Python loops: for, while and range()

Why loops?

A loop repeats a block of code. Without a loop, printing the 5 times table needs ten lines; with a loop it needs two:

for i in range(1, 11):
    print(5, "x", i, "=", 5 * i)

Python has two loops: for (repeat for each item) and while (repeat while a condition is true).

range(): a ready-made sequence of numbers

for loops

total = 0
for month in range(1, 13):
    total += 500
print("Saved in a year:", total)    # 6000

Each time round, month takes the next value, and the indented block runs once.

for works on anything you can go through one item at a time: strings, lists, tuples, dictionaries and files.

for letter in "Ranchi":
    print(letter)

for name in ["Asha", "Ravi", "Meena"]:
    print("Hello", name)

while: repeat as long as a condition is true

n = 5
while n > 0:
    print(n)
    n -= 1          # without this line the loop never ends
print("Lift-off!")

Use while when you do not know in advance how many times to repeat, for example "keep asking until the PIN is right".

Infinite loop: if the condition never becomes false, the loop runs forever. Press Ctrl + C to stop it.

break and continue

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

for n in range(1, 11):
    if n == 7:
        break          # stops at 7: prints 1 to 6
    print(n)

for n in range(1, 11):
    if n % 2 == 0:
        continue       # skips even numbers: prints 1 3 5 7 9
    print(n)

The while True menu pattern

Real programs often loop until the user chooses to quit:

while True:
    print("1. Balance  2. Deposit  3. Exit")
    choice = input("Choose: ")
    if choice == "1":
        print("Balance: Rs 5000")
    elif choice == "2":
        print("Deposit accepted")
    elif choice == "3":
        print("Goodbye")
        break
    else:
        print("Invalid choice")

Nested loops: tables and patterns

A loop inside a loop. The inner loop runs completely for each turn of the outer loop.

for row in range(1, 4):           # 3 rows
    for seat in range(1, 3):      # 2 seats in each row
        print(f"R{row}S{seat}", end=" ")
    print()                       # new line after each row

Output:

R1S1 R1S2
R2S1 R2S2
R3S1 R3S2

Star pyramid

n = 4
for i in range(1, n + 1):
    print(" " * (n - i) + "*" * (2 * i - 1))

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

Example: ATM PIN with 3 attempts

correct = "4321"
attempts = 0
while attempts < 3:
    pin = input("Enter PIN: ")
    if pin == correct:
        print("Access granted")
        break
    attempts += 1
    print("Wrong PIN,", 3 - attempts, "tries left")
else:
    print("Card blocked")

The else on a loop runs only when the loop ends without a break. Here it means all three tries were wrong.

Common mistakes

• Expecting range(1, 10) to include 10.
• Forgetting to change the variable in a while loop, so it never ends.
• Wrong indentation: a line meant to run once after the loop is indented inside it.
• Changing a list while looping over it; loop over a copy instead: for x in items[:].

Key points

• for repeats for each item; while repeats while a condition is true.
• range(start, stop, step) stops before stop.
• break ends the loop; continue jumps to the next turn.
• In nested loops, the inner loop finishes completely for every turn of the outer loop.

Practice questions

total = 0
for n in range(2, 101, 2):
    total += n
print(total)     # 2550

n = 1234
rev = 0
while n > 0:
    rev = rev * 10 + n % 10
    n //= 10
print(rev)

for i in range(3):
    for j in range(4):
        print("Hi")

for i in range(1, 5):
    for j in range(1, i + 1):
        print(j, end=" ")
    print()