4. Functions

Write a calculation once, give it a name, and use it many times.

Summary

def clamp(x, low, high):            # parameters in the parentheses
    return max(low, min(high, x))   # return sends a value back

speed = clamp(1.4, -1, 1)           # call with arguments
print(speed)                        # 1

Rules and tools:

• The code inside a function runs only when someone calls it. Defining it changes nothing.
• Parameters are the names in the definition (x, low, high). Arguments are the values in the call (1.4, -1, 1). Arguments fill parameters in order.
• You can also name the arguments: clamp(1.4, low=-1, high=1). Naming them makes tuning code easy to read. Our motor config calls do this.
• return sends a value back and ends the function. Code under return never runs. Without return, the caller gets None. Forgetting return and then printing None is a classic bug.
• Variables you make inside a function are local: they do not exist outside it.
• Default arguments let callers skip parameters:

def get_motor_config(k_p=0.0, current_limit=40):
    return (k_p, current_limit)

get_motor_config()              # (0.0, 40): both defaults
get_motor_config(k_p=8)         # (8, 40): changes one

• A function of the form def set_position(self, position): '''position is in degrees''' has a docstring: a string as the first line that documents the function. intake.py uses these.

In our robot code

robot/robot.py has one function that configures every TalonFX motor. Eight parameters, nearly all defaults:

def get_motor_config(self, inverted=0, k_p=0.0, k_i=0.0, k_d=0.0, ...):
    motor_config = configs.TalonFXConfiguration()
    ...
    return motor_config

A subsystem only passes what it changes:

self.intake_motor_config = self.robot.get_motor_config(0, 8, 0, 0, 0.0161, 0, 0, 2.5)

A small helper keeps alliance-flip math in one place:

def flip_X_coord(self, x):
    return self.fieldConstants.fieldLength - x

Try it

Try it

Try it: call a function

✓ solved

Run the code. Then change the gear ratio to 12.1 and run again.

Your code def rotate_distance(rotations, gear_ratio, wheel_circ): wheel_turns = rotations / gear_ratio return wheel_turns * wheel_circ d = rotate_distance(56.8, gear_ratio=5.68, wheel_circ=0.319) print(f'robot drove {d:.1f} meters')

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Exercises

4.1 Write clamp

Try it

Write clamp

✓ solved

Write the body of clamp(x, low, high). It returns x, but never smaller than low and never bigger than high.

Your code def clamp(x, low, high): # return x, but hold it in [low, high] return x

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4.2 Config with defaults

Try it

Config with defaults

✓ solved

This is the shape of get_motor_config in robot/robot.py. Both parameters have defaults.

Write the body: return the tuple (k_p, current_limit).

Your code def get_motor_config(k_p=0.0, current_limit=40): # return the tuple (k_p, current_limit) pass

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4.3 Alliance flip

Try it

Alliance flip

✓ solved

The same flip_X_coord helper as robot.py. The field is 16.54 meters long.

Write flip_x(x): return the red-alliance mirror of x.

Your code field_length = 16.54 # meters def flip_x(x): # return the red-alliance mirror of x pass

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Next lesson: 5. Classes and Objects.