0020-while-loops-in-python.md

While Loops in Python

A while loop in Python repeatedly executes a block of code as long as a condition remains True.
The loop stops automatically when the condition becomes False.

while loops are most useful when:

• The number of iterations is unknown
• Execution depends on a changing condition
• You want to keep running until something happens (user input, event, state change)

while condition:
    # code runs while condition is True

---

Core Concepts Behind while Loops

Concept	Meaning
Condition	Expression evaluated as True or False
Loop body	Code executed repeatedly
Iteration	One cycle of execution
Exit condition	Condition that stops the loop
Infinite loop	Condition never becomes False

---

1. Basic while Loop

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

Output:

1
2
3
4
5

count += 1 is critical — without it, the loop would run forever.

---

2. Infinite while Loop

An infinite loop runs forever unless explicitly stopped.

while True:
    print("This loop will run forever")
    break

Output:

This loop will run forever

True never becomes False, so you must use break.

---

3. while Loop with else

The else block executes only if the loop ends normally (not via break).

x = 0
while x < 3:
    print(x)
    x += 1
else:
    print("Loop has finished")

Output:

0
1
2
Loop has finished

---

while-else with break

x = 0
while x < 3:
    if x == 1:
        break
    print(x)
    x += 1
else:
    print("Loop has finished")

Output:

0

else does not run because the loop exited via break.

---

4. Using break in a while Loop

break immediately terminates the loop.

x = 0
while x < 10:
    if x == 5:
        break
    print(x)
    x += 1

Output:

0
1
2
3
4

---

5. Using continue in a while Loop

continue skips the current iteration and jumps back to the condition check.

x = 0
while x < 5:
    x += 1
    if x == 3:
        continue
    print(x)

Output:

1
2
4
5

3 is skipped.

---

6. while Loop with User Input

Common real-world use case: keep asking until correct input.

password = ""
while password != "secret":
    password = input("Enter the password: ")

print("Access granted!")

Loop continues until the correct password is entered.

---

7. Nested while Loops

A while loop inside another while loop.

i = 1
while i <= 3:
    j = 1
    while j <= 2:
        print(f"i = {i}, j = {j}")
        j += 1
    i += 1

Output:

i = 1, j = 1
i = 1, j = 2
i = 2, j = 1
i = 2, j = 2
i = 3, j = 1
i = 3, j = 2

---

8. Real-World Example: Sum Until User Quits

total = 0

while True:
    user_input = input("Enter a number (or 'q' to quit): ")

    if user_input == 'q':
        break

    total += int(user_input)

print("Total sum is:", total)

This pattern is extremely common in real applications.

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9. Common Use Cases of while Loops

1. Waiting for a condition

   • User input
   • File availability
   • Network response

2. Unknown number of iterations

   • Reading data until EOF
   • Retrying until success

3. Real-time monitoring

   • Background services
   • Event listeners
   • Game loops

---

Common Mistakes & Errors

Mistake	Why It Happens
Infinite loop	Condition never changes
Forgetting increment	Counter not updated
Wrong exit condition	Loop never stops
Overusing while	for loop might be simpler
Nested infinite loops	Program becomes unresponsive

---

Comparison: for vs while

Feature	for Loop	while Loop
Known iterations	Yes	No
Condition-based	No	Yes
Cleaner syntax	Yes	Depends
Risk of infinite loop	Low	High

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Practice Tasks

1. Print numbers from 10 to 1 using a while loop.
2. Ask the user to enter numbers until they enter 0, then print the sum.
3. Create a password checker with a maximum of 3 attempts.
4. Print all even numbers between 1 and 50 using while.
5. Use a nested while loop to print a multiplication table (1–5).