6.1: Defining Functions

Learning Objectives

By the end of this section you should be able to

• Identify function calls in a program.
• Define a parameterless function that outputs strings.
• Describe benefits of using functions.

Calling a function

Throughout the book, functions have been called to perform tasks. Ex: print() prints values, and sqrt() calculates the square root. A function is a named, reusable block of code that performs a task when called.

Checkpoint: Example: Simple math program

Concepts in Practice \(\PageIndex{1}\): Identifying function calls

Which line has a function call?

1	input_num = 14
2	offset_num = input_num - 10
3	print(offset_num)

Answer

c. The print() function is called to output the variable, offset_num.

Concepts in Practice \(\PageIndex{2}\): Identifying function calls

How many times is print() called?

print("Please log in")
username = input("Username:")
password = input("Password:")
print("Login successful")
print("Welcome,", username)

1. 1
2. 3
3. 5

Answer

b. Lines 1, 4, and 5 each call print() once. Lines 2 and 3 call a different function, input().

Concepts in Practice \(\PageIndex{3}\): Identifying function calls

How many function calls are there?

# Use float() to convert input for area calculation
width = float(input("Enter width:"))
height = float(input("Enter height:"))
print("Area is", width*height)

1. 3
2. 5
3. 6

Answer

b. Line 1 is a comment that is not executed. Lines 2 and 3 each call float() and input(). Line 3 calls print() for a total of five function calls.

Defining a function

A function is defined using the def keyword. The first line contains def followed by the function name (in snake case), parentheses (with any parameters—discussed later), and a colon. The indented body begins with a documentation string describing the function's task and contains the function statements. A function must be defined before the function is called.

Checkpoint: Example: Welcome message function

Concepts in Practice \(\PageIndex{4}\): Defining functions

What's wrong with the first line of the function definition?

def water_plant:

1. A docstring should go after the colon.
2. Parentheses should go after water_plant.
3. def should be define before water_plant.

Answer

b. The corrected line is def water_plant(): .

Concepts in Practice \(\PageIndex{5}\): Defining functions

What is the output?

def print_phone_num():
  print("Phone: (", 864, ")", 555, "-", 0199)

print("User info:")
print_phone_num()

1. Phone: ( 864 ) 555 - 1000
User info:
Phone: ( 864 ) 555 - 1000
2. User info:
3. User info:
Phone: ( 864 ) 555 - 1000

Answer

c. The main program outputs "User info:" and then calls print_phone_num(), which outputs the phone number.

Concepts in Practice \(\PageIndex{6}\): Defining functions

Which statement calls a function named print_pc_specs()?

1. print_pc_specs
2. print_pc_specs()
3. print_pc_specs():

Answer

b. A function call includes the function name and parentheses.

Concepts in Practice \(\PageIndex{7}\): Defining functions

Which is an appropriate name for a function that calculates a user's taxes?

1. calc_tax
2. calculate user tax
3. c_t

Answer

a. calc_tax is concise, in snake case, and indicates the function's task.

Benefits of functions

A function promotes modularity by putting code statements related to a single task in a separate group. The body of a function can be executed repeatedly with multiple function calls, so a function promotes reusability. Modular, reusable code is easier to modify and is shareable among programmers to avoid reinventing the wheel.

Checkpoint: Improving a program with a function

Concepts in Practice \(\PageIndex{8}\): Improving programs with functions

Consider the code above. If the points were changed from floats to integers, how many statements would need to be changed in the original and revised programs respectively?

1. 3, 1
2. 4, 4
3. 12, 4

Answer

c. Each calculation uses four statements to read in the x,y values of a pair of points. The original program performs three calculations by repeating code for a total of 12 input statements. The revised program performs three calculations by executing the four input statements multiple times with function calls.

Concepts in Practice \(\PageIndex{9}\): Improving programs with functions

Consider the code above. How many times can calc_distance() be called?

1. 1
2. 3
3. many

Answer

c. The benefits of a function's reusability is highlighted by many function calls that drastically reduce the amount of cluttered, redundant code. Calling calc_distance() 10 times is equivalent to 60 statements in the original program.

Try It: Cinema concession stand

Write a function, concessions(), that prints the food and drink options at a cinema.

Given:

    concessions()

The output is:

    Food/Drink Options:
    Popcorn: $8-10
    Candy: $3-5
    Soft drink: $5-7

Interactive Code

# Define concessions()


print("Welcome to the cinema.")
concessions()

Answer

def concessions():
print("Food/Drink Options:")
print("Popcorn: $8-10")
print("Candy: $3-5")
print("Soft drink: $5-7")

print("Welcome to the cinema.")
concessions()

Try It: Terms and conditions prompt

Write a function, terms(), that asks the user to accept the terms and conditions, reads in Y/N, and outputs a response. In the main program, read in the number of users and call terms() for each user.

Given inputs 1 and "Y", the output is:

    Do you accept the terms and conditions?
    Y
    Thank you for accepting.

Given inputs 2, "N", and "Y", the output is:

    Do you accept the terms and conditions?
    N
    Have a good day.
    Do you accept the terms and conditions?
    Y
    Thank you for accepting.

Interactive Code

Input

3
Y
Y
N

Answer

def terms():
print("Do you accept the terms and conditions?")
response = input()
if response == "Y":
print("Thank you for accepting.")
else:
print("Have a good day.")

count = int(input())
for i in range(0, count):
terms()