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3.4. Conditionals (Continued)

In section 3.2. Basic Conditionals, we learned about some basic constructs which are used to make decisions on the basis of some condition, more specifically, the if and else constructs.

In this section, we'll dig deeper into these two statements and learn a new construct, elif.

Nested if statements

You can write if statements inside if statements.

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age = int(input('Enter your age: '))
test_score = float(input('Enter driving test score: '))

if age >= 18:
    if test_score > 7:
        print('You are over 18 and passed driving test so you can drive.')
    else:
        print('You failed driving test so you cannot drive.')
else:
    print('You cannot drive because you are under 18.')

Enter your age: 18
Enter driving test score: 10
You are over 18 and passed driving test so you can drive.

Enter your age: 18
Enter driving test score: 3
You failed driving test so you cannot drive.

Enter your age: 15
Enter driving test score: 10
You cannot drive because you are under 18.

Note that each if statement has its own code block and is indented accordingly. The second if statement is part of code block of first if statement.

Using and instead of nested if

Nesting if statements is equivalent to using and on the two logical expressions. The above code can be shortened to following:

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age = int(input('Enter your age: '))
test_score = float(input('Enter driving test score: '))

if age >= 18 and test_score > 7:
    print('You are over 18 and passed driving test so you can drive.')
else:
    print('You cannot drive.')

Enter your age: 18
Enter driving test score: 10
You are over 18 and passed driving test so you can drive.

Enter your age: 18
Enter driving test score: 3
You cannot drive.

Enter your age: 15
Enter driving test score: 10
You cannot drive.

The elif statement

The code shown below takes three numbers and outputs the largest number from the three given numbers.

a = int(input('Enter first number: '))
b = int(input('Enter second number: '))
c = int(input('Enter third number: '))

if a >= b and a >= c:
    print(a, 'is largest number from given numbers')
else:
    if b >= a and b >= c:
        print(b, 'is largest number from given numbers')
    else:
        if c >= a and c >= b:
            print(c, 'is largest number from given numbers')

Enter first number: 20
Enter second number: 14
Enter third number: 24
24 is largest number from given numbers

The code works as expected. However, if you look at the lines below the first else statement, there are so many nested if statements that it makes the code unnecessarily confusing.

To overcome this problem, we'll use the elif construct.

a = int(input('Enter first number: '))
b = int(input('Enter second number: '))
c = int(input('Enter third number: '))

if a >= b and a >= c:
    print(a, 'is largest number from given numbers')
elif b >= a and b >= c:
    print(b, 'is largest number from given numbers')
elif c >= a and c >= b:
    print(c, 'is largest number from given numbers')

Enter first number: 20
Enter second number: 14
Enter third number: 24
24 is largest number from given numbers

As you can see, the code looks much easier to read now.

elif is short for "else if" and is equivalent to a nested if inside an else block:

if condition_1:
    print('do something')
else:
    if condition_2:
        print('do something else')

if condition_1:
    print('do something')
elif condition_2:
    print('do something else')

elif is a way of saying "if previous condition didn't satisfy, check this condition"

You can stack as many elif statements as you want but note that first statement is always an if.

color = input('Enter a color: ')
color = color.lower()  # (1)!

if color == 'red':
    print('Color code for red: #FF0000')
elif color == 'blue':
    print('Color code for blue: #0000FF')
elif color == 'green':
    print('Color code for green: #008000')
else:
    print(color, 'color is not supported')

  1. color.lower() is a method that converts the string to lowercase. This is done to make the comparison easier as user might give input in mixed case (e.g. rEd or Red).

    If we don't do this, then comparison would fail if user provides a mixed case input. (e.g. "Red" == "red" evaluates to False)

    These methods are covered in detail in a later section.

Enter a color: Red
Color code for red is #FF0000

Enter a color: Green
Color code for green is #008000

Enter a color: Magenta
magenta color is not supported

The final else is executed when none of the conditions above it are satisfied.

Exercise

Exercise

Write a program that takes two numbers and calculate their sum.

  • if both numbers are even, add them
  • if both numbers are odd, subtract larger number with smaller
  • if one number is odd and other is even, multiply them

Output the result of operation.

HINT:

If a number is even, the remainder of dividing it with 2 is always 0 otherwise it is 1. See section 2.4. Mathematical Operations: Other Operations for more information.

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n1 = int(input('Enter first number: '))
n2 = int(input('Enter second number: '))

if (n1 % 2 == 0) and (n2 % 2 == 0):
    # both numbers are even
    result = n1 + n2
elif (n1 % 2 == 1) and (n2 % 2 == 1):
    # both numbers are odd
    if n1 > n2:
        result = n1 - n2
    else:
        result = n2 - n1
else:
    result = n1 * n2

print(result)

Alternative approaches:

  • Line 9-12 could be replaced with result = abs(n2 - n1) which returns the absolute (unsigned) value of subtraction result.