Loops

Overview

Loops repeat code over sequences or while a condition holds. Python favors for loops over index-based iteration—use range(), enumerate(), and zip() instead of manual counter variables when possible.

Syntax / Usage

# for loop over a sequence
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

# range(start, stop, step)
for i in range(3):
    print(i)  # 0, 1, 2

# while loop
count = 0
while count < 3:
    count += 1

# break and continue
for n in range(10):
    if n == 5:
        break      # exit loop
    if n % 2 == 0:
        continue   # skip to next iteration
    print(n)

# enumerate for index + value
for index, value in enumerate(["a", "b"]):
    print(index, value)

Examples

Sum numbers until the user types quit:

total = 0
while True:
    line = input("Enter a number (or 'quit'): ")
    if line == "quit":
        break
    total += int(line)
print(f"Total: {total}")

Find the first even number in a list:

numbers = [1, 3, 5, 8, 9]
for n in numbers:
    if n % 2 == 0:
        print(f"First even: {n}")
        break
else:
    print("No even numbers found")

Common Mistakes

• Modifying a list while iterating over it—iterate over a copy instead
• Infinite while True loops without a break condition
• Using range(len(items)) when enumerate(items) is clearer
• Off-by-one errors with range stop values (stop is exclusive)

See Also

python-list-comprehension python-functions python-dictionaries