07-loops

07 · Loops

Part 1 — Fundamentals · Estimated time: 30–40 min · Prerequisite: 06 · Conditionals

A loop repeats a block of code so you don't have to copy-paste it. Want to greet 100 users, add up a column of numbers, or keep asking until the answer is right? That's a loop. Python has two: the for loop (repeat once per item) and the while loop (repeat as long as something stays true).

What you'll learn

• for loops over a range of numbers and over the characters of a string
• The range() function with one, two, and three arguments
• while loops — and how to avoid the dreaded infinite loop
• break (stop early) and continue (skip to the next round)
• The accumulator pattern (building a running total)

---

1. The for loop

A for loop runs its body once for each item in a sequence.

for number in range(1, 4):
    print(number)
# prints 1, then 2, then 3

for letter in "Hi":
    print(letter)
# prints H, then i

number and letter are just names you choose — they hold the current item each time around. Like if, the body is indented under a line ending in :.

▶️ Run it: python examples/01_for_loop.py

---

2. The range() function

range() generates a sequence of numbers — perfect for counting.

range(5)        # 0, 1, 2, 3, 4   (stop only: starts at 0, stops BEFORE 5)
range(1, 6)     # 1, 2, 3, 4, 5   (start, stop)
range(0, 10, 2) # 0, 2, 4, 6, 8   (start, stop, step)

The stop value is never included — range(1, 6) gives you 1 through 5.

▶️ Run it: python examples/02_range_function.py

💡 Try it yourself: print the even numbers from 2 to 10 using a single range(...).

---

3. The while loop

A while loop keeps going as long as its condition is true.

count = 1
while count <= 3:
    print(count)
    count += 1     # <-- without this, the loop never ends!

The danger: if the condition never becomes false, the loop runs forever. Always make sure something inside the loop moves it toward stopping.

▶️ Run it: python examples/03_while_loop.py

---

4. break and continue

• break stops the loop immediately.
• continue skips the rest of this round and jumps to the next one.

for n in range(1, 10):
    if n == 5:
        break        # stop entirely when we hit 5
    if n % 2 == 0:
        continue     # skip even numbers
    print(n)         # prints 1, 3

▶️ Run it: python examples/04_break_and_continue.py

---

Common mistakes

Mistake	What happens	Fix
while condition never goes false	infinite loop (program hangs)	Change something inside the loop (e.g. count += 1).
Expecting range(1, 5) to include 5	it stops at 4	Use range(1, 6) to reach 5.
Forgetting the : or indentation	SyntaxError / IndentationError	End the for/while line with : and indent the body.
Editing the counter in two places	hard-to-follow bugs	Update the loop variable in one obvious spot.

---

Recap / cheat-sheet

for item in range(1, 6):   # 1..5
    print(item)

for char in "abc":         # loop over a string
    print(char)

range(stop)                # 0..stop-1
range(start, stop)         # start..stop-1
range(start, stop, step)   # ...stepping by step

count = 0
while count < 5:           # repeat while True
    count += 1             # move toward stopping!

break      # exit the loop now
continue   # skip to the next iteration

---

Exercises

Run a file with python exercises/<file>.py, then compare with the matching file in solutions/.

1. exercises/01_count_to_five.py — print 1 to 5 with a for loop.
2. exercises/02_sum_to_n.py — add up 1..n with an accumulator.
3. exercises/03_countdown.py — count down with a while loop, then "Liftoff!".

Try each yourself before opening the solution.

---

Next → 08 · Input & Output (coming soon)