chapter 5.1

Loops

Repeating actions with for and while

Loops let you repeat code without writing it over and over. Python has two kinds: for loops (when you know how many times) and while loops (when you don't).

for loops iterate over a sequence — a list, a string, a range of numbers:

# Loop through a range of numbers
for i in range(5):
    print(f"Count: {i}")

# range(5) gives: 0, 1, 2, 3, 4  (NOT 5!)

⚠warning

range(5) produces 0 through 4 — five numbers, but it stops *before* 5. This trips up everyone at first. range(1, 6) gives 1 through 5.

# Loop through a list
colors = ["red", "green", "blue"]
for color in colors:
    print(f"Color: {color}")

# Loop through a string
for char in "Python":
    print(char)

while loops keep running as long as a condition is True:

countdown = 5
while countdown > 0:
    print(f"T-minus {countdown}...")
    countdown -= 1    # Same as: countdown = countdown - 1
print("Liftoff!")

⚠warning

If you forget to update the loop variable (like countdown -= 1), the condition never becomes False and you get an infinite loop. Your program hangs forever.

break exits a loop early. continue skips to the next iteration:

# break — stop when we find it
numbers = [1, 3, 7, 12, 5, 9]
for n in numbers:
    if n > 10:
        print(f"Found {n}, stopping!")
        break
    print(f"Checked {n}")

# continue — skip odd numbers
for i in range(10):
    if i % 2 != 0:
        continue
    print(f"Even: {i}")

enumerate() gives you both the index and the value — use it instead of tracking an index manually:

fruits = ["apple", "banana", "cherry"]
for i, fruit in enumerate(fruits):
    print(f"{i}: {fruit}")

your turn

challenge

Write a loop that prints the multiplication table for the number 7 (7 x 1 = 7, 7 x 2 = 14, etc. up to 7 x 10). Use a `for` loop with `range()` and an f-string.

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output

$ Press RUN to execute your code

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