Sum 1..n

1 到 n 求和

≈ 15 min · python-loops · Open in WolfCode

The accumulator pattern — initialize / update / return — appears in every Chapter 3 lesson and most of the rest of the course.

The problem

Implement sum_to_n(n) returning 1 + 2 + 3 + ... + n. The autograder requires a for loop — no closed-form formula.

The accumulator pattern

Python · runnable

def sum_to_n(n):
    total = 0                      # initialize before the loop
    for i in range(1, n + 1):      # iterate 1, 2, ..., n
        total += i                 # update inside the loop
    return total

print(sum_to_n(5))      # → 15
print(sum_to_n(100))    # → 5050
print(sum_to_n(0))      # → 0  (empty sum)

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The `+ 1` gotcha

What goes wrong without it?

def sum_to_n_BUGGY(n):
    total = 0
    for i in range(1, n):          # ✗ missing the + 1
        total += i
    return total

print(sum_to_n_BUGGY(5))    # → 10, not 15 — last term (5) is missing

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Why initialize before the loop?

Predict the output — then run.

def sum_to_n_BUGGY2(n):
    for i in range(1, n + 1):
        total = 0                  # ✗ resets every iteration
        total += i
    return total

print(sum_to_n_BUGGY2(5))    # returns 5 (the last i), not 15

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Check yourself

sum_to_n(0) returns:

Now you try

Open 01-sum-to-n.py in WolfCode. The autograder runs 5 tests including n = 0 and a check that you actually used a for loop (not the closed-form formula).

Sum 1..n

The problem

The accumulator pattern

The + 1 gotcha

Why initialize before the loop?

Now you try

The `+ 1` gotcha