4 Decisions: if.. elif.. else¶

Decision making plays a key role in programming and Python provides several ways to do that.

4.1 The If statement¶

The if statement is used to check if a condition is fulfilled and then a task (or set of tasks) is executed. The following example shows how it is used to check if a number is even:

In [ ]:

number = 2
if number % 2 == 0:
    print("number is even.")

The operator % is called modulus and gives the remainder of an integer division (see section 1). The conditional expression follows directly after the word if and ends with a colon : (just as in the syntax for loops, see section 3). The task(s) to be executed in case the condition is True, need to be indented (again as in the syntax for loops). Note that if the condition is not True, the task is simply not executed.

In [ ]:

number = 3
if number % 2 == 0:
    print("number is even.")

4.2 The if.. else statement¶

An else statement can be combined with an if statement. An else statement contains the block of code that is executed if the conditional expression in the if statement resolves as False.

In [ ]:

number = 3
if number % 2 == 0:
    print("number is even.")
else:
    print("number is odd.")

Just as loops can be nested within loops, if statements can be nested within if statements.

Now that you know the concept of loops and decision making, by combining them you can do a range of different tasks!

Exercise 4.2.1¶

While n < 10, print whether each number is odd or even.

In [ ]:

# Exercise 4.2.1
for n in range(1,11):
    if n % 2 == 0:
        print('%d is even' % n)
    else:
        print('%d is odd' % n)

Exercise 4.2.2¶

While n < 10, find the number of values that are even.

In [ ]:

# Exercise 4.2.2
counter = 0
for n in range(1,11):
    if n % 2 == 0:
        counter += 1
print(f"Number of even values: {counter}")

If there is more than one condition you want to check, you can add elif statements to check each condition. You can have as many elifs between an if and else as you want.

In [ ]:

for n in range(-2, 2):
    if n < 0:
        print(f'{n} is negative')
    elif n > 0:
        print(f'{n} is positive')
    else:
        print(f'{n} is zero')

It is worth noting that in an if.. elif.. else block, the first statement that returns as True is executed.

Review:¶

In this section, we have covered:

How to do decision making
The if statement.
The if .. else statment.
The if .. elif .. else statement.