> If Demo

For reference here is the code:

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

If-Statement Deconstruction

Here is the key line of code:

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

1. bit.get_color() - Returns

• Calling bit.get_color() function, returns the color value of the current square
  'red' 'green' 'blue', or None if square is empty
• Look at code above
• Like bit.get_color() is replaced by 'blue' or 'red' or whatever, then the line is evaluated

2. == Compares Two Values - Boolean

• The operator == compares two values for equality
  -(two equal signs next to each other)
• Returns True if equal, False otherwise
• e.g. 'blue' == 'blue' is True
• Comparison does not use single equal sign (=)
• Easy to type = by accident, Python gives an error fortunately
• Not Equal Test: !=
  e.g. if num != 6:

3. How If Demo Works

• Look at the if-demo code
• For each moved-to square
• run an if-test on color of that square
• If the test is True, paints it green

If Statement Syntax

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

if boolean-test:
  body lines inside the if
  indented

If Statement Operation

• If statement evaluates the test code, 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

With the basic if-structure in place, here are more boolean features, then we'll put it all together.

Bit Test: bit.left_clear()

• bit.left_clear()
• Returns True if square to left is clear, false otherwise
  -So if True, could turn left and move
• Also have bit.right_clear()

Boolean Expressions: and or not

• Something like bit.left_clear() returns a "boolean" True/False
• These operators combine boolean values: and or not
• xx and yy = both sides must be True, then it is True
• xx or yy = either side or both sides are True, it is True
• not xx = inverts True/False

Boolean Examples..

Q1: when is the following if-test True?

    if bit.left_clear() and bit.right_clear():
        # when does this happen?

Q2: when is the following if-test True? (just changing and to or)?

    if bit.left_clear() or bit.right_clear():
        # when does this happen?

Q3: when is the following if-test True?

    if not bit.left_clear():
        # when does this happen?

Answers..

A1: When either both left and right are clear

A2: When either left or right are clear (or both)

A3: When the left is blocked (i.e. lear_clear() returns False)

Put It All Together - Applied Boolean Code

> Bit Logic1 Exercises

First 3 are demos - walk through them

Then last 3 are little exercises to try .. see how many we get through today