Loops: For and While

Loops let you repeat code multiple times without writing it over and over.

For Loops

Use for when you know how many times to loop:

# Print numbers 0 to 4
for i in range(5):
    print(i)

Output:

Looping Over Lists

fruits = ["apple", "banana", "cherry"]

for fruit in fruits:
    print(f"I like {fruit}")

Range Variations

# Start at 1, go to 5
for i in range(1, 6):
    print(i)  # 1, 2, 3, 4, 5

# Start at 0, go to 10, step by 2
for i in range(0, 10, 2):
    print(i)  # 0, 2, 4, 6, 8

# Count backwards
for i in range(5, 0, -1):
    print(i)  # 5, 4, 3, 2, 1

While Loops

Use while when you don't know how many iterations you need:

count = 0

while count < 5:
    print(count)
    count += 1  # Don't forget this!

Common Pattern: User Input

password = ""

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

print("Access granted!")

Break and Continue

Break - Exit the loop early

for i in range(10):
    if i == 5:
        break  # Stop the loop
    print(i)
# Prints: 0, 1, 2, 3, 4

for i in range(5):
    if i == 2:
        continue  # Skip this iteration
    print(i)
# Prints: 0, 1, 3, 4

Nested Loops

Loops inside loops:

for i in range(3):
    for j in range(3):
        print(f"({i}, {j})")

Example: Multiplication Table

for i in range(1, 6):
    for j in range(1, 6):
        print(f"{i * j:3}", end=" ")
    print()  # New line after each row

Output:

  1   2   3   4   5 
  2   4   6   8  10 
  3   6   9  12  15 
  4   8  12  16  20 
  5  10  15  20  25

Loop with Else

Python has an unusual feature - loops can have an else clause:

for i in range(5):
    if i == 10:
        break
else:
    print("Loop completed without break")

The else block runs only if the loop completes normally (no break).

Common Patterns

Sum of numbers

numbers = [1, 2, 3, 4, 5]
total = 0

for num in numbers:
    total += num

print(f"Sum: {total}")  # 15

Find an item

names = ["Alice", "Bob", "Charlie"]
search = "Bob"

for name in names:
    if name == search:
        print(f"Found {search}!")
        break
else:
    print(f"{search} not found")

Warning: Be careful with while loops - make sure your condition will eventually become False, or you'll create an infinite loop!