Week 1 • Wednesday

Block 5: Defining Functions

Define and call reusable functions with parameters and return values.

Concepts

def function_name(params): syntax
return keyword vs print()
Default parameter values
Calling functions with positional vs keyword args

Code Examples

Basic function

def add(a, b):
    return a + b
print(add(3, 5))  # 8

Default parameters

def greet(name, greeting='Hello'):
    return f'{greeting}, {name}'
print(greet('Alice'))      # Hello, Alice
print(greet('Bob', 'Hi'))  # Hi, Bob

Exercise

Define compute_bmi(weight_kg, height_m) and test on 3 people. Write a greet(name, greeting='Hello') function.

Solution

def compute_bmi(weight_kg, height_m):
    return weight_kg / (height_m ** 2)

def greet(name, greeting='Hello'):
    return f'{greeting}, {name}!'

# Test
people = [(70, 1.75), (60, 1.65), (90, 1.80)]
for w, h in people:
    print(f'BMI: {compute_bmi(w, h):.1f}')
print(greet('Alice'))
print(greet('Bob', 'Hi'))

Practice Problems

Problem 1: Write a function celsius_to_fahrenheit(c) that converts and returns the value.

Hint: F = C * 9/5 + 32

Problem 2: Write a function with a default argument that can be overridden.

Hint: def f(x, multiplier=2): return x * multiplier

Application

Functions modularize code—data cleaning, feature engineering, and model evaluation are typically separate functions.

Case Study

A ML pipeline has functions: load_data(), preprocess(), train_model(), evaluate(). Each is tested independently and composed in main().

Homework

Write a function that takes a list of numbers and returns (min, max, average) as a tuple.