Python While Loops

1. What is a While Loop?

A while loop runs a block of code repeatedly as long as a condition is True.

x = 0
while x < 5:
    print(x)
    x += 1
# Output: 0 1 2 3 4

Syntax

while <condition>:
    # code runs while condition is True

2. For Loop vs While Loop

# These do the same thing:
for x in range(5):     # for loop
    print(x)

x = 0                  # while loop equivalent
while x < 5:
    print(x)
    x += 1

Use a for loop when the number of iterations is known. Use a while loop when it depends on a condition at runtime.

3. Practical Use Case — Validating User Input

A while loop shines when you don't know how many times a user will enter invalid input.

num = input("Enter an integer: ")
while not num.isdigit():
    num = input("Enter an integer: ")

This keeps asking until the user gives a valid integer — something a for loop can't easily do.

4. Infinite Loops & break

A while True loop runs forever unless you break out of it.

while True:
    num = input("Enter an integer: ")
    if num.isdigit():
        break   # exits the loop as soon as valid input is given

⚠️ Infinite loop warning: If you use while True and never break, your program will crash. Always make sure there's a reachable exit condition.

5. break and continue

Work exactly the same as in for loops.

break — exit the loop immediately

while condition:
    if something:
        break   # stops the loop entirely

continue — skip to the next iteration

while condition:
    if something:
        continue   # skips the rest of this iteration

6. Iterating Through a List with While

lst = [2, 3, 3, 2, 1]
result = 0
i = 0

while result < 9:
    num = lst[i]
    result += num
    print(num)
    i += 1

⚠️ Index Out of Range Bug

If the condition is never met, i can exceed the list length and cause a crash.

# ❌ Crashes if values never sum to 9
while result < 9:
    ...

# ✅ Add a second condition to guard against it
while result < 9 and i < len(lst):
    ...

Always guard against going out of bounds when iterating a list with a while loop.

7. While-Else Statement

The else block runs only if the loop ended normally (i.e. the condition became False without hitting break).

lst = [2, 3, -2, 4]
target = -2
i = 0

while i < len(lst):
    if lst[i] == target:
        print("Found it!")
        break
    i += 1
else:
    print("Didn't find it.")

• break hit → else is skipped
• Loop finishes normally → else runs

Same behavior as for-else. Avoids needing a found = False boolean flag.

8. Common Mistakes

Forgetting to increment the iterator

i = 0
while i < len(lst):
    print(lst[i])
    # ❌ forgot i += 1 → infinite loop!

Not guarding against index out of range

# ❌ will crash if condition is never met
while result < 9:
    result += lst[i]
    i += 1

# ✅ guard with a second condition
while result < 9 and i < len(lst):
    result += lst[i]
    i += 1

Cheat Sheet

# Basic while loop
while condition:
    # body

# Infinite loop with break
while True:
    if exit_condition:
        break

# Loop with compound condition
while x < 10 and i < len(lst):
    ...

# While-else
while condition:
    if found:
        break
else:
    # runs only if break was never hit

# Loop control
break       # exit the loop entirely
continue    # skip to the next iteration