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Comment by magicalhippo | Hacker News Reader

The article states a Bloom filter is "guaranteed to report correctly if it contains the item", however a Bloom filter cannot do this. The bits set by the various hash functions could very well be set due to some other key. What the Bloom filter _can_ say, is that if none of the bits are set, then clearly the key was never inserted.

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> The article states a Bloom filter is "guaranteed to report correctly if it contains the item", however a Bloom filter cannot do this.

I think what's going on here is that you're reading "if" as meaning "whether". In fairness, this is common English usage: "I'll tell you tomorrow if I'm going" usually means "I'll tell you tomorrow whether I'm going".

That's not what the OP means. The correct reading is perhaps clarified with a comma:

> A Bloom filter is guaranteed to report correctly, if it contains the item.

or perhaps better

> If it contains the item, a Bloom filter is guaranteed to report correctly.

Ah yes, that would be it.

IMO it is better to present the Bloom filter by asking/stating what one can be certain of given each return value of the Bloom filter.

I think it might just be a strangely formed sentence where the part "if it contains the item" is not the subject of the report, but a condition:

If it contains the item -> reports that it contains the item (correctly)

If it doesn't contain the item -> may report that it contains the item (incorrectly)

Nothing wrong with being pedantic when dealing with definitions.

Since we're on the topic of being pedantic, "well, you know, that's just, like... your opinion, man". I'd argue that there's nothing excessive about arguing if A may, in fact, be the opposite of A :)

Certainly not, I'm all for some arguing on the internet.

The logic you wrote is exactly the _opposite_ of what a Bloom filter does.

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