New to Programming? No Worries! Start Learning Python from Scratch — Part 3 : Mastering Python Loops

Loops are a fundamental concept in Python, enabling you to automate repetitive tasks and handle data efficiently. With `for` and `while` loops, you can iterate over collections, repeat actions until conditions are met, and significantly reduce code complexity. Mastering loops allows you to write more efficient and elegant Python code, making your programming tasks easier and more powerful.

Loops in Python

Mastering Loops in Python

For Loop

The for loop in Python is used to iterate over elements in a sequence, such as a list, and perform actions on each element.

Here’s a block of code to sum numbers in a list using a for loop:

numbers = [1, 2, 3, 4, 5]  # List of numbers to be summed
total_sum = 0              # Initialize the sum variable

for num in numbers:        # Loop through each number in the list
    total_sum += num       # Add the current number to the total sum

# After the loop, total_sum contains the sum of all numbers

While Loop

As mentioned above, we use a `for` loop when we know beforehand how many times we want to repeat a block of code. It’s great for iterating over items in a list, a range of numbers, or any sequence where the number of iterations is fixed.

But what about when we do not know beforehand?

The `while` loop is used when we do not know in advance how many times the loop should run. Instead of iterating over a fixed sequence, the `while` loop continues running as long as a certain condition is true. This makes it ideal for situations where we need the loop to continue until something specific happens, like waiting for a user to input a negative number to stop the loop.

Here’s a program that uses a while loop to continuously ask the user for a number and adds it to a sum until the user enters a negative number:

total_sum = 0  
# Start with zero as the total sum
number = int(input("Enter a number (negative to stop): "))
# Get the first number

while number >= 0:  
# Continue as long as the number is not negative
    total_sum += number  
    # Add the current number to the total sum
    number = int(input("Enter another number (negative to stop): ")) 
    # Get the next number

# Now, total_sum contains the sum of all the positive numbers you entered

Controlling Loop Execution with `break` and `continue`

The break and continue statements in Python are used to control the flow of loops. The break statement is used to exit the loop entirely when a certain condition is met, while the continue statement skips the rest of the code inside the loop for the current iteration and moves on to the next iteration.

Flowchart of continue in Python

Flowchart of break in Python

Here’s an example of how both break and continue work:

for number in range(1, 10):
    if number == 5:
        break           # Exit the loop when number is 5
    if number % 2 == 0:
        continue        # Skip the rest of the loop if number is even
    print(number)

Explanation Of The Above Code

The loop starts by iterating over numbers from 1 to 9.

For each number:

• If the number is 5, the break statement is triggered, and the loop exits immediately, so no further numbers are processed.
• If the number is even (like 2, 4), the continue statement is triggered, which skips the print statement for that iteration and moves to the next number.
• If the number is odd and not 5, it gets printed.

So, the numbers 1 and 3 are printed before the loop reaches 5 and breaks, stopping further execution.