2.2 Python Loops

Loops are a fundamental programming concept that allows you to repeatedly execute a block of code. They are essential for automating repetitive tasks and processing collections of data efficiently.

What Are Loops?

Normally, Python executes code sequentially, from top to bottom. Loops provide a way to break this linear execution, enabling you to repeat actions multiple times. This is incredibly useful for tasks like printing values, performing calculations on multiple items, or processing data stored in sequences.

Types of Loops in Python

Python offers three primary types of loops:

• while loop: Repeats a block of code as long as a specified condition remains True.
• for loop: Iterates over items in a sequence (like lists, strings, or ranges) or any iterable object.
• Nested loops: One loop placed inside another, often used for working with multi-dimensional data structures.

1. while Loop

The while loop continues to execute its block of code as long as the given condition evaluates to True. It's crucial to ensure that the condition eventually becomes False, otherwise, you'll create an infinite loop.

Syntax:

while condition:
    # Block of code to be executed
    # Ensure the condition will eventually become False

Example:

count = 1
while count <= 3:
    print("Loop iteration:", count)
    count += 1

Output:

Loop iteration: 1
Loop iteration: 2
Loop iteration: 3

In this example, the loop starts with count as 1. It prints the current value of count and then increments count by 1. The loop continues as long as count is less than or equal to 3. Once count becomes 4, the condition count <= 3 is no longer True, and the loop terminates.

2. for Loop

The for loop is designed to iterate over the items of any sequence or other iterable object in the order that they appear in the sequence.

Syntax:

for variable in sequence:
    # Block of code to be executed for each item in the sequence

Example with a List:

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

Output:

I like apple
I like banana
I like cherry

Here, the loop iterates through each element in the fruits list. In each iteration, the current element is assigned to the fruit variable, and the code block prints a statement including that fruit.

Example with range():

The range() function is commonly used with for loops to generate a sequence of numbers. range(start, stop, step) generates numbers from start up to (but not including) stop, with an optional step.

for i in range(1, 4):  # Generates numbers 1, 2, 3
    print("Number:", i)

Output:

Number: 1
Number: 2
Number: 3

This loop iterates from 1 up to (but not including) 4, printing each number.

3. Nested Loops

A nested loop occurs when you place one loop inside another loop. This is particularly useful when you need to perform actions based on combinations of items or work with multi-dimensional data structures like matrices.

Example:

for i in range(1, 4):  # Outer loop
    for j in range(1, 3):  # Inner loop
        print(f"i={i}, j={j}")

Output:

i=1, j=1
i=1, j=2
i=2, j=1
i=2, j=2
i=3, j=1
i=3, j=2

In this example, the outer loop iterates three times (for i = 1, 2, 3). For each iteration of the outer loop, the inner loop completes all of its iterations (for j = 1, 2).

Python Loop Control Statements

Python provides special keywords that allow you to alter the normal flow of loops:

• break: Terminates the loop entirely, even if the loop's condition is still met or there are more items in the sequence.
• continue: Skips the rest of the current iteration and proceeds to the next iteration of the loop.
• pass: Acts as a placeholder. It does nothing but is syntactically required when a statement is needed but no action should be taken. This is often used during development.

1. break Statement

The break statement is used to exit a loop prematurely.

Example:

for num in range(1, 10):
    if num == 5:
        break  # Exit the loop when num is 5
    print(num)

Output:

1
2
3
4

When num reaches 5, the break statement is executed, and the loop terminates immediately. The numbers 5 through 9 are not printed.

2. continue Statement

The continue statement skips the current iteration of the loop and moves to the next one.

Example:

for num in range(1, 6):
    if num == 3:
        continue  # Skip the rest of the code for num = 3
    print(num)

Output:

1
2
4
5

When num is 3, the continue statement is executed. This causes the print(num) line in the current iteration to be skipped, and the loop proceeds directly to the next iteration (where num becomes 4).

3. pass Statement

The pass statement is a null operation; when it is executed, nothing happens. It's used as a placeholder.

Example:

for i in range(3):
    pass  # Placeholder for future logic
    # print("This line will not be reached if pass is executed")

This code will run without error. If you uncomment the print statement, it would execute. However, as it stands, the pass statement satisfies the requirement for a statement within the loop block, but no actual operation is performed for each iteration.

Summary Table: Python Loops and Controls

Conclusion

Loops are an indispensable tool in Python for automating repetitive tasks and making your code more efficient and readable. Mastering the different loop types (while, for) and control statements (break, continue, pass) is crucial for writing effective and robust Python programs.

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Interview Questions

• What is the difference between a for loop and a while loop in Python?
• How does a while loop work in Python? Provide an example.
• When should you use a for loop instead of a while loop?
• What is a nested loop? Give a Python example.
• What is the purpose of the break statement in Python loops?
• Explain the difference between break and continue in Python.
• How does the pass statement work in Python loops?
• Write a Python program using a loop to print numbers 1 to 10, but skip 5.
• Can we use else with loops in Python? How does it behave?
• How do you iterate through a list in Python using a loop?