3. Lists and Loops

Store many values in order. Do the same thing to all of them without copy-pasting.

Summary

A list keeps values in order:

currents = [38.2, 41.0, 39.5, 31.4]

Tools for lists:

Read one value with an index. Indexes start at 0: currents[0] is 38.2.
currents[-1] reads the last value: 31.4.
An index that is too big gives an IndexError.
len(currents) gives the count: 4.
currents.append(40.1) adds a value to the end.

A loop visits each value:

for c in currents:                  # each value in the list
    print(c)

for i in range(4):                  # i = 0, 1, 2, 3
    print(i)

for i, c in enumerate(currents):    # index and value together
    print(f'module {i}: {c} A')

tick = 0
while tick < 100:                   # repeats while the condition is true
    tick += 1

Which one to use:

for x in list: you need the values. Most common.
for i in range(n): you need to count, or index into the list.
for i, x in enumerate(list): you need the index and the value.
while: you do not know how many repeats. Be careful: a wrong condition loops forever.
break inside a loop exits it at once.

In robot code, do not use a bare while to wait for time to pass. The robot runs its loop 50 times per second, and a blocking while stops all of it. Waiting uses coroutines with yield (lesson 6).

In our robot code

robot/robot.py registers every subsystem with one loop instead of copy-pasted lines:

self.subsystems = [
    self.drivetrain, self.poseEstimator,
    self.intake, self.shooter, self.hopper,
]
for subsystem in self.subsystems:
    self.scheduler.registerSubsystem(subsystem)

The hopper fuel simulation uses range:

for fuel_num in range(1, self.max_fuel_in_hopper + 1):
    ...

Try it

Run the code. Add 'climber' to the list. Run again.

Your code

subsystems = ['drivetrain', 'shooter', 'intake', 'hopper']

for s in subsystems:
    print(f'registering {s}')

for i, s in enumerate(subsystems):
    print(f'module {i}: {s}')

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

3.1 Total current and hot modules

currents holds the amps of the four swerve modules.

PART A: add every current into total with a for loop. Do not use sum().

PART B: count the modules that use more than 40 amps into hot_count.

Your code

currents = [38.2, 41.0, 39.5, 31.4]  # amps, one per swerve module

# PART A: add them up with a for loop
total = 0.0

# PART B: count the modules over 40 amps
hot_count = 0

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.

3.2 Find the module with the most current

A swerve module that draws much more current than the others often has a mechanical problem. Find it.

PART A: set best_index to the index of the module with the most current.

PART B: set best_current to that module’s value.

Your code

currents = [38.2, 41.0, 39.5, 31.4]

# PART A: the index of the biggest current
best_index = -1

# PART B: the biggest current
best_current = 0.0

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.

3.3 Count robot frames

One robot frame is 20 ms.

PART A: count tick up to 100 with a while loop.

PART B: set seconds to tick * 0.02.

Your code

# One robot frame is 20 ms.
tick = 0

# PART A: count tick up to 100 with a while loop


# PART B: set seconds to tick * 0.02
seconds = 0.0

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: 4. Functions.