Loops

A loop repeats a block of code. That's how a program prints a thousand receipts or checks every pixel in a photo without a thousand lines of code.

A while loop repeats as long as its condition is true. Something inside it has to change, or the condition stays true forever and the loop never ends.

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

A for loop goes through the items of something one at a time. With range() it counts: range(5) gives 0, 1, 2, 3, 4. It starts at 0 and stops before 5, so it gives exactly five numbers. range(2, 6) gives 2 to 5, and a third number sets the step: range(0, 10, 3) gives 0, 3, 6, 9.

for i in range(1, 4):
    print(i * 10)

A common pattern keeps a running total: start a variable at 0 before the loop and add to it each time round. break leaves a loop straight away, and continue skips to the next time round.

total = 0
for i in range(1, 5):
    total = total + i
print(total)