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`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.

🎨 `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

🎨 `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

Holes 👩🏼‍🎨¶

`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 👩🏾‍🎨¶

`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.

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

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