Python Programming Loops For While And Loop Control Statements Complete Guide

1. For Loop

Example: Printing Numbers from 1 to 5

# Step 1: Understand the range() function
# The range(start, stop, step) generates numbers starting from 'start' up to but not including 'stop',
# incrementing by 'step'. If only one argument is given, it generates from 0 up to but not including that number.

# Step 2: Use a for loop to iterate over the range of numbers
for i in range(1, 6):
    print("Number:", i)

# Output:
# Number: 1
# Number: 2
# Number: 3
# Number: 4
# Number: 5

Explanation:

• In this example, range(1, 6) generates numbers from 1 to 5.
• for i in range(1, 6): starts a loop where i takes each successive value (1, 2, 3, 4, 5).
• print("Number:", i) prints the current value of i.

Example: Iterating Over a List

# Step 1: Create a list of fruits
fruits = ["apple", "banana", "cherry"]

# Step 2: Use a for loop to iterate over the list
for fruit in fruits:
    print(f"Fruit: {fruit}")

# Output:
# Fruit: apple
# Fruit: banana
# Fruit: cherry

Explanation:

• fruits is a list containing three elements.
• for fruit in fruits: iterates over each element in the list and assigns it to the variable fruit.
• print(f"Fruit: {fruit}") prints each fruit name.

2. While Loop

Example: Counting down from 5 to 1

# Step 1: Initialize the counter variable
count = 5

# Step 2: Start the while loop
while count > 0:
    print("Count:", count)
    count -= 1  # Decrement the counter in each iteration

# Output:
# Count: 5
# Count: 4
# Count: 3
# Count: 2
# Count: 1

Explanation:

• count is initialized to 5.
• while count > 0: continues to execute as long as count is greater than 0.
• print("Count:", count) prints the current value of count.
• count -= 1 decrements count by 1 in each iteration.

Example: Reading Input Until a Condition is Met

# Step 1: Initialize an empty string to store user input
user_input = ""

# Step 2: Start the while loop
while user_input != "exit":
    user_input = input("Enter something ('exit' to quit): ")
    if user_input != "exit":
        print("You entered:", user_input)

# Output:
# Enter something ('exit' to quit): hello
# You entered: hello
# Enter something ('exit' to quit): world
# You entered: world
# Enter something ('exit' to quit): exit

Explanation:

• user_input is initialized as an empty string.
• while user_input != "exit": keeps looping until the user enters "exit".
• input("Enter something ('exit' to quit): ") prompts the user for input.
• if user_input != "exit": print("You entered:", user_input) checks if the user didn't enter "exit" and prints the input otherwise.

3. Loop Control Statements

Break Statement: Stops the loop entirely.

Example: Break out of a for loop when a condition is met

# Step 1: Iterate over numbers from 1 to 10
for num in range(1, 11):
    if num == 5:  # Step 2: Check condition
        break  # Step 3: Exit the loop if condition is met
    print(num)  # Step 4: Print the current number

# Output:
# 1
# 2
# 3
# 4

Explanation:

• The loop runs from 1 to 10.
• When num equals 5, the break statement terminates the loop.
• Thus, only numbers less than 5 are printed.

Continue Statement: Skips the current iteration but does not terminate the loop.

Example: Skip printing even numbers using a continue statement

# Step 1: Iterate over numbers from 1 to 10
for num in range(1, 11):
    if num % 2 == 0:  # Step 2: Check if the number is even
        continue  # Step 3: Skip the current iteration if condition is met
    print(num)  # Step 4: Print the current number if it's not even

# Output:
# 1
# 3
# 5
# 7
# 9

Explanation:

• The loop runs from 1 to 10.
• When num is even (num % 2 == 0), the continue statement skips the rest of the loop body and moves to the next iteration.
• Only odd numbers are printed.

Pass Statement: Does nothing and is often used as a placeholder.

Example: Using pass to skip certain conditions

# Step 1: Iterate over numbers from 1 to 5
for num in range(1, 6):
    if num == 3:  # Step 2: Check if the number is equal to 3
        pass  # Step 3: Do nothing if condition is met
    else:
        print(num)  # Step 4: Print the current number if condition is not met

# Output:
# 1
# 2
# 4
# 5

Explanation:

• The loop runs from 1 to 5.
• When num equals 3, the pass statement does nothing and the loop proceeds to the next iteration without printing anything.
• All other numbers are printed.

Summary

• For Loop: Iterates over a sequence (like a range or list).
• While Loop: Repeats as long as a condition is true.
• Break Statement: Terminates the loop immediately.
• Continue Statement: Skips the current iteration and moves to the next.
• Pass Statement: Does nothing; serves as a placeholder.