Contents

• Overview
• General Form
• Example
• Practice Problems

Overview

So far we have learned two decisions structures, the if and the if-else. Now we'll consider the if-elif-else which allows us to chain numerous decision conditions into one decision structure. In programming, we often need to decide from more than just two possible options. For example, if we need to write a program that determines students' letter grades based on their scores, a True/False decision structure (if-else) will not suffice. This is where the if-elif-else structure comes in.

The general form of the if-elif-else statement looks like this:

if condition_1:
    code_block_based_on_condition_1
elif condition_2:
    code_block_based_on_condition_2
elif condition_3:
    code_block_based_on_condition_3
    .
    .
    .
elif condition_n:
    code_block_based_on_condition_n
else
    code_block_based_on_else

The key details to notice about this general form are the following:

• The initial if keyword which indicates to Python that it must evaluate an expression (condition_1) in order to make a decision.
• The condition_1 that follows the if keyword is the expression that must evaluate to True in order for the code_block_based_on_condition_1 to be run. If the condition evaluates to False, the elif condition (condition_2) is evaluated. If condition_2 is True then code_block_based_on_condition_2) after the elif: will be run instead.
• If condition_2 evaluates to False, then then elif condition_3 is evaluated. If condition_3 is True, then code_block_based_on_condition_3) after the elif: will be run instead.
• The progression each successive condition continues until one of the conditions evaluate to True.
• If none of the conditions are True, then the else code block will run. If there is no else included in the code, then the if statement will exit with none of the code blocks being run.
• The colons (:) at the end of the if statement, the elif's and the else statement are required syntactic element that indicates the end of those statements and the beginning of their associated code blocks.
• The indentation of the code blocks is a requirement in Python that indicates what line(s) of code are included in the code block.
• The code blocks can be from one to many lines of code.

For these problems, you may need to refer to the Comparison Operators and Logical Operators tables on the previous if statement page. You may also need the table operators on the Basic Operators page as well. These tables are also available in the Reference Materials.

Please enter an integer: -5
Your integer is negative.

int_type = ""
user_int = int(input("Please enter an integer: "))
if user_int < 0:
    int_type = "negative"
elif user_int == 0:
    int_type = "zero"
else:
    int_type = "positive"
print("Your integer is " + int_type + ".")

Please enter an integer: 55
Your integer is positive.

--------------------------------------------------
This program will prompt you for two integers,
then it will tell you their relationship:
--------------------------------------------------
Please enter your first integer: 10
Please enter your second integer: 20
Your first integer is smaller than your second integer.

response = ""
print("-" * 50)
print("This program will prompt you for two integers,")
print("then it will tell you about their relationship:")
print("-" * 50)
user_int_1 = int(input("Please enter your first integer: "))
user_int_2 = int(input("Please enter your second integer: "))
if user_int_1 < user_int_2:
    response = "smaller than"
elif user_int_2 == user_int_2:
    response = "equal to"
else:
    response = "larger than"
print("Your first integer is " + response + " your second integer.")

--------------------------------------------------
This program will prompt you for two integers,
then it will tell you their relationship:
--------------------------------------------------
Please enter your first integer: 10
Please enter your second integer: 20
Your first integer is smaller than your second integer.