Loops in Python - For and While

 

Welcome to The Python Playground: Loops in Python - For and While

Introduction

Loops are an essential part of programming that allow you to repeat a block of code multiple times without writing it repeatedly. In Python, the two primary types of loops are for loops and while loops. These loops help make your code more efficient, readable, and flexible.

In this blog, we’ll break down the concept of loops, learn how to use them effectively, and explore real-world examples to understand their practical applications. By the end, you'll know how to leverage loops to write cleaner and more powerful Python programs.

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What Are Loops?

A loop is a programming construct that allows a block of code to run repeatedly as long as a specified condition is met. Python supports two types of loops:

1. for loops: Used to iterate over a sequence (like a list, tuple, dictionary, or string).

2. while loops: Used to repeat code as long as a condition is True.

Let’s dive into each type and see how they work.

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The for Loop

A for loop is used to iterate over a sequence (like a list or string). It’s perfect for cases where you know the number of iterations beforehand.

Syntax:

for item in sequence:
    # Code to execute

Example 1: Iterating Over a List

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

Output:

apple
banana
cherry

Here, the loop goes through each item in the list fruits and prints it.

Example 2: Using range()

The range() function generates a sequence of numbers, which is useful for running a loop a specific number of times.

for i in range(5):
    print(i)

Output:

0
1
2
3
4

By default, range(n) generates numbers from 0 to n-1.

Real-World Example: Sending Emails to Multiple Users

users = ["Alice", "Bob", "Charlie"]
for user in users:
    print(f"Sending email to {user}")

Output:

Sending email to Alice
Sending email to Bob
Sending email to Charlie

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The while Loop

A while loop runs as long as a specified condition is True. It’s ideal for cases where you don’t know in advance how many times the loop should run.

Syntax:

while condition:
    # Code to execute

Example 1: Counting Down

count = 5
while count > 0:
    print(count)
    count -= 1

Output:

5
4
3
2
1

Here, the loop continues until count becomes 0.

Example 2: Validating User Input

password = "python123"
user_input = ""

while user_input != password:
    user_input = input("Enter the password: ")

print("Access granted!")

Output:

Enter the password: wrongpassword
Enter the password: python123
Access granted!

This loop keeps prompting the user until they enter the correct password.

Real-World Example: Monitoring a Sensor

sensor_reading = 100
threshold = 80

while sensor_reading > threshold:
    print("Sensor value too high! Taking corrective action.")
    sensor_reading -= 5

Output:

Sensor value too high! Taking corrective action.
Sensor value too high! Taking corrective action.
Sensor value too high! Taking corrective action.

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Combining Loops with Conditional Statements

Loops often work hand-in-hand with conditional statements like if to create more complex logic.

Example: Printing Even Numbers

for num in range(10):
    if num % 2 == 0:
        print(f"{num} is even")

Output:

0 is even
2 is even
4 is even
6 is even
8 is even

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Breaking Out of Loops

Sometimes, you may want to exit a loop before it has completed all its iterations. Python provides two keywords for this:

1. break: Terminates the loop entirely.

2. continue: Skips the current iteration and moves to the next one.

Example: Using break

for num in range(10):
    if num == 5:
        break
    print(num)

Output:

0
1
2
3
4

Example: Using continue

for num in range(10):
    if num % 2 == 0:
        continue
    print(num)

Output:

1
3
5
7
9

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Infinite Loops

An infinite loop runs forever unless stopped manually or programmatically. Be cautious when writing loops to avoid accidentally creating infinite loops.

Example:

while True:
    print("This will run forever unless stopped!")
    break  # Adding a break to prevent infinite looping

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FAQs

Q1: Can I use else with loops?

Yes, Python allows an optional else block after for or while loops. The else block executes only if the loop completes normally (i.e., it doesn’t encounter a break).

Example:

for num in range(5):
    print(num)
else:
    print("Loop completed successfully!")

Output:

0
1
2
3
4
Loop completed successfully!

Q2: How do I loop through a dictionary?

You can loop through the keys, values, or both using .keys(), .values(), or .items().

Example:

student_scores = {"Alice": 85, "Bob": 92, "Charlie": 78}
for student, score in student_scores.items():
    print(f"{student} scored {score}")

Output:

Alice scored 85
Bob scored 92
Charlie scored 78

Q3: What’s the difference between for and while loops?

• for loops are used when you know the number of iterations in advance.

• while loops are used when the number of iterations depends on a condition.

Q4: Can I nest loops?

Yes, you can nest loops, but be cautious about complexity and performance.

Example:

for i in range(3):
    for j in range(2):
        print(f"i: {i}, j: {j}")

Output:

i: 0, j: 0
i: 0, j: 1
i: 1, j: 0
i: 1, j: 1
i: 2, j: 0
i: 2, j: 1

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Conclusion

Loops are a powerful feature in Python that allow you to automate repetitive tasks, iterate over data, and implement complex logic with ease. Whether you’re processing lists, validating inputs, or controlling hardware, loops are an essential tool in your programming arsenal.

Experiment with the examples provided and practice creating your own loops. Stay curious and keep exploring new ways to use loops in your projects. Happy coding on The Python Playground.