alt Hacker News> A city can also exist in multiple zip codes. And there can be multiple cities with the same name in the same state.
These are reasons you cannot deduce the Zip from the city, not the opposite. A ZIP+4 actually encodes all other information for a US address.
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A ZIP+4 does not encode all information.
Proof: a post office has its own zip code, for PO Boxes.
The +4 is the last four digits of the post office box.
If the Post Office has more than 10,000 boxes, the +4 will be duplicated.
ZIP+4? I think that's literally enough digits to give every house in the US (about 150,000,000 apparently) its own identifier.
Wrong. There can also be multiple cities in the same ZIP code. There is not a 1:1 relationship with a 5-digit ZIP as everyone is assuming here.
Nobody knows their +4 code. You cannot ask for information 90%+ of people won't have.