Making Choices

Overview

Teaching: 20 min
Exercises: 20 min

Questions

• How can my programs do different things based on data values?

Objectives

• Write conditional statements including if, elif, and else branches.

• Correctly evaluate expressions containing and and or.

A key part of programming is making the computer do different things depending on the outcome of a test. This functionality is provided by the conditional statements if, elif, and else.

Conditionals

We can ask Python to take different actions, depending on a condition, with an if statement:

num = 37
if num > 100:
    print('greater')
else:
    print('not greater')
print('done')

not greater
done

The second line of this code uses the keyword if to tell Python that we want to make a choice. If the test that follows the if statement is true, the body of the if (i.e., the lines indented underneath it) are executed. If the test is false, the body of the else is executed instead. Only one or the other is ever executed:

Conditional statements don’t have to include an else. If there isn’t one, Python simply does nothing if the test is false:

num = 53
print('before conditional...')
if num > 100:
    print(num,' is greater than 100')
print('...after conditional')

before conditional...
...after conditional

We can also chain several tests together using elif, which is short for “else if”. The following Python code uses elif to print the sign of a number.

num = -3

if num > 0:
    print(num, 'is positive')
elif num == 0:
    print(num, 'is zero')
else:
    print(num, 'is negative')

"-3 is negative"

Note that to test for equality we use a double equals sign == rather than a single equals sign = which is used to assign values.

That’s Not Not What I Meant

Sometimes it is useful to check whether some condition is not true. The Boolean operator not can do this explicitly.

if not (a < b):
    print('a is NOT less than b')
if not c == d:
    print('c is NOT equal to d')

We can also combine tests using and and or. and is only true if both parts are true:

if (1 > 0) and (-1 > 0):
    print('both parts are true')
else:
    print('at least one part is false')

at least one part is false

while or is true if at least one part is true:

if (1 < 0) or (-1 < 0):
    print('at least one test is true')

at least one test is true

What Is Truth?

True and False are special words in Python called booleans, which represent truth values. A statement such as 1 < 0 returns the value False, while -1 < 0 returns the value True. However, these True and False booleans are not the only values in Python that are true and false. In fact, any value can be used in an if or elif. The code below shows how some of these conditionals work:

if 'word':
    print('word is true')
if not '':
    print('empty string is NOT true')
if [1, 2, 3]:
    print('non-empty list is true')
if not []:
    print('empty list is NOT true')
if 1:
    print('one is true')
if not 0:
    print('zero is NOT true')

You can also perform special tests against lists and dictionaries to see if an object is contained in them by using in. For lists, this is simply if the object is in the list. For dictionaries, it checks if the value is in the list of keys, not values.

my_list = ["apple", "orange", "pear"]
if "pear" in my_list:
    print("I've found a pear!")

I've found a pear!

How Many Paths?

Consider this code:

if 4 > 5:
    print('A')
elif 4 == 5:
    print('B')
elif 4 < 5:
    print('C')

Which of the following would be printed if you were to run this code? Why did you pick this answer?

1. A
2. B
3. C
4. B and C

Solution

C gets printed because the first two conditions, 4 > 5 and 4 == 5, are not true, but 4 < 5 is true.

Close Enough

Write some conditions that print True if the variable a is within 10% of the variable b and False otherwise. Hint: You can make the condition easier to understand if you use the abs function — help will tell you what it does!

Solution 1

a = 5
b = 5.1

if abs(a - b) < 0.1 * abs(b):
    print('True')
else:
    print('False')

Solution 2

print(abs(a - b) < 0.1 * abs(b))

This works because the Booleans True and False have string representations which can be printed.

Key Points

• Use if condition to start a conditional statement, elif condition to provide additional tests, and else to provide a default.

• The bodies of the branches of conditional statements must be indented.

• Use == to test for equality.

• X and Y is only true if both X and Y are true.

• X or Y is true if either X or Y, or both, are true.

• Zero, the empty string, and the empty list are considered false; all other numbers, strings, and lists are considered true.

• True and False represent truth values.