Conditional Repetition With The While Loop

The Python while loop is the natural choice when you don't know in advance how many times a block must repeat. It keeps running while a condition is true — perfect for menu-driven apps, retry logic, polling sensors, or any code that must wait for something to happen.

• Syntax: while condition: followed by an indented body.
• Modify the condition's variables inside the body so the loop can eventually stop.
• while True: creates an intentional infinite loop — exit it with break.
• Python has no built-in do-while; emulate it with while True: + break.

# basic while
while condition:
    # body

# intentional infinite loop
while True:
    cmd = input("> ")
    if cmd == "quit":
        break

# do-while equivalent
while True:
    value = compute()
    if not value:
        break

• How it works: Python evaluates the condition at the top of every iteration. If it is true, the body executes; otherwise control jumps past the loop. The condition is evaluated before the body — if it is false initially, the body never runs.
• Updating the loop variable: Beginners often forget to mutate the variable that controls the condition, creating an infinite loop. Always plan how the loop will terminate before writing the body.
• Infinite loops on purpose: Servers, game loops, and CLI menus often write while True: and rely on break, return, or an exception to exit. This is idiomatic and not a bug.
• Sentinel-controlled loops: Read input until a special 'sentinel' value is encountered — e.g. read numbers until the user types -1. Sentinels make the stopping rule explicit.
• Loop-else clause: Just like for, a while may have an else block that runs when the condition becomes false without a break.
• Performance: while is slightly slower than for over the same number of iterations because each iteration re-evaluates the condition expression. Prefer for for fixed-size loops.

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

total = 0
while True:
    n = int(input("Number (0 to stop): "))
    if n == 0:
        break
    total += n
print("Total:", total)

pwd = ""
attempts = 3
while attempts > 0 and pwd != "secret":
    pwd = input("Password: ")
    attempts -= 1
print("Welcome!" if pwd == "secret" else "Locked out")

n = 1234
rev = 0
while n > 0:
    rev = rev * 10 + n % 10
    n //= 10
print("Reversed:", rev)

while True:
    print("1.Add 2.Sub 3.Quit")
    choice = input("> ")
    if choice == "3":
        print("Bye"); break
    a = int(input("a: ")); b = int(input("b: "))
    print(a+b if choice=="1" else a-b)

n = 17
d = 2
while d * d <= n:
    if n % d == 0:
        print(n, "is composite"); break
    d += 1
else:
    print(n, "is prime")

• Initialise loop variables before the while statement.
• Always ensure the condition can become false — otherwise you'll loop forever.
• Use while True: + break when the exit condition is complex.
• Add a maximum-attempts counter to retry loops to avoid hanging indefinitely.
• Use time.sleep() inside polling loops to avoid 100% CPU usage.
• Test edge cases: zero iterations, single iteration, and termination conditions.

• Infinite loop: forgetting to update the counter inside the body.
• Off-by-one: using < instead of <= (or vice versa).
• Wrong condition order: while True: with no break hangs.
• Heavy work inside the condition: the condition is re-evaluated every iteration.
• Mutating the wrong variable: updating a different name leaves the condition unchanged.
• Tight CPU loops: a polling loop without sleep can pin a CPU core to 100%.

• Problem 1: Compute the sum of digits of a positive integer using a while loop.
• Problem 2: Read numbers until -1 is entered and print the average of all entered values.
• Problem 3: Use a while loop to compute the GCD of two numbers (Euclidean algorithm).
• Problem 4: Print the Fibonacci series up to a given limit n.
• Problem 5: Build a password-strength prompt that keeps asking until the user enters a strong one.
• Problem 6: Simulate a simple ATM with options for deposit, withdraw, balance, and exit.

• Q1. When should you prefer while over for?
• Q2. How do you simulate a do-while loop in Python?
• Q3. What is an infinite loop and how is it used in practice?
• Q4. Explain the loop-else clause with a while example.
• Q5. How can you avoid 100% CPU usage in a continuous polling loop?
• Q6. How do you implement retry logic with a maximum number of attempts?

• Q1: Is while True considered bad practice?
  Not at all. Many production servers and CLIs use while True with a break or return on the exit condition. It is clear and idiomatic.
• Q2: Does Python have a do-while loop?
  No. Emulate it using while True: ... if not condition: break.
• Q3: Why does my while loop never stop?
  Likely the variable in the condition is not updated inside the body. Add a print statement to debug.
• Q4: Can a while loop have an else?
  Yes — it runs when the condition becomes false naturally (no break).
• Q5: How do I exit a nested while?
  Use a flag variable, wrap them in a function and return, or raise an exception.
• Q6: Is while faster than for?
  Usually for is slightly faster for fixed iteration counts. Use the loop that expresses your intent most clearly.