Contents

• Overview
• General Form
• Comparison Operators
• Compound Conditions
• Practice Problem

Overview

The most fundamental decision construct we have in Python is the if statement, which will execute a block (one or more lines) of code if a specified condition evaluates to True. In English, we could describe this concept as "if this specific thing is true, then do the following." As an example, "if your score is 100, then print 'Perfect!'" In Python code, we would write this as:

if score == 100:
      print("Perfect!")

There are some new syntactic details to consider in this code (which we will address below) but, for now, pay more attention to the concept of "if this then do that." Keeping this in mind i sthe start of constructing decisions in Python.

The general form of the if statement looks like this:

if condition:
      code_block

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

• The if keyword which indicates to Python that it must evaluate an expression (condition) in order to make a decision.
• The condition that follows the if keyword is the expression that must evaluate to True in order for the code block to be run. If the condition evaluates to False, the code block will be skipped, it will not run, and program execution continues with any code that appears on the next non-indented line below the code block.
• The colon (:) at the end of the if statement line is a required syntactic element that indicates the end of the if statement and the beginning of the code block on the next line.
• The indentation of the code block is a requirement in Python that indicates what line(s) of code are included in the code block.
• The code block is from one to many lines of code that will run if the condition evaluates to True.

if condition:
    code_block_based_on_true_condition
else:
    code_block_based_on_false_condition

Notes:

• In this example, I included an initial print statement to display the start of the program to depict the execution flow.
• The user prompt collects a number from the user. Remember that anything entered at a user prompt defaults to a string, so I used the int() function to convert it to an integer.
• The if statement uses the double-equal comparison operator (==) to compare the user-entered guess with the secret_number.
• A common mistake in code like this is to use a single-equal symbol instead of the double-equal symbol. See the full list of comparison operators below for more information.
• Note that the print statement on line 7 is indented, in Python this is how we indicate what lines of code are part of the code_block "inside" of the decision statement (as indicated on the flowchart above).
• The last line is not indented, which means it is not part of the code_block inside of the decision statement.

In the example above we used the double-equal comparison symbol in our if statement condition. That symbol is one of the numerous comparison operators that exist in Python. Here is a list of all of them:

The if statement examples provided above contain a single condition (x == y for example), however, our conditions can be more complex and make several comparisons in one compound condition. To create more complex conditions, we need a way to connect more than one condition together, which we do with logical operators. Here is a list of the logical operators in Python:

Logical Operators

x = 10
y = 20
z = 30
if (x < y) and (z > x):

x = 10
y = 20
z = 30
if (x < y) and (z < x):

x = 10
y = 20
z = 30
if (x < y) and (z > y) and (z == (x + y)):

x = 10
y = 20
z = 30
if (x < y) or (z > x):

x = 10
y = 20
z = 30
if (x < y) or (z < x):

x = 10
y = 20
z = 30
if (x < y) or (z > y) or (z == (x + y)):

x = 10
y = 20
z = 30
if not((x < y) or (z > x)):

x = 10
y = 20
z = 30
if not((x < y) or (z < x)):

x = 10
y = 20
z = 30
if not((x < y) or (z > y) or (z == (x + y))):

Please enter your age: 43
You are middle-aged.

age = int(input("Please enter your age: "))
if (age > 39) and (age < 51):
    print("You are middle-aged.")

Please enter your age: 43

You are middle-aged.

The if statement works well when we have a single decision to make. That decision can be a single condition or a compound condition. However, the simple if only executes code if the condition result is True. We often need one code_block if the result is True and a different block of code if the result is False. To accomplish this, we will learn the if-else statement on the next page.