Add up to n, instantly

Write sum_to(n) returning 1 + 2 + … + n — this time without a loop.

Pair the first number with the last: 1 + n, 2 + (n − 1), … Every pair adds up to n + 1, and there are n / 2 pairs.

def sum_to(n):
    total = 0
    for i in range(1, n + 1):
        total += i
    return total

Run adds print(sum_to(100)) after your code, to try it.

Stuck on the idea rather than the code? The Counting steps lesson walks through it.