Loops

Step 1: The for loop

Imagine printing “Hello” five times by writing print("Hello") five times. A loop does it for you.

for i in range(5):
    print("Hello", i)

Hello 0
Hello 1
Hello 2
Hello 3
Hello 4

range(5) makes the numbers 0, 1, 2, 3, 4. The loop variable i takes each one in turn. Notice:

• Counting starts at 0, not 1.
• range(5) stops before 5.
• The lines inside the loop are indented, just like an if.

Step 2: Start, stop and step

range() can take up to three numbers: where to start, where to stop (not included) and how big each step is.

for n in range(1, 6):
    print(n)

1
2
3
4
5

for n in range(10, 0, -2):
    print(n)

10
8
6
4
2

Step 3: Keeping a running total

A very common pattern: create a variable before the loop, then update it inside the loop.

total = 0
for n in range(1, 5):
    total = total + n
    print("Added", n, "total is", total)

print("Final total:", total)

Added 1 total is 1
Added 2 total is 3
Added 3 total is 6
Added 4 total is 10
Final total: 10

Common mistake: If you write total = 0 inside the loop, it resets to zero on every trip. Set it once, before the loop.

total += n is a shorter way to write total = total + n.

Step 4: The while loop

A for loop runs a set number of times. A while loop runs as long as its condition is true. It is perfect when you don’t know in advance how many times you need.

count = 3
while count > 0:
    print(count)
    count = count - 1
print("Go!")

3
2
1
Go!

The loop changes count each time. If it didn’t, the condition would stay true forever and the program would never stop: an infinite loop. If that happens, press the Stop button in your editor.

Step 5: Stopping early with break

break ends a loop immediately. It is often used with while True, a loop that runs until you tell it to stop.

while True:
    word = input("Type a word (or 'quit'): ")
    if word == "quit":
        break
    print("You typed", word)
print("Bye!")

Type a word (or 'quit'): apple
You typed apple
Type a word (or 'quit'): quit
Bye!