Python Break and Continue Statements

When working with loops in Python, there are situations where you might want to:

• Skip the current iteration and move to the next one
• Exit the loop completely before its normal completion

Python provides two special statements for these situations: continue and break. These statements give you more fine-grained control over your loops, making your code more efficient and expressive.

The break Statement

The break statement allows you to exit a loop immediately, regardless of the loop condition.

How break Works

When Python encounters a break statement:

1. It immediately terminates the loop containing it
2. Program control flows to the statement immediately after the loop
3. If the break is inside nested loops, it only exits from the innermost loop

Basic Example of break

for number in range(1, 10):
    if number == 5:
        print("Found 5! Breaking the loop...")
        break
    print(f"Current number: {number}")
    
print("Loop ended")

Output:

Current number: 1
Current number: 2
Current number: 3
Current number: 4
Found 5! Breaking the loop...
Loop ended

Notice how the loop exits as soon as number equals 5, and the program continues with the statements after the loop.

The continue Statement

The continue statement skips the rest of the code inside the current loop iteration and jumps to the next iteration.

How continue Works

When Python encounters a continue statement:

1. It skips the remaining code in the current loop iteration
2. Control returns to the top of the loop for the next iteration
3. The loop condition is re-evaluated as usual

Basic Example of continue

for number in range(1, 10):
    if number % 2 == 0:  # If number is even
        continue  # Skip the rest of the loop for even numbers
    print(f"Current number: {number}")
    
print("Loop ended")

Output:

Current number: 1
Current number: 3
Current number: 5
Current number: 7
Current number: 9
Loop ended

In this example, the continue statement skips printing even numbers, so only odd numbers are displayed.

Practical Applications

Example 1: Input Validation with break

Here's how you might use break to validate user input:

while True:
    user_input = input("Enter a positive number (or 'q' to quit): ")
    
    if user_input.lower() == 'q':
        print("Exiting the program...")
        break
    
    try:
        number = float(user_input)
        if number <= 0:
            print("Please enter a POSITIVE number.")
        else:
            print(f"You entered: {number}")
    except ValueError:
        print("That's not a valid number. Try again.")

print("Program terminated.")

Sample Output:

Enter a positive number (or 'q' to quit): -5
Please enter a POSITIVE number.
Enter a positive number (or 'q' to quit): hello
That's not a valid number. Try again.
Enter a positive number (or 'q' to quit): 10.5
You entered: 10.5
Enter a positive number (or 'q' to quit): q
Exiting the program...
Program terminated.

Example 2: Processing a List with continue

Here's how you might use continue to filter items while processing a list:

def process_scores(scores):
    passing_scores = []
    
    for score in scores:
        # Skip invalid scores
        if score < 0 or score > 100:
            print(f"Invalid score detected: {score}. Skipping.")
            continue
            
        # Skip failing scores
        if score < 60:
            continue
            
        # Process passing scores
        passing_scores.append(score)
    
    if passing_scores:
        average = sum(passing_scores) / len(passing_scores)
        return f"Average of passing scores: {average:.2f}"
    else:
        return "No passing scores found."

# Test the function
test_scores = [75, 90, -5, 45, 110, 60, 82]
result = process_scores(test_scores)
print(result)

Output:

Invalid score detected: -5. Skipping.
Invalid score detected: 110. Skipping.
Average of passing scores: 76.75

Using break and continue with else in Loops

Python has a unique feature where you can use an else clause with loops. The else block executes when the loop completes normally (without encountering a break).

for number in range(2, 10):
    for x in range(2, number):
        if number % x == 0:
            print(f"{number} equals {x} * {number//x}")
            break
    else:
        # This runs if no break occurred (number is prime)
        print(f"{number} is a prime number")

Output:

2 is a prime number
3 is a prime number
4 equals 2 * 2
5 is a prime number
6 equals 2 * 3
7 is a prime number
8 equals 2 * 4
9 equals 3 * 3

Common Pitfalls and Best Practices

Pitfalls to Avoid

1. Infinite Loops: Be careful when using continue in while loops. Ensure your loop condition eventually becomes False.

# Problematic code - infinite loop!
counter = 0
while counter < 5:
    if counter < 3:
        continue  # Counter never increases if less than 3!
    counter += 1

2. Missing Loop Termination: Ensure loops with break will eventually terminate.

Best Practices

1. Use Comments: Always document why you're using break or continue.

2. Limit Usage: Excessive use of break and continue can make code harder to read.

3. Consider Alternatives: Sometimes restructuring your loop logic is clearer than using break or continue.

# Instead of:
for item in items:
    if not some_condition(item):
        continue
    # process item...

# Consider:
for item in items:
    if some_condition(item):
        # process item...

Summary

• break immediately exits the loop entirely
• continue skips the current iteration and moves to the next one
• These statements help you control loop flow with greater precision
• When combined with conditions, they make your loops more efficient and expressive
• The loop's else clause executes only if the loop completes normally (without break)

Practice Exercises

1. Write a program that asks the user for numbers until they enter 0, but skips negative numbers.

2. Create a program that looks for the first palindrome word in a list of words.

3. Implement a simple calculator that continues until the user enters "exit".

4. Write a program that generates random numbers until it gets a number divisible by both 3 and 5.

Additional Resources

• Python Documentation on Control Flow
• Python Loop Techniques
• Python Style Guide (PEP 8) for best practices