Functions

4.8. Functions#

So far we have used a lot of Python functions. You might have noticed that functions use parentheses (), e.g. the print() function or the input() function.

We’ll now see that we can write our own functions! Functions are very useful for

  • Reusing blocks of code

  • Isolating parts of your program, which can make testing and debugging easier as each function can be tested individually

  • Composing programs as the logic and structure of the program can be broken down into smaller tasks that are easier to solve

  • Readability as functions generally result in less code which can make your program easier to read

Functions are defined using the following syntax

def function_name(parameter_1, parameter_2, ... parameter_n):
    # code to execute
    return result

Take note of the following:

  • def is a keyword which signifies that this is a function definition

  • function_name is chosen by you and follows the same naming rules as variables

  • each parameter_i has a name chosen by you and is available as a variable in the function

  • functions don’t necessarily need to be provided a parameter

  • : is placed after the parameter list

  • the function’s code block must be indented (just like we do for if statements and loops)

  • return signifies that the next thing will be “given back” to the caller

Example 1: One parameter

Here is a simple function called greet that constructs a greeting. This function returns a string with Hello <name>.

def greet(name):
    return 'Hello {}'.format(name)

print(greet('Alison'))

Hello Alison

This function meets the following specifications. Note that function and variable names are formatted in grey and the variable types are given in brackets.

  • Name: greet

  • Parameters: name (string)

  • Return: greeting message (string)

Example 2: Two parameters

Here is an add function, which takes 2 parameters, x and y. When you use the function you need to provide it two values. The first value that is provided to the function will be saved under the variable name x and the second value will be saved under the variable name y.

def add(x, y):
    return x + y

print(add(2, 4))

6

Example 3: Two parameters

Note that the order of the parameters matter. Here our divide function will divide x by y. So divide(2, 4) gives 2/4 while divide(4, 2) gives 4/2.

def divide(x, y):
    return x/y

print(divide(2, 4))
print(divide(4, 2))

0.5
2.0

This function meets the following specifications.

  • Name: divide

  • Parameters: x (int or float), y (int or float)

  • Return: x divided by y (float)

Example 4: No parameters

Functions don’t always need parameters. Here is an example of a function that gives us a list of students. Note that we still need the parentheses!

def get_class_list():
    return ['Alice', 'Bob', 'Charlie', 'David']

print(get_class_list())

['Alice', 'Bob', 'Charlie', 'David']

This function meets the following specifications.

  • Name: get_class_list

  • Parameters: None

  • Return: x divided by y (float)

Question 1

What do you think the output of the following code will be?

def power(x, exponent):
    y = x**exponent
    return y

z = power(2, 3)
print(z)

  1. 6

  2. 23

  3. 8

  4. 9
Solution

C.

The power function takes two parameters, x and exponent. power(2, 3) means that x = 2 and exponent = 3. x**exponent will then be 2**3, which is 2 to the power of 3. This means the result we return is 8.

Question 2

Which of the following best describes the purpose of the function given below.

import random

def five_tosses():
    tosses = []
    for i in range(5):
        r = random.random()
        if r < 0.5:
            tosses.append('Heads')
        else:
            tosses.append('Tails')
    return tosses

  1. This program simulates 5 random coin tosses for a fair coin. The results will be stored in a list containing the values 'Heads' or 'Tails'. The program will give a different result each time.

  2. This program simulates 5 random coin tosses for a fair coin. The results will be stored in a list containing the values 'Heads' or 'Tails'. The program will give the same result each time.

  3. This program simulates 5 random coin tosses for a fair coin. The results will be stored in a list containing the values 0 or 1 to indicate (0 for heads and 1 for tails). The program will give a different result each time.

  4. This program simulates 5 random coin tosses for a fair coin. The results will be stored in a list containing the values 0 or 1 to indicate (0 for heads and 1 for tails). The program will give the same result each time.

Solution

Solution is locked

Question 3

Consider the function below.

def calculate(x, y, z):
    return x + y - 2*z

Which of the following will return 8? Select all that apply.

  1. calculate(8, 8, 4)

  2. calculate(2, 6, 0)

  3. calculate(2, 5, 3)

  4. calculate(5, 7, 2)

  5. calculate(0, 10, 1)

  6. calculate(7, 3, 4)
Solution

Solution is locked

Question 4

What’s wrong with the following code snippet?

def multiply(a, b, c)
    return a * b * c

print(multiply(2, 4, 7))

  1. This will result in a IndentationError because line 2 should not be indented

  2. This will result in a SyntaxError because line 1 should have a : at the end

  3. This will result in a NameError because the function multiply hasn’t been imported

  4. This will result in a ValueError because the variables a, b and c have not been defined

  5. This will result in a TypeError because the variables a, b and c are strings, not integers

Solution

Solution is locked

Question 5

Write a function that converts temperature in Celsius to Fahrenheit.

The conversion formula is as follows:

\(T_{\text{Fahrenheit}} = T_{\text{Celsius}}\times \cfrac{9}{5}+ 32\)

Function specification

  • name: celsius_to_fahrenheit

  • parameters: temperature (int or float)

  • return: temperature in Fahrenheit (float)

Solution

Solution is locked