for Loops and range

Big idea

A for loop takes the items of a sequence one at a time. With range(), it's the easy way to repeat something a fixed number of times.

Looping over a string

for variable in sequence:
    # runs once per item

for letter in "code":
    print(letter.upper())

output
C
O
D
E

Each round, letter refers to the next character. No counter to set up, no index to update, nothing to forget.

Watch it

range: counting

range makes a sequence of numbers to loop over:

• range(5) → 0, 1, 2, 3, 4 (five numbers, starting at 0)
• range(2, 6) → 2, 3, 4, 5 (stops before 6)
• range(0, 20, 5) → 0, 5, 10, 15 (a step of 5)
• range(5, 0, -1) → 5, 4, 3, 2, 1 (counting down)

The stop number is never included — the same rule as slicing.

n = 7
for i in range(1, 11):
    print(f"{n} x {i} = {n * i}")

output
7 x 1 = 7
7 x 2 = 14
7 x 3 = 21
7 x 4 = 28
7 x 5 = 35
7 x 6 = 42
7 x 7 = 49
7 x 8 = 56
7 x 9 = 63
7 x 10 = 70

Adding things up

A loop plus a running total is one of the most common patterns in all of programming:

total = 0
for i in range(1, 101):
    total += i
print(total)

5050

Off by one

for i in range(1, 5):
    print(i)

output
1
2
3
4
5

range(1, 5) stops before 5. To include 5, use range(1, 6).

Loops inside loops

A loop body can contain another loop. The inner loop runs completely for each round of the outer one:

for row in range(1, 5):
    print("*" * row)

output
*
**
***
****

for or while?

• You know the items or the count → for.
• You're waiting for something to happen → while.

Think of it like

Dealing cards: "for each player at the table, deal one card." You don't count players by hand — you just go round until everyone has had a turn.

Key terms

• for loop — Runs its block once for each item in a sequence.
• range(start, stop, step) — A sequence of numbers; stop isn't included.
• Loop variable — The name that refers to the current item, like i.