Accumulating: totals and counts

The accumulator pattern

Here is the pattern that unlocks most beginner programs. To add up the numbers 1 through 5:

total = 0
for n in range(1, 6):
    total = total + n
print(total)

Three parts, all of them ideas you already own:

1. Start an accumulator variable before the loop: total = 0 (lesson 3-1)
2. Update it inside the loop body each iteration: total = total + n (lesson 3-2)
3. Use it after the loop: print(total)

Trace it: total goes 0 → 1 → 3 → 6 → 10 → 15. The final print shows 15.

The same shape counts things. To count how many numbers from 1 to 20 are even, start count = 0 and add 1 only when a condition holds, using if inside the loop (lesson 4-2) and the remainder operator % from lesson 2-2: a number is even when n % 2 == 0.