Python Loops

Contents:

1. Introduction to Loops in Python
2. for Loop in Python
3. While Loop in Python
4. Loop Control Statements in Python
5. break Statement in Python
6. continue Statement in Python
7. pass Statement in Python
8. Nested Loops in Python
9. Looping Through Different Data Structures in Python
10. Difference between for loop and while loop in Python
11. Best Practices for Using Loops in Python
12. FAQs on Loops in Python

Introduction to Loops in Python

Loops in Python are used to repeatedly execute a block of code until a condition is met. They reduce the need for manual repetition and make code more efficient. In Python, we primarily use for and while loops.

Types of Loops in Python

• for Loop: Iterates over a sequence (like lists, tuples, or strings).
• while Loop: Repeats as long as a condition is True.

for Loop in Python

The for loop iterates over a sequence (like a list, tuple, string, or range) and executes a block of code for each element.

Syntax of for Loop

for variable in sequence:
    # Code to execute in each iteration

Example:

subjects = ["Python", "Java", "C++"]
 
# Loop through subjects
for subject in subjects:
    print(f"Learning {subject}")

Output:

Learning Python
Learning Java
Learning C++

Using range() with for Loop:

The range() function generates a sequence of numbers, commonly used for looping a specific number of times.

Example:

for attempt in range(1, 4):
    print(f"Attempt {attempt}")

Output:

Attempt 1
Attempt 2
Attempt 3

Looping Through a String:

A for loop can be used to iterate through each character in a string. This is useful for performing operations on individual characters.

course = "Sanfoundry"
 
# Loop through each character
for char in course:
    print(char)

Output:

S
a
n
f
o
u
n
d
r
y

While Loop in Python

The while loop in Python is used to repeatedly execute a block of code as long as a given condition is True.

Syntax of while Loop

while condition:
    # Code to execute

Example:

# Sanfoundry Quiz Attempts
attempts = 1
 
# Loop until 3 attempts
while attempts <= 3:
    print(f"Attempt {attempts}: Retake Quiz")
    attempts += 1

Output:

Attempt 1: Retake Quiz
Attempt 2: Retake Quiz
Attempt 3: Retake Quiz

Loop Control Statements in Python

Loop control statements modify the normal execution flow of loops (for and while), allowing better control over iteration.

Types of Loop Control Statements

• break Statement: Terminates the loop immediately.
• continue Statement: Skips the current iteration and moves to the next one.
• pass Statement: Does nothing and acts as a placeholder.

break Statement in Python

The break statement in Python is used to exit a loop prematurely when a certain condition is met. It works with both for and while loops, stopping execution immediately and moving to the next line of code outside the loop.

Syntax of break Statement

for/while loop:
    if condition:
        break

Example:

# Sanfoundry Quiz Attempts
for attempt in range(1, 6):
    if attempt == 3:
        print("Quiz passed on attempt", attempt)
        break
    print(f"Attempt {attempt}: Retake required")

Output:

Attempt 1: Retake required
Attempt 2: Retake required
Quiz passed on attempt 3

Example: Using break in a while Loop

score = 40
 
# Increase score until passing
while score < 50:
    print("Score too low. Retake required.")
    score += 5
    if score >= 50:
        print("Passed!")
        break

Output:

Score too low. Retake required.
Score too low. Retake required.
Passed!

continue Statement in Python

The continue statement is used to skip the current iteration of a loop and move to the next iteration. It does not terminate the loop but skips any remaining code for that iteration.

Syntax of continue Statement

for/while loop:
    if condition:
        continue
    # Code to execute if condition is False

Example

# Sanfoundry Quiz Results
subjects = {"Python": 85, "Java": 40, "C++": 90, "DBMS": 45}
 
# Skip failed subjects
for subject, score in subjects.items():
    if score < 50:
        continue
    print(f"{subject}: Passed with score {score}")

Output:

Python: Passed with score 85
C++: Passed with score 90

Example: Using continue in a while Loop

# List of Course Progress
progress = [100, 50, 75, 30, 90]
i = 0
 
# Skip incomplete courses
while i < len(progress):
    if progress[i] < 60:
        i += 1
        continue
    print(f"Course {i + 1}: Completed with {progress[i]}% progress")
    i += 1

Output:

Course 1: Completed with 100% progress
Course 3: Completed with 75% progress
Course 5: Completed with 90% progress

pass Statement in Python

The pass statement is a placeholder that does nothing when executed. It is used when a block of code is syntactically required but not implemented yet.

Syntax of pass Statement

if condition:
    pass  # Placeholder for future code

Example:

# Course List
courses = ["Python", "Java", "C++"]
 
for course in courses:
    if course == "Java":
        pass  # Code for Java will be added later
    else:
        print(f"Learning {course}")

Output:

Learning Python
Learning C++

Example with Function

# Placeholder function for future logic
def check_score(score):
    if score < 50:
        pass  # Logic to handle low scores will be added later
    else:
        print("Score is acceptable!")

Output:

(No output if score < 50, otherwise prints message)

Pass in a Class Example

# Placeholder class for future features
class Quiz:
    pass
 
# Create object
q = Quiz()
print("Quiz object created!")

Output:

Quiz object created!

Nested Loops in Python

Nested loops are loops inside another loop. The inner loop is executed completely for each iteration of the outer loop.

Syntax of Nested Loops

for outer_variable in outer_sequence:
    for inner_variable in inner_sequence:
        # Code to execute

Example 1:

# Sanfoundry Quiz Attempts for Subjects
subjects = ["Python", "DBMS"]
attempts = [1, 2, 3]
 
for subject in subjects:
    for attempt in attempts:
        print(f"{subject} Quiz - Attempt {attempt}")

Output:

Python Quiz - Attempt 1  
Python Quiz - Attempt 2  
Python Quiz - Attempt 3  
DBMS Quiz - Attempt 1  
DBMS Quiz - Attempt 2  
DBMS Quiz - Attempt 3

Example 2:

# Sanfoundry Retake Attempts
subject_count = 2
attempt_count = 3
 
i = 1
while i <= subject_count:
    j = 1
    while j <= attempt_count:
        print(f"Subject {i} - Retake {j}")
        j += 1
    i += 1

Output:

Subject 1 - Retake 1  
Subject 1 - Retake 2  
Subject 1 - Retake 3  
Subject 2 - Retake 1  
Subject 2 - Retake 2  
Subject 2 - Retake 3

Example 3: Multiplication Table

# Multiplication Table for 2 and 3
for num in [2, 3]:
    for i in range(1, 4):
        print(f"{num} x {i} = {num * i}")
    print("----")

Output:

2 x 1 = 2  
2 x 2 = 4  
2 x 3 = 6  
----  
3 x 1 = 3  
3 x 2 = 6  
3 x 3 = 9  
----

Looping Through Different Data Structures in Python

Python provides various ways to iterate through different data structures like lists, tuples, dictionaries, and sets.

1. Looping Through a List

# List of Sanfoundry Subjects
subjects = ["Python", "Java", "DBMS"]
 
for subject in subjects:
    print(f"Learning {subject}")

Output:

Learning Python  
Learning Java  
Learning DBMS

2. Looping Through a Tuple

topics = ("OS", "DSA", "Networks")
 
for topic in topics:
    print(f"Studying {topic}")

Output:

Studying OS  
Studying DSA  
Studying Networks

3. Looping Through a Dictionary

scores = {"Python": 85, "Java": 90, "DBMS": 78}
 
for subject, score in scores.items():
    print(f"{subject} Score: {score}")

Output:

Python Score: 85  
Java Score: 90  
DBMS Score: 78

4. Looping Through a Set

# Set of Sanfoundry Certifications
certs = {"AI", "ML", "Cloud"}
 
for cert in certs:
    print(f"Certified in {cert}")

Output:

Certified in AI  
Certified in ML  
Certified in Cloud

Difference between for loop and while loop in Python

Here is the comparison between for loop and while loop in Python:

Best Practices for Using Loops in Python

• Use for loops when looping through a list or range and while loops when the number of repeats is unknown.
• Always set a clear stopping condition in while loops to avoid infinite loops.
• Keep loops simple by reducing extra calculations inside them.
• Use list comprehensions to write shorter and faster loops when creating new lists.
• Try enumerate() instead of a manual counter when looping with an index.
• Use break and continue only when necessary to keep the code easy to follow.
• For large data, use generator expressions to save memory and run loops efficiently.

FAQs on Loops in Python

1. What are loops in Python?
Loops in Python allow executing a block of code multiple times until a condition is met. The two main types are for loops and while loops.

2. What is the difference between for and while loops?

• A for loop iterates over a sequence (like a list, tuple, or string) and runs for a fixed number of times.
• A while loop runs as long as a specified condition is True.

3. How do I exit a loop early in Python?
You can use the break statement to immediately stop a loop when a certain condition is met.

4. What is the purpose of the continue statement?
The continue statement skips the current iteration and moves to the next one, without exiting the loop entirely.

5. Can loops be nested in Python?
Yes, you can place one loop inside another. This is useful for working with multi-dimensional data, like nested lists.

6. How can I make loops more efficient?
Avoid unnecessary calculations inside loops, use list comprehensions when possible, and prefer generator expressions for handling large data sets.

7. What is an infinite loop in Python?
An infinite loop occurs when the loop condition always evaluates to True, causing the loop to run indefinitely.
Example:

while True:
    print("This is an infinite loop")

To stop it, press Ctrl + C or close the program.