if and else

NOTES

Today we're going to look at some more of the fundamental tools in programming.

🎨 if

red_dots.py

Turn the red dots into blue dots.

NOTES

• Step through red_dots.py

• Look at control flow

  • When does the if block run? When is it skipped?

• Comment on event stream pattern: the outer loop moves Bit along towards a goal, while the if handles events that come up along the way.

%%file red_dots.py
from byubit import Bit

@Bit.run('red-dots')
def go(bit):
    while bit.front_clear():
        bit.move()
        if bit.is_red():
            bit.paint('blue')

🎨 else

more_red_dots.py

Turn the red into blue and the empty spaces into green.

NOTES

• Step through
• When is the if block run? When is the else block run?
• Note the exclusivity of the two blocks

%%file more_red_dots.py
from byubit import Bit

@Bit.run('more-red-dots')
def go(bit):
    while bit.front_clear():
        bit.move()
        if bit.is_red():
            bit.paint('blue')
        else:
            bit.paint('green')

🎨 elif

turns.py

• While Bit is not blocked, move forward
• If the square is red, turn left
• If the square is green, turn right
• Otherwise, paint the square blue

NOTES

• Step through
• Show that elif block checks conditions
• Show that else block doesn't check a condition
• All blocks are exclusive

%%file turns.py
from byubit import Bit

@Bit.run('turns')
def go(bit):
    while bit.front_clear():
        bit.move()
        if bit.is_red():
            bit.left()
        elif bit.is_green():
            bit.right()
        else:
            bit.paint('blue')

Holes 👩🏼‍🎨

# Solution 
from byubit import Bit

@Bit.run('holes')
@Bit.pictures('images/')
def run(bit):
    bit.paint('blue')
    while bit.front_clear():
        bit.move()
        if bit.right_clear():
            bit.paint('red')
        elif bit.left_clear():
            bit.paint('green')
        else:
            bit.paint('blue')

holes.py

Bit is in the pipes, and the pipes have holes. Bit's job is to mark where the holes are so someone else can fix them.

• Mark holes on the right with red
• Mark holes on the left with green
• Otherwise paint blue

NOTES

Draw it out! Give the students time to discuss how they would solve this.

Demonstrate general strategy: how we can use a while to move Bit to the end goal and use if to handle events along the way. Event stream pattern

Errors:

• Mix left and right
• Don't put blue in elif (put it outside the if/else control)
• First square/last square

Explore:

• move-then-ifblock, ifblock-then-move

Fly 👩🏾‍🎨

# Solution
from byubit import Bit

@Bit.run('fly')
@Bit.pictures('images/')
def run(bit):
    while not bit.is_red():
        if bit.is_blue():
            bit.left()
        elif bit.is_green():
            bit.right()

        bit.move()

fly.py

Bit is out flying around.

When Bit finds a blue square, he turns left.

When Bit finds a green square, he turns right.

When Bit finds a red square, he stops.

NOTES

Draw it out!

Errors: how do you identify and fix them?

• Turn right on blue, left on green (runs into wall after wandering)
• While front_clear() instead of not is_red() (misses red square)
• Else instead of elif (turns on blank square)
• Move in the else block (infinite loop)

Does it matter if you move-then-turn vs turn-then-move?

Elevators 👨🏼‍🎨

elevators.py

Bit needs to climb to the top of the building, using the green elevators.

NOTES

This activity prepares the students for waterfall.py in the lab.

• Draw it out!
• Pose the problem as an event stream problem
• What is the outer loop?
• What is the condition that triggers the event?
  • What is the event?
• How do we rise?
  • What is the while condition?
  • What is the glue code before and after the "rise" loop?
• Use comments to indicate subgoals

More Elevators

Now write a single function that can allow Bit to travel in either direction.

NOTES

Add in 'more-elevators' as a test case. Observe the problem: Bit turns into the floor when heading from left to right.

How do we know which way to turn to rise?

Similar "rise" blocks: each has a while loop, but the turns are different.

Nested: while if if while

Key Ideas

• if, else, and elif
• event stream pattern:
  • handle specific events that come up while moving towards a goal
    • outer while with inner if
• Boundary conditions