Repeated calculations

A loop can repeat more than printing; it can repeat a calculation on the same variable. To work out 3×3×3×3, start with num = 1 and multiply by 3 four times:

num = 1
for i in range(4):
    num = num * 3
print(num)

81

Where you put print matters: indented, it prints on every turn; not indented, it prints once at the end:

num = 1
for i in range(4):
    num = num * 3
    print(num)

3
9
27
81

Using i in the calculation

The loop variable changes on each turn, so you can use it in the calculation. range(2, 7) gives 2, 3, 4, 5, 6, so this works out 2×3×4×5×6:

num = 1
for i in range(2, 7):
    num = num * i
print(num)

720

Running totals and +=

Same idea with adding: a variable starts at 0 and grows on each turn. total += i is short for total = total + i:

total = 0
for i in range(1, 11):
    total += i
print("1 + 2 + ... + 10 = " + str(total))

1 + 2 + ... + 10 = 55

if inside the loop

Put an if inside the loop to count or add only the numbers that pass a test. Here we count the multiples of 3 from 1 to 20:

count = 0
for i in range(1, 21):
    if i % 3 == 0:
        count += 1
print(count)

6