Loops

Looping statements repeat a block of code multiple times. They are one of the most powerful tools in programming — without loops, you'd have to write the same code 100 times to do 100 things.

1. The for Loop

Used to iterate over a sequence — going through items one by one. Most commonly used when you know how many times to repeat.

for variable in sequence:
    code block

From your file (control-flow.py):

# Iterating over a string — one character at a time
for char in "Hello":
    print(char)
# Output: H e l l o (each on its own line)

Using range()

range() generates a sequence of numbers. It has three forms:

# Count down from 10 to 1 (from   file)
for num in range(10, 0, -1):
    print(num)

Nested for loops

A loop inside a loop. From the star pattern example:

# Print a 5×5 star grid
for _ in range(5):         # outer: 5 rows
    for _ in range(5):     # inner: 5 stars per row
        print("*", end=" ")
    print()                 # newline after each row

2. The while Loop

Repeats a block of code as long as a condition stays True. Use when you don't know in advance how many iterations you need.

initialization
while condition:
    code block
    updation    # important! avoid infinite loop

From file:

# Print from 11 down, by 2 (odd numbers descending)
num = 11
while num > 0:
    print(num)
    num -= 2    # updation — num decreases each time

Reverse a number (from file)

number = 1234
reverse = 0

while number > 0:
    remainder = number % 10        # get last digit
    reverse = reverse * 10 + remainder
    number = number // 10          # remove last digit

print(reverse)  # 4321

# How it works step by step:
# number=1234 → remainder=4, reverse=4,   number=123
# number=123  → remainder=3, reverse=43,  number=12
# number=12   → remainder=2, reverse=432, number=1
# number=1    → remainder=1, reverse=4321, number=0 → stop

3. else with Loops

Python allows an else after a loop. The else block runs only if the loop finishes normally (not interrupted by break).

for i in range(3):
    print(i)
else:
    print("Loop finished normally")
# Output: 0 1 2 Loop finished normally

4. Jumping Statements

Used to change the normal flow of a loop. Your teacher covers three: break, continue, and pass.

break — Exit the loop immediately

for i in range(5):
    if i == 3:
        break
    print(i)
else:
    print("completed the loop")
# Output: 0 1 2
# else does NOT run — loop was broken

continue — Skip this iteration, keep going

for i in range(5):
    if i == 3:
        continue       # skip i=3
    print(i)
else:
    print("completed the loop")
# Output: 0 1 2 4 (3 is skipped)
# else DOES run — loop completed normally

pass — Placeholder, do nothing

for i in range(5):
    if i == 3:
        pass           # does nothing — loop continues normally
    print(i)
else:
    print("completed the loop")
# Output: 0 1 2 3 4 completed the loop
# All numbers print including 3 — pass didn't skip anything

n = int(input("Enter a number: "))
for i in range(1, 11):
    print(f"{n} x {i} = {n * i}")

i = 1
total = 0
while i <= 100:
    total += i
    i += 1
print("Sum:", total)   # 5050

*
* *
* * *
* * * *
* * * * *

for row in range(1, 6):
    for col in range(row):
        print("*", end=" ")
    print()

for i in range(1, 16):
    if i % 3 == 0:
        continue
    print(i)
# Output: 1 2 4 5 7 8 10 11 13 14

number = int(input("Enter a number: "))
reverse = 0

while number > 0:
    remainder = number % 10
    reverse = reverse * 10 + remainder
    number = number // 10

print("Reversed:", reverse)