CSE 160 Section 3

Functions

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CSE 160

CSE 160: Section 3

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Logistics

• Check Due Dates on Website and Gradescope
• Canvas isn’t always up to date
• Written Check-In 3 due Fri Jul 10
• Programming Practice 2 due Sun Jul 12
• HW2 due Mon Jul 13
• Submitting on Gradescope
• Wait for autograder!

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CSE 160: Section 3

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Lecture Review: If/ Else Statements

CSE 160: Section 3

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If/ Else Structure

• Checks that condition is True/False, and executing code based on that condition

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is_raining = True

is_sprinkling = False

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if is_raining:

print(“Bring an umbrella”)

elif is_sprinkling:

print(“Bring a raincoat”)

else:

print(“Bring sunglasses”)

Output:

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Bring an umbrella

CSE 160: Section 3

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Boolean Zen

• Minimize the use of if statements where possible!
• More readable
• Easier to debug

num = 0

def is_num_2(num):

if num == 2:

return True

else:

return False

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num = 0

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def is_num_2(num):

return num == 2

Without boolean zen:

With boolean zen:

CSE 160: Section 3

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Lecture Review: Nested Loops

CSE 160: Section 3

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Nested Loops

• A for loop within the body of another for loop

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Example:

for i in [0, 1, 2, 3]:

for j in [0, 1, 2]:

print(i)

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CSE 160: Section 3

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Nested Loops

• Nested loop structure
• Outer loop
• Inner loop

for i in [0, 1, 2, 3]:

for j in [0, 1, 2]:

print(i)

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• Variable names i and j should be different for clarity

CSE 160: Section 3

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Nested Loops

• Lets see what this outputs!

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Example:

for i in [0, 1, 2, 3]:

for j in [0, 1, 2]:

print(i)

PythonTutor

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CSE 160: Section 3

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Lecture Review: Functions

CSE 160: Section 3

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Functions: an overview

• A function is a block of code which only runs when it is called.
• You can pass data, known as parameters, into a function.
• A function can return data as a result (but it doesn’t have to)

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CSE 160: Section 3

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Why do we use functions?

• Don’t Repeat Yourself (DRY)
• If you’re doing the same thing over and over again, it might be easier to make it into a function
• Abstractions
• It’s easier to call a function (like sqrt() or sum()) than writing all the code
• It’s easier to reason about your program when you break it down into smaller chunks

CSE 160: Section 3

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Function Syntax

# Writing a function

def func_name(parameter1,...):

function body

return value

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# To call that function:

func_name(parameter1,...)

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CSE 160: Section 3

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Functions run only when called

• What would be the output of this program?

Output:

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def fall():

x = “Happy Fall!”

print(x)

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fall()

Output:

Happy Fall!

CSE 160: Section 3

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Functions run only when called

• What would be the output of this program?

def winter():

x = “Happy Winter!”

print(x)

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Output:

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def fall():

x = “Happy Fall!”

print(x)

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fall()

Output:

Happy Fall!

Include parentheses in function call

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Parameters

• We can pass a function a value to work with, called a “parameter”

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def print_twice(x):

print(x)

print(x)

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print_twice(“Hi!”)

Output:

Hi!

Hi!

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Return Value

This function does have a return value, what will this print?

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def two_times_seven():

x = 2 * 7

return x

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print(two_times_seven())

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Output:

14

CSE 160: Section 3

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Return Value

This function does have a return value, what will this print?

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def two_times_seven():

x = 2 * 7

return x

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print(two_times_seven())

Output:

14

You must have a return statement if you want your function call to have access to x in two_times_seven()! Otherwise, it will return None

CSE 160: Section 3

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Return vs Print in Functions

Function with a return

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Function with a print statement

Output: ?

Can you see the difference?

def add(a, b):

return a + b

def add_print(a, b):

print(a + b)

CSE 160: Section 3

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Function with a Return

Function with a return

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def add(a, b):

return a + b

• When you call add(a, b):
• Nothing will show up unless you print the result
• It only returns a value, it doesn't display it.

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• You need to print to see the output:
• print(add(1,2))

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print(add(1,2))

Output:

3

CSE 160: Section 3

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Function with a Print

Function with a print statement

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print(add_print(1,2))

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Output:

3

None

def add_print(a, b):

print(a + b)

• On the other hand, add_print(a, b) will immediately print the result when called:
• add_print(1,2)

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• But it doesn't return anything, so if you try:
• print(add_print(1,2))
• Gives you None because it does not return anything

CSE 160: Section 3

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Docstrings

• You should comment your functions so others (and yourself) can understand what they do without having to read through the code. You should include:
• What are the inputs? What types should they be?
• What does it do (does it print something?)
• What does it return?

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def abs(x):

‘’’Takes in a number (x), and returns its absolute value’’’

if x < 0:

return x * -1

else:

return x

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CSE 160: Section 3

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Mod Operator Overview

CSE 160: Section 3

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Mod

Modulo operator, denoted as %, is a mathematical operation that calculates the remainder when one number is divided by another.

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• 2 % 5

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• 3 % 7

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• 5 % 2

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• 8 % 3

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• 20 % 5

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CSE 160: Section 3

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Mod

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• 2 % 5 = 2

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• 3 % 7 = 3

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• 5 % 2 = 1

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• 8 % 3 = 2

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• 20 % 5 = 0

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CSE 160: Section 3

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Mod

What is the output of the following code?

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for i in range(5):

print(i % 3)

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CSE 160: Section 3

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Mod

What is the output of the following code?

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for i in range(5):

print(i % 3)

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Answer:

0

1

2

0

1

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CSE 160: Section 3

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Section Handout Problems

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Problem 1

Write a function avg_age(ages) that calculates and returns the average of a list of ages. You are not allowed to use Python’s built in sum() function. Your function should take the list ages as a parameter and return the average. For example, given ages = [20, 21, 22], avg_age(ages) should return 21.

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Python Tutor

Python Tutor

CSE 160: Section 3

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Problem 2

Given a list of students’ names (student_lst), write a function max_height(student_lst) that returns the maximum height in inches. Assume a function get_height(student) is given to you, and will return the height (in inches) of any student when given a student name.

For example: get_height(“Nicholas”) will return 75.

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a) Implement the function max_height(student_lst)

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Python Tutor

b) What is the return type of max_height(student_lst)?

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c) Suppose you printed the max height instead of returning it. What would be the return

type of max_height(student_lst)

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PythonTutor

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CSE 160: Section 3

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Problem 5

Write a function called count_character(word, search), where given a string

called word, return the number of times the character called search appears.

For example, count_character(“apple pie”, “p”) return 3.

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Python Tutor

CSE 160: Section 3

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Additional Problems

CSE 160: Section 3

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Problem 3

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Write a function count_letters(word_list, target) that counts the number of times a target letter appears in a list of strings and returns it. For example, given word_list = [“this”, “is”, “a”, “list”] and target = “s”, count_letters(word_list, target) should return 3.

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PythonTutor

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CSE 160: Section 3

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Problem 4

Write a function called budget_saver(cost, budget) that takes the price of a product and a budget. The function should return “too expensive” if the price is more than the budget, “great deal” if the price is less than the budget, and “okay” if the cost and budget are equal.

For example, budget_saver(250, 100) returns “too expensive”.

Python Tutor

CSE 160: Section 3

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Written Check-In Due Friday

CSE 160: Section 3

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