2. Conditionals and Logic

Do different things for different states. This is how a subsystem decides what to do each frame.

Summary 

if voltage < 11:
    print('CRITICAL')
elif voltage < 12:
    print('LOW')
else:
    print('GOOD')

Rules and tools:

  • A condition ends with a colon :. The lines under it are indented with four spaces. Indented lines belong to the if. A wrong indent gives a syntax error.
  • elif means “else if”. The conditions run top to bottom. The first true one runs, and the rest are skipped. Order sets priority.
  • else runs when no condition above it was true. It is optional.
  • Comparisons: == equal, != not equal, <, <=, >, >=.
  • Chain comparisons like math: 2.5 < time_left < 3 means both parts are true. Our hub-shift rumble code uses this.
  • Combine conditions with and, or, not:
is_intaking = True
fuel_in_hopper = 10

is_intaking and fuel_in_hopper < 24   # True: both parts are true
is_intaking or fuel_in_hopper == 0    # True: one part is true
not is_intaking                       # False
  • None means: no value yet. Example: self.auto_win = None until the FMS sends the auto winner. Test it with is None or is not None, not with ==.
  • Name bool variables like questions: is_intaking, at_default, has_fuel. The name should tell you what True means.

Common mistake: if is_intaking = True: assigns a value. It does not compare. Python shows a SyntaxError. Use == to compare, or just write if is_intaking:.

In our robot code

robot/subsystems/intake.py, periodic(). This runs every frame. The order sets the priority:

if self.robot.intake_at_default:
    self.stop_intake()
elif self.robot.clear_jam:
    self.set_intake_speed(-0.3 * 100)
elif self.robot.is_intaking:
    self.set_intake_speed(0.85 * 100)

robot/robot.py, the Auto Win logging:

if self.auto_win is None:
    SmartDashboard.putString("Auto Win", "UNKNOWN")

Try it

Try it
Try it: if, elif, else
✓ solved

Run the code. Then change match_time to 5 and to 130. Run again each time.

The code operates in your browser. The page sends nothing to a server and saves your work on this computer. Press Ctrl+Enter to run it.

Exercises

2.1 Battery warnings and shift windows

Try it
Battery warnings and shift windows
✓ solved

Two small if/elif/else chains, the same pattern as the shift logic in robot.py.

voltage = 11.8 and match_time = 75.

PART A: set level to 'CRITICAL' if voltage is under 11, 'LOW' if under 12, else 'GOOD'.

PART B: set shift to 'TRANSITION' if match time is 10 or less, 'SHIFT' if 110 or less, else 'ENDGAME'.

The code operates in your browser. The page sends nothing to a server and saves your work on this computer. Press Ctrl+Enter to run it.

2.2 The intake state machine

Try it
The intake state machine
✓ solved

periodic() in robot/subsystems/intake.py is a chain of flags. The order sets the priority.

The four states are: 'stowed', 'reversing', 'intaking', 'stopped'.

SCENARIO A: at_default False, clear_jam True, is_intaking False. Fill in state_a with an if/elif/else chain.

SCENARIO B: all three flags are True. Write the same chain for state_b.

The code operates in your browser. The page sends nothing to a server and saves your work on this computer. Press Ctrl+Enter to run it.

2.3 Label the auto winner

Try it
Label the auto winner
✓ solved

robot.py logs who won autonomous, but until the FMS sends the result there is no answer. That is Python’s None.

auto_win: None means unknown, True means RED, False means BLUE.

PART A: auto_win_a = None. Set label_a with if/elif/else.

PART B: auto_win_b = True. Set label_b the same way.

The code operates in your browser. The page sends nothing to a server and saves your work on this computer. Press Ctrl+Enter to run it.

Next lesson: 3. Lists and Loops.