Walkthrough: Print-Debug an Off-by-One

例题: 用 print 调试 off-by-one

≈ 8 min · python-debugging · Open in WolfCode

A function that runs fine but returns the wrong number. The hardest bug type — no exception, no traceback. Walk through finding it with print debugging.

The bug

sum_to_n(5) should return 15, but returns 10

def sum_to_n_BUGGY(n):
    total = 0
    for i in range(1, n):       # bug: should be range(1, n + 1)
        total += i
    return total

print(sum_to_n_BUGGY(5))   # 10  ← wrong, expected 15

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Step 1 — Add print scaffolding

Python · runnable

def sum_to_n_BUGGY(n):
    total = 0
    for i in range(1, n):
        print(f"DEBUG i={i}, adding to total={total}")
        total += i
    print(f"DEBUG final total={total}")
    return total

sum_to_n_BUGGY(5)

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Step 2 — Fix and verify

Python · runnable

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

print(sum_to_n(5))     # 15 ✓
print(sum_to_n(100))   # 5050 ✓

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