7. Conditionals & Relational Operators

Conditional statements are fundamental in programming, enabling a program to make decisions and execute different code based on specific conditions. This allows the program to respond to various situations, ensuring adaptive behavior.

These structures are the primary mechanism used to guide the flow of execution. Mastering them is essential for building dynamic and interactive programs based on Boolean conditions.

1. Basic Syntax and Flow Control

1.1. The if-else Syntax

Conditional structures allow a program to make decisions based on Boolean conditions (True or False). The two main components are the if and else blocks, which manage the flow of execution.

Key Elements:

• The if statement checks a condition; if True, its indented code block runs.
• The else block runs when the preceding if condition is False. (Note: The else block is optional.)
• Syntax Requirement: The basic if-else structure requires a Boolean condition and a colon (:), followed by an indented code block. Indentation is mandatory to define which statements belong to each conditional block.

if condition_is_true:
    # Code runs if the condition is True
    statement_1
else:
    # Code runs if the condition is False
    statement_2

1.2. Conditional Flow (if-else)

The following flowchart illustrates how a condition (the diamond) guides the execution flow. Only one path, the True or the False path, is ever taken before the program flow continues.

flowchart TD
    A[Start Program] --> B{Condition is True?};
    B -- True --> C[Execute Code in if Block];
    B -- False --> D[Execute Code in else Block];
    C --> E[Continue Program];
    D --> E;

1.3. Relational Operators

Relational operators allow Python to compare values and evaluate conditions. The most common ones are:

Operator	Description	Example
==	Equals	x == 10 → True if x is 10
!=	Not equal	x != 5 → True if x is not 5
>	Greater than	x > 3 → True if x is greater than 3
<	Less than	x < 7 → True if x is less than 7
>=	Greater than or equal	x >= 18 → True if x is 18 or older
<=	Less than or equal	x <= 100 → True if x is 100 or less

1.4. Conditional Expression (Ternary Operator)

Python's Conditional Expression provides a compact way to assign a value to a variable based on a single condition, all in one line of code.

The general syntax is:

value_if_true if condition else value_if_false

It simplifies the common if-else structure where the only action is assigning a value.

2. Commented Examples

2.1. Basic if Structure (No else)

# 1. Input: Get age from user and convert to integer
age = int(input("How old are you? "))

# 2. Decision: Check if the user meets the age requirement
if age >= 18:
    # 3. Output: Executes ONLY if the condition is True
    print("You can enter the party!") # Example Output: You can enter the party!
# If the condition is False, the program moves on without action.

Explanation:

• The basic if statement checks a single condition.
• The code inside the if block is only executed if the condition evaluates to True.
• If the condition is False, no action is taken.

2.2. Using if-else for Two Outcomes

# 1. Input: Get age from user and convert to integer
age = int(input("How old are you? "))

# 2. Decision: Check the condition
if age >= 18:
    # 3. Output (True Path): Executes if age is 18 or older
    print("You can enter the party!") # Example Output (True): You can enter the party!
else:
    # 4. Output (False Path): Executes if the condition is False
    print("Sorry, you cannot enter the party.") # Example Output (False): Sorry, you cannot enter the party.

Explanation:

• The if-else structure handles two mutually exclusive paths.
• The if block runs if the condition is True.
• The else block provides the default action and runs if the condition is False.

2.3. Conditional Expression (Ternary Operator)

# 1. Input: Get age from user and convert to integer
age = int(input("How old are you? "))

# 2. Decision and Assignment in One Line (Ternary Operator)
# Syntax: result_if_true if condition else result_if_false
status = "Adult" if age >= 18 else "Minor"

# 3. Output
print("Your status is:", status) # Example Output (age 20): Your status is: Adult

Explanation:

• The Conditional Expression is a shortcut for simple if-else statements where the primary goal is to assign a value to a variable.
• It is read from the middle: check the condition (age >= 18).
• If the condition is True, the expression returns the value before the if ("Adult").
• If the condition is False, the expression returns the value after the else ("Minor").

3. Short Practice Exercises

3.1. Prediction of Output

What is the final output of the following code block?

x = 10
if x > 5:
    print("A")
else:
    print("B")

Show solution

"A"

Explanation: The if condition (x > 5) is True, so the else block is skipped.

3.2. Error Correction

How should the code be corrected to check if current_state is equal to "running"?

current_state = "running"
# if current_state = "running":
#     print("System active")

Show solution

current_state = "running"
if current_state == "running":
    print("System active")

Explanation: The comparison operator == must be used instead of the assignment operator =:

3.3. Indentation Error

Identify the error in the following conditional structure:

user_status = True
if user_status:
print("Welcome")

Show solution

IndentationError

Explanation: The statement inside the if block (print("Welcome")) must be indented (usually 4 spaces).

3.4. Conditional Expression Output

What value is assigned to the variable message in the following line of code?

score = 95
message = "Passed" if score >= 60 else "Failed"

Show solution

"Passed"

Explanation: The condition (score >= 60) is True (95 is greater than 60), so the value before the if ("Passed") is assigned to the variable message.

4. Google Colab: Try It Yourself

Practice basic conditional statements (if and else) and relational operators with this Colab notebook:

👉 Open the Conditionals & Relational Operators Colab notebook ↗

First time using Google Colab? Read the quick beginner guide ↗

5. Mini-Quiz

5.1. What happens when an if condition evaluates to True?

A) The program stops immediately.
B) The else block is executed anyway.
C) Only the if block runs.
D) Both blocks run.

Show answer

C) Only the if block runs.

5.2. Which operator would you use to check if a number of absences is greater than or equal to 6?

A) =
B) ==
C) !=
D) >=

Show answer

D) >=

5.3. What is the primary role of the else block in a conditional structure?

A) Executes when the if condition is True.
B) Executes when the if condition is False.
C) Handles syntax errors.
D) Always executes.

Show answer

B) Executes when the if condition is False.

5.4. Which operator checks for equality in Python?

A) =
B) ==
C) !=
D) >=

Show answer

B) ==

5.5. What happens if the if block is not indented correctly?

A) Python ignores it.
B) Python raises an IndentationError.
C) It runs anyway.
D) The else block runs instead.

Show answer

B) Python raises an IndentationError.

6. Common Mistakes

• Using = instead of == for comparisons.
• Forgetting the colon : after if or else statements.
• Incorrect indentation of conditional blocks.
• Not converting input to int/float before comparing.

7. Summary Diagram

mindmap
  root((Conditional Structures))
    Flow Structures
      Block Structure
        if / else
      Single-Line Assignment
        Ternary Operator: A if C else B
    Building Blocks
      Relational Operators
        ==, <, >, >=, etc.
      Boolean Conditions
        True / False
    Key Rules
      Proper Indentation
      Only One Block Runs

8. Optional Extensions

• Write a program that asks the user for a number and prints whether it is positive or negative using if and else.
• Modify the age example above (Commented Example 2.2) to include a category for children (e.g., below 12) and print "Child" or "Adult".
• Research nested if statements and create a program that evaluates two conditions at once (e.g., age and temperature) and prints different messages accordingly.