Control Flow

We can use control flow statements to change the execution of a program based on certain conditions, such as the value of a variable.

Booleans and Comparison

A Boolean is another data type in Python. It can have one of two values: True or False. They can be a literal constant or the result of a comparison operator.

is_true = True
is_false = False

We can use a set of operators to compare two values and output a Boolean value. These operators include:

• > greater than,
• < less than,
• >= greater than or equal to,
• <= less than or equal to,
• == equal to,
• != not equal to.

Here are some examples of comparison operators in action:

# Greater than
4 > 3 # True
10 > 10 # False

# Greater than or equal to

4 >= 3 # True
10 >= 10 # True

If Statements

An if statement executes a block of code if given a boolean expression that evaluates to True.

if True:
    print('It is true!')

Indentation is used to define the scope of the if statement. A code block must be indented by the same amount, but the amount of indentation is up to you.

if True:
    # This is a code block
    print('this will print')
    print('this will also print')
    # This is the end of the code block

if False:
    # This code block will not be executed
    print('this will not print')
    print('this will not print either')

if x == 10:
    # This code block will be executed if x is equal to 10
    print('this might print')

You can even nest if statements inside of each other.

if True:
    print('this will print')
    if False:
        # this is a nested code block
        print('this will not print')
    print('this will also print')

Exercise

Provide a value for x such that only the last print statement is executed.

Else and Else If

An if statement can also be extended by adding an else statement.

x = 6
if x > 7:
    print('x is greater than 7')
else:
    print('x is less than 7')

In the example above, the else statement is only executed if the if statement's expression evaluates to False.

Additionally, you can add an elif statement to extend an if statement further. An elif is short for "else if" and is used to check for additional conditions. You can have as many elif statements as you want. The statements are evaluated from top to bottom, and the first one that evaluates to True is executed.

x = 6
if x > 7:
    print('x is greater than 7')
elif x > 5:
    print('x is greater than 5')
else:
    print('x is less than 5')

In the example above, the elif statement only tries to run if the initial if statement evaluates to False.

You need to be careful when ordering your elif statements, or this could result in unreachable code.

x = 6
if x > 5:
    print('x is greater than 5')
elif x > 7:
    # this code will never run
    print('x is greater than 7')
else:
    print('x is less than 5')

Exercise

Given a number x print out the following:

1. If x is positive, print out x is positive.
2. If x is negative, print out x is negative.
3. If x is 0, print out x is 0.

While loops

A while statement is similar to an if statement. However, a while statement will continue to execute its code block as long as the expression evaluates to True.

while True:
    print("This will print over and over again")

It's important to be careful when writing while loops. If the expression never evaluates to False, the loop will never terminate and the program will not end.

Using a variable to control the number of times a loop executes is a common pattern.

times = 3
while times > 0:
    print("This will print 3 times")
    times -= 1

In the example above, the times variable is 3. On each iteration of the loop the value is reduced by 1. Once the value of times is 0 the expression no longer evaluates to True.

Exercise

Write a while loop that prints out the numbers 0 to 9.

For loops

A for loop is used to iterate over a sequence such as a string, tuple, list, or dictionary (which we will learn about later). For now, we will use the range function to create loops that execute a specific number of times.

for i in range(3):
    print("This will print 3 times")

In the example above, the range function creates a sequence of numbers from 0 to 2. On each iteration of the loop, the next value in the sequence is assigned to the variable i.

Exercise

Write a for loop that prints out the numbers 0 to 9.

Break and Continue

break and continue can be used in while and for loops to alter their behavior.

A break statement will immediately exit the loop.

while True:
    print("This will only print once")
    break

A continue statement starts the next iteration of the loop and skips the rest of the code block.

while True:
    print("This will print over and over again")
    continue
    print("This will never print")

break and continue are often used in conjunction with if statements.

for i in range(10):
    if i % 2 == 0:
        continue
    print(i)

The example above will print out the odd numbers from 1 to 9. The continue statement skips the rest of the code block when i is even.

Exercise

Given a number x, use continue to print out even numbers from 0 to x. Use break Stop if you reach a number greater than 20.

None

None is a value that represents the absence of a value. It is often used with if statements to check if a variable has a value.

x = None
if x is None:
    x = "Default value"

In the example above, we check if x is None before assigning a value to it. This is most useful when the value of the variable is not known until later in the program.

message = None
if y == 1:
    message = "y is 1"
elif y == 2:
    message = "y is 2"
if message:
    print(x)

In the example above, we assign message a value based on the value of y. Then check if message has a value before printing it out.

Exercise

Given the variables x,y, and z, print the sum of the values that are not None.

Boolean Operations

Boolean operations are used to make more complex boolean expressions.

if x > 0 and y > 0:
    print("Both x and y are positive")

In the example above, we use the and operator to check if both x and y are positive. You could achieve the same result with two if statements, but this is an example of how boolean operations can make your code more concise.

if x > 0:
    if y > 0:
        print("Both x and y are positive")

There are three boolean operations:

1. and - Returns True if both the operands are True, False otherwise.

print(True and True) # True

2. or - Returns True if any of the operands are True, False otherwise.

print(True or False) # True

3. not - Returns True if the operand is False, False otherwise.

print(not True) # False

Exercise

Given the variables x,y, and z print the following:

1. if x and y are greater than 10, print step 1 is True
2. if z or y is greater than x, print step 2 is True
3. if step 2 is False, print step 2 is False