Control Flow

Learning Objectives

At the end of this lesson you will be able to:

• Discuss the benefits of thinking about a problem before starting to code

• Understand the basics of control flow

• Explain the basic conditional statements, such as if, else, and, not, and or

• Write some simple expressions using these statements

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.”

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 set of lines indented underneath it) is executed, and “greater” is printed. If the test is false, the body of the else is executed instead, and “not greater” is printed. Only one or the other is ever executed before continuing on with program execution to print “done”:

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.

Comparing in Python

Along with the > and == operators we have already used for comparing values in our conditionals, there are a few more options to know about:

• >: greater than

• <: less than

• ==: equal to

• !=: does not equal

• >=: greater than or equal to

• <=: less than or equal to

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

True and False

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.

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?

What Is Truth?

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. After reading and running the code below, explain what the rule is to determine which values are considered true and which are considered false.

word is true
non-empty list is true
one is true

That’s Not Not What I Meant

Sometimes it is useful to check whether a condition is not true. The Boolean operator not can do this explicitly. After reading and running the code below, write some if statements that use not to test the rule that you formulated in the previous challenge.

empty string is not true
not not True is true

Exercise

Write some conditions that print True if the variable a is within 10% of the variable b and False otherwise. Compare your implementation with your partner’s: do you get the same answer for all possible pairs of numbers?

Hint

There is a [built-in function abs][abs-function] that returns the absolute value of a number:

12

Exercise Solution 1

a = 5
b = 5.1

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

Exercise 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.

Summary Quiz