PCEP-30-02 Control Flow - Conditional Blocks and Loops

Control Flow - Conditional Blocks and Loops Detailed Explanation

1. Conditional Statements

Conditional statements allow your program to make decisions based on certain conditions. This is achieved using the if, if-else, and if-elif-else constructs.

1.1 If Statement

The if statement is the simplest form of a conditional. It checks a condition and executes the code block only if the condition is True.

Syntax:

if condition:
    # code block (executed if condition is True)

Example:

x = 10
if x > 5:
    print("x is greater than 5")

Explanation:

• The condition x > 5 is evaluated.
• If the condition is True, the indented code block will run.

More Examples:

#Check if a number is positive
number = 7
if number > 0:
    print("The number is positive")

1.2 If-Else Statement

The if-else statement provides an alternative block of code to execute if the condition is False.

Syntax:

if condition:
    # code block (executed if condition is True)
else:
    # alternative code block (executed if condition is False)

Example:

x = 3
if x > 5:
    print("x is greater than 5")
else:
    print("x is 5 or less")

Explanation:

• If x > 5 evaluates to True, the first block runs.
• If x > 5 evaluates to False, the else block runs.

More Examples:

#Check if a number is even or odd
number = 4
if number % 2 == 0:
    print("The number is even")
else:
    print("The number is odd")

1.3 If-Elif-Else Statement

When there are multiple conditions to check, use if-elif-else. It allows you to test several conditions in order, executing the first block with a True condition.

Syntax:

if condition1:
    # code block (executed if condition1 is True)
elif condition2:
    # code block (executed if condition1 is False and condition2 is True)
else:
    # alternative code block (executed if all conditions are False)

Example:

x = 10
if x > 15:
    print("x is greater than 15")
elif x > 5:
    print("x is greater than 5 but not greater than 15")
else:
    print("x is 5 or less")

Explanation:

• The program checks each condition in sequence.
• If a condition is True, its block runs, and the rest are skipped.

More Examples:

#Grading system based on score
score = 85
if score >= 90:
    print("Grade: A")
elif score >= 80:
    print("Grade: B")
elif score >= 70:
    print("Grade: C")
else:
    print("Grade: F")

1.4 Nested Conditions

Conditions can be nested inside each other to check multiple levels of logic.

Syntax:

if condition1:
    if condition2:
        # nested block (executed if condition1 and condition2 are True)

Example:

x = 10
if x > 5:
    if x % 2 == 0:
        print("x is greater than 5 and even")

Explanation:

• First, x > 5 is checked.
• If True, the nested condition x % 2 == 0 is checked.

More Examples:

#Check if a number is positive and even
number = 12
if number > 0:
    if number % 2 == 0:
        print("The number is positive and even")
    else:
        print("The number is positive but odd")
else:
    print("The number is not positive")

2. Loops

Loops allow you to repeat a block of code multiple times. Python supports two types of loops: for and while.

2.1 For Loop

The for loop iterates over a sequence (like a list, string, or range).

Syntax:

for item in sequence:
    # code block (executed for each item in the sequence)

Example:

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

Explanation:

• The loop goes through each item in the sequence (fruits).
• On each iteration, the variable fruit holds the current item.

More Examples:

#Print numbers from 0 to 4
for i in range(5):  # range(5) generates 0, 1, 2, 3, 4
    print(i)

2.2 While Loop

The while loop repeats a block of code as long as the condition is True.

Syntax:

while condition:
    # code block (executed while condition is True)

Example:

x = 0
while x < 5:
    print(x)
    x += 1  # Increment x to avoid an infinite loop

Explanation:

• The loop checks the condition x < 5.
• If True, the block runs, and x is incremented.
• The loop stops when x < 5 becomes False.

More Examples:

#Print numbers until the user enters "stop"
while True:
    user_input = input("Enter a number (or 'stop' to quit): ")
    if user_input == "stop":
        break
    print(f"You entered: {user_input}")

3. Loop Control Statements

Control statements modify the flow of loops.

3.1 Break

The break statement exits the nearest enclosing loop immediately.

Example:

for i in range(10):
    if i == 5:
        break  # Exit the loop when i equals 5
    print(i)

Output:

0
1
2
3
4

3.2 Continue

The continue statement skips the rest of the code in the current iteration and moves to the next iteration.

Example:

for i in range(5):
    if i == 3:
        continue  # Skip the iteration when i equals 3
    print(i)

Output:

0
1
2
4

3.3 Else in Loops

The else block in a loop runs if the loop is not terminated by a break.

Example:

for i in range(3):
    print(i)
else:
    print("Loop completed!")

Output:

0
1
2
Loop completed!

Example with break:

for i in range(3):
    if i == 2:
        break
    print(i)
else:
    print("Loop completed!")  # This won't execute

Output:

0
1

Control Flow - Conditional Blocks and Loops (Additional Content)

1. The else Clause with a while Loop

In Python, the else block can be used with both for and while loops. The else clause executes only if the loop completes normally, that is, without hitting a break statement.

While many learners see it used with for loops, it works the same way with while loops.

Example: while with else

x = 0
while x < 3:
    print(x)
    x += 1
else:
    print("While loop completed")

Output:

0  
1  
2  
While loop completed

Explanation:

• The loop runs while x < 3.

• After x reaches 3, the condition becomes false.

• Since the loop was not interrupted by a break, the else block runs.

With break inside the loop:

x = 0
while x < 5:
    if x == 3:
        break
    print(x)
    x += 1
else:
    print("This will NOT be printed")

Output:

0  
1  
2

In this case, the else block is skipped because the loop was terminated with break.

2. The pass Statement

The pass statement in Python is used as a placeholder. It does nothing when executed, but is syntactically required when a statement is needed, such as in an if, for, while, or function body.

Syntax:

if condition:
    pass  # Do nothing

Example:

x = 10
if x > 0:
    pass  # Placeholder for future logic
print("Program continues...")

Explanation:

• In situations where you're required to have a statement (e.g., after if, for, etc.) but haven't implemented logic yet, pass keeps the code syntactically valid.

This is especially common in:

• Empty function definitions during development

• Class method placeholders

• Branches in control flow yet to be implemented

Important for PCEP: PCEP might include a question with this option to test if you understand that pass is valid and does not produce an error.

3. while True and break: Simulating Indefinite Loops

This is a very common pattern in Python to simulate an indefinite loop that continues until a specific condition is met. It often replaces the traditional do-while loop found in other languages.

Pattern:

while True:
    user_input = input("Enter something (or 'exit' to quit): ")
    if user_input == "exit":
        break
    print(f"You entered: {user_input}")

Explanation:

• while True creates an infinite loop.

• The loop continues endlessly unless a break statement is encountered.

• It's useful when the termination condition depends on user input or runtime behavior, not something easily written in a while condition.

Summary Table

Feature	Syntax	Purpose
while ... else	while condition: ... else:	Executes else only if the loop is not broken
pass	if x > 0: pass	Does nothing, used as a placeholder
while True + break	while True: if condition: break	Infinite loop with manual termination