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Loops: for and while

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In this section, we will learn about loops in Python, specifically the for and while loops. Loops are fundamental control structures that allow us to execute a block of code multiple times. This is particularly useful for tasks that require repetitive actions, such as iterating over a list or performing a calculation multiple times.

Key Concepts

  1. For Loop: Used for iterating over a sequence (such as a list, tuple, dictionary, set, or string).
  2. While Loop: Repeats a block of code as long as a condition is true.
  3. Loop Control Statements: break, continue, and else clauses in loops.

For Loop

The for loop in Python is used to iterate over a sequence of elements. The syntax is:

for variable in sequence:
    # code block to be executed

Example: Iterating Over a List

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

Explanation:

  • fruits is a list containing three elements.
  • The for loop iterates over each element in the list and assigns it to the variable fruit.
  • The print(fruit) statement is executed for each element in the list.

Example: Iterating Over a String

for letter in "Python":
    print(letter)

Explanation:

  • The for loop iterates over each character in the string "Python".
  • Each character is printed on a new line.

While Loop

The while loop in Python is used to execute a block of code as long as a condition is true. The syntax is:

while condition:
    # code block to be executed

Example: Counting with a While Loop

count = 0
while count < 5:
    print(count)
    count += 1

Explanation:

  • The while loop continues to execute as long as count is less than 5.
  • The print(count) statement prints the current value of count.
  • The count += 1 statement increments count by 1 in each iteration.

Loop Control Statements

Break Statement

The break statement is used to exit a loop prematurely.

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

Explanation:

  • The loop iterates over numbers from 0 to 9.
  • When number equals 5, the break statement is executed, and the loop terminates.
  • Numbers 0 to 4 are printed.

Continue Statement

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

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

Explanation:

  • The loop iterates over numbers from 0 to 9.
  • If number is even (number % 2 == 0), the continue statement skips the current iteration.
  • Only odd numbers are printed.

Else Clause in Loops

The else clause in a loop is executed when the loop terminates normally (i.e., not by a break statement).

for number in range(5):
    print(number)
else:
    print("Loop completed")

Explanation:

  • The loop iterates over numbers from 0 to 4.
  • After the loop completes, the else clause is executed, printing "Loop completed".

Practical Exercises

Exercise 1: Sum of Numbers

Write a program to calculate the sum of numbers from 1 to 10 using a for loop.

# Solution
total = 0
for number in range(1, 11):
    total += number
print("Sum:", total)

Exercise 2: Factorial Calculation

Write a program to calculate the factorial of a given number using a while loop.

# Solution
number = 5
factorial = 1
while number > 0:
    factorial *= number
    number -= 1
print("Factorial:", factorial)

Exercise 3: Find Prime Numbers

Write a program to find all prime numbers between 1 and 20 using a for loop and else clause.

# Solution
for num in range(2, 21):
    for i in range(2, num):
        if num % i == 0:
            break
    else:
        print(num, "is a prime number")

Common Mistakes and Tips

  • Infinite Loops: Ensure that the condition in a while loop will eventually become false. Otherwise, the loop will run indefinitely.
  • Off-by-One Errors: Be careful with the range boundaries in loops to avoid missing elements or iterating too many times.
  • Using Break and Continue: Use break and continue statements judiciously to control the flow of loops.

Conclusion

In this section, we covered the basics of for and while loops in Python. We learned how to iterate over sequences, control loop execution with break and continue statements, and use the else clause in loops. By practicing the provided exercises, you should now have a solid understanding of how to use loops effectively in your Python programs.

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Python Programming Course

Module 1: Introduction to Python

Module 2: Control Structures

Module 3: Functions and Modules

Module 4: Data Structures

Module 5: Object-Oriented Programming

Module 6: File Handling

Module 7: Error Handling and Exceptions

Module 8: Advanced Topics

Module 9: Testing and Debugging

Module 10: Web Development with Python

Module 11: Data Science with Python

Module 12: Final Project

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