Find the Max

找最大值

≈ 15 min · python-loops · Open in WolfCode

The tracking-variable pattern. Initialize with the FIRST element — not zero — and your function survives the all-negatives case.

The pattern

Python · runnable

def find_max(numbers):
    best = numbers[0]              # initialize with the FIRST element
    for x in numbers[1:]:          # loop over the REST
        if x > best:               # update when we see something bigger
            best = x
    return best

print(find_max([3, 1, 4, 1, 5, 9, 2, 6]))   # → 9
print(find_max([-7, -1, -100]))             # → -1
print(find_max([42]))                       # → 42

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The trap: initializing to 0

What does this return for all-negative input?

def find_max_BUGGY(numbers):
    best = 0
    for x in numbers:
        if x > best:
            best = x
    return best

print(find_max_BUGGY([-7, -1, -100]))   # → 0   ← not even in the input!

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Check yourself

What's the SAFEST initial value if the input could be ANY positive or negative numbers (but is non-empty)?

The pattern generalizes

The same three-step shape — initialize / update conditionally / return — drives find_min, find_longest_word, count_evens, and a dozen others. Once you can write find_max, you can write all of them.

Now you try

Open 03-find-max.py. 6 tests including the all-negatives edge case and a check that you didn't use the built-in max().