Loop Control: break, continue, pass

Why read this? Sometimes you need to exit a loop early (found what you needed), skip an iteration (invalid data), or do nothing at all (placeholder). Python gives you break, continue, and pass for exactly these situations.

🎯 Learning Objectives

1. Exit loops early with break
2. Skip to the next iteration with continue
3. Use pass as a placeholder for empty code blocks
4. Use the else clause with loops (triggered on normal completion)
5. Write robust input validation loops

📋 Prerequisites

• While Loops and For Loops
• Understanding of loop mechanics.

📖 Core Content

12.1 The break Statement — Exiting Early

python
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# runnable
numbers = [3, 7, 1, 9, 4, 2, 8]
target = 9
for num in numbers:
    print(f"Checking {num}")
    if num == target:
        print(f"Found {target}! Stopping.")
        break

pseudo
Checking 3
Checking 7
Checking 1
Checking 9
Found 9! Stopping.

12.2 The continue Statement — Skip This Iteration

python
Run Inline
# runnable
for num in range(1, 11):
    if num % 3 == 0:
        continue  # skip multiples of 3
    print(num, end=" ")

pseudo
1 2 4 5 7 8 10

12.3 The pass Statement — Do Nothing

python
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# runnable
for i in range(5):
    pass  # TODO: implement later
if True:
    pass  # placeholder
def not_implemented_yet():
    pass  # allows empty function body

12.4 Loop else — "No Break Occurred"

python
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# runnable
# Search for a prime factor
n = 17
for i in range(2, int(n**0.5) + 1):
    if n % i == 0:
        print(f"{n} is divisible by {i}")
        break
else:
    print(f"{n} is prime!")  # runs because no break

12.5 Worked Example 1: Input Validation with while-break

python
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# runnable
while True:
    age = input("Enter your age (or 'quit' to exit): ")
    if age.lower() == 'quit':
        print("Goodbye!")
        break
    try:
        age = int(age)
        if 0 <= age <= 150:
            print(f"You are {age} years old.")
            break
        else:
            print("Age must be 0-150.")
    except ValueError:
        print("Invalid input. Enter a number or 'quit'.")

12.6 Worked Example 2: Processing Data with continue

python
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# runnable
data = ["apple", "", "banana", None, "cherry", "", "date"]
for item in data:
    if not item:  # skip None, empty strings
        continue
    print(f"Processing: {item}")

pseudo
Processing: apple
Processing: banana
Processing: cherry
Processing: date

12.7 Worked Example 3: Early Exit in Search

python
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# runnable
def is_prime(n):
    if n < 2:
        return False
    for i in range(2, int(n**0.5) + 1):
        if n % i == 0:
            return False  # like break + return
    return True
# Find first 10 primes
count = 0
num = 2
while count < 10:
    if is_prime(num):
        print(num, end=" ")
        count += 1
    num += 1

12.8 Worked Example 4: Nested Loop Break

python
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# runnable
# Find first pair whose product > 50
for a in range(1, 11):
    for b in range(1, 11):
        if a * b > 50:
            print(f"Found: {a} × {b} = {a*b}")
            break  # breaks inner loop only
    else:
        continue  # if inner didn't break, continue outer
    break  # if inner broke, break outer too

pseudo
Found: 6 × 9 = 54

12.9 Worked Example 5: Menu System

python
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# runnable
while True:
    print("\n--- MENU ---")
    print("1. Say Hello")
    print("2. Show date")
    print("3. Exit")
    choice = input("Choose (1-3): ")
    if choice == '1':
        print("Hello!")
        continue  # go back to menu
    elif choice == '2':
        import datetime
        print(f"Today: {datetime.date.today()}")
        continue
    elif choice == '3':
        print("Goodbye!")
        break
    else:
        print("Invalid choice, try again.")

📐 Key Concepts Reference

⚠️ Common Pitfalls

Pitfall 1: break Only Breaks the Innermost Loop

Pitfall 2: continue in While Loop Forgets Update

python
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i = 0
while i < 10:
    if i % 2 == 0:
        continue  # skips i += 1!
    i += 1

Pitfall 3: Confusing pass with continue

📝 Practice Questions

Q1: What does this output?

python
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for i in range(5):
    if i == 3:
        break
    print(i)

Answer:

pseudo
0
1
2

When i=3, break exits the loop. So 3 and 4 are never printed. Q2: What does this output?

python
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for i in range(5):
    if i == 3:
        continue
    print(i)

Answer:

pseudo
0
1
2
4

continue skips printing for i=3, but loop continues with i=4. Q3: What's the difference between pass and continue?

Answer:

• pass does nothing. Execution continues to the next line. Used for empty blocks.
• continue skips the rest of the current iteration and goes to the next loop iteration. Used only inside loops. Q4: When does the else clause of a loop run?

Answer: The else runs when the loop condition becomes False (for while) or when the sequence is exhausted (for for), AND no break statement was executed. Q5: Write a loop that finds and prints the first number divisible by 7 between 1 and 100.

Answer:

python
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# runnable
for i in range(1, 101):
    if i % 7 == 0:
        print(f"First number divisible by 7: {i}")
        break

Q6: What's wrong with this code?

python
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i = 0
while i < 5:
    if i == 2:
        continue
    print(i)
    i += 1

Answer: Infinite loop! When i=2, continue skips i += 1, so i stays 2 forever. Fix: increment before the continue, or use a for loop. Q7: Write code that prints all numbers 1-20 EXCEPT multiples of 3 (use continue).

Answer:

python
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# runnable
for i in range(1, 21):
    if i % 3 == 0:
        continue
    print(i, end=" ")

Output: 1 2 4 5 7 8 10 11 13 14 16 17 19 20 Q8: What does this output?

python
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for i in range(3):
    for j in range(3):
        if j == 1:
            break
        print(f"({i},{j})", end=" ")
    print()

Answer:

pseudo
(0,0)
(1,0)
(2,0)

The inner break exits the inner loop when j=1, so only j=0 is printed for each i. Q9: Write a while loop that keeps asking for input until "yes" or "no" is entered.

Answer:

python
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# runnable
while True:
    answer = input("Enter yes or no: ").lower()
    if answer in ("yes", "no"):
        print(f"You entered: {answer}")
        break
    print("Invalid, try again.")

Q10: Write a program that prints all prime numbers up to 50 using a loop with else.

Answer:

python
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# runnable
for n in range(2, 51):
    for i in range(2, int(n**0.5) + 1):
        if n % i == 0:
            break
    else:
        print(n, end=" ")

Output: 2 3 5 7 11 13 17 19 23 29 31 37 41 43 47

🔗 Cross-References

• Next Topic: Pattern Printing & Formatted Output
• Previous Topic: Nested Loops & Loop Patterns
• Reference: Python for Everybody, Chapter 5 (Section 5.4)
• Video: L41: Break continue & pass in python explained with examples Join Discord Previous11. Nested LoopsNext13. Pattern Printing