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Comment by ghostly_s | Hacker News Reader

> 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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Nobody knows their +4 code. You cannot ask for information 90%+ of people won't have.

Of course they do.

Let me uh just grab my utility bill...

If asking for the zip first was more common then we quickly learn those four extra digits because the auto fill benefits would be immediately obvious

Why?

I have a 4 digit postcode, I have to look it up every time I have to fill in an address form for delivery.

I've had people screw 1 digit up in that postcode and their items (a laptop in one case) went to the completely wrong city.

A code sounds foolproof, until you realise most people don't engage with them for most of their lives - you don't tell the uber driver the zip/post code you are waiting in, and travelling to, nobody does.

edit: just to add - Magic numbers are bad. Software engineers know that a number that's undocumented in code is unmaintainable, a zip code is worse.

> I have a 4 digit postcode, I have to look it up every time I have to fill in an address form for delivery.

> A code sounds foolproof, until you realise most people don't engage with them for most of their lives - you don't tell the uber driver the zip/post code you are waiting in, and travelling to, nobody does.

When the above comments said +4, they meant knowing the second half of the nine digit zip code.

Basically everyone in the US knows the first 5 digits. It's really easy to memorize them. If you can remember your city, you can remember your zip code. And in the US you use it all the time, so it stays memorized.

> edit: just to add - Magic numbers are bad. Software engineers know that a number that's undocumented in code is unmaintainable, a zip code is worse.

That complaint about magic numbers is completely off base. Magic strings are just as bad in software. "Beverly Hills" and 90210 are equal sins on the magic front.

> Basically everyone in the US knows the first 5 digits. It's really easy to memorize them. If you can remember your city, you can remember your zip code. And in the US you use it all the time, so it stays memorized.

What's the 5 digits for Yonkers New York (edited because I originally had NYC)

> That complaint about magic numbers is completely off base. Magic strings are just as bad in software. "Beverly Hills" and 90210 are equal sins on the magic front.

For the same reasons, that's why it would be: Beverly Hills, Los Angelos County, California, USA, 90210

> What's the 5 digits for Yonkers New York (edited because I originally had NYC)

Nobody sends packages where the destination is an entire city. If someone gives me an address inside Yonkers, it'll have the zip code in the address. I've never had to look up a zip code in my life.

> For the same reasons, that's why it would be: Beverly Hills, Los Angelos County, California, USA, 90210

Which reasons? That has nothing to do with magic numbers, except that a 'magic full mailing address' is still bad, you don't shove that into the middle of your code either. If you're looking at the "made a typo" reason then that's where showing the address after putting in the zip code will give you the same verification but faster.

Heh, you should take a minute to look the answer up -

ZIP Codes 10701, 10702 (post office), 10703–10705, 10707 (shared with Tuckahoe, NY), 10708 (shared with Bronxville, NY), 10710, 10583 (shared with Scarsdale, NY)

Is that information supposed to change my mind about something?

If you put in any of those numbers it can prefill the city name, with enough accuracy that you don't need to change it.

Did I imply anywhere that cities only have one zip code before you asked about Yonkers? I said if you can remember your city name you can remember your zip code. That doesn't imply you would use a list to get from one to the other.

It would have improved your response was all - nobody accused you of anything, but this response of yours... way off

Improved how?

I picked up the implication that you thought my response could be improved, so I tried to guess what your criticism was and respond to that. If it feels "way off" because I framed it as disagreement, then I dunno, that feels like the right framing? Unless it's something else I did? I could have made it clearer I was guessing but that doesn't seem way off.

Your zip plus 4 changes. It isn't worth trying to know as it isn't supposed to be constlnt. If you send a lot of mail there is a discount for using it but you have to update everyone's address often (iirt at least 4x per year)

Source? The numbers correspond to the USPS distribution centers and carrier routes. If the numbers are changing that would imply an increase in zip code subdivisions, making each zip code a better address predictor for a given individual.

https://faq.usps.com/s/article/ZIP-Code-The-Basics

I don't know how to link to the correct question but there is why did my code change.

10% of the US population is 35 million people. That's a pretty weird version of "nobody".

This is a bad hill to die on for a ux conversation. "10% can feel kind a big number when 100% is huge" is a funny argument, as is trying to be pedantic about "nobody knows" as a shortcut for "most people won't know and you can't rely on any particular user knowing". If 10% is big enough to matter I can't wait to tell you about 90%!

I'm not suggesting that one would design for the 10%, but I also think that writing off 10% - "nobody" - (particularly of a large number of people) is pretty dumb too.

> "10% can feel kind a big number when 100% is huge" is a funny argument

That would be a funny argument if it were the one being made.

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.

Yeah, anyone who has had to work with USPS bar codes should know that internally these are called routing codes, and they come in 5, 9 and even 11 digit variants. The 11-digit one narrows down to a specific delivery point, but even that isn’t enough to derive an address (just enough to know whether you’re looking at the right one or not). Zip+4 codes also change frequently because they aren’t based on locations but on delivery routes and sequencing.

> Zip+4 codes also change frequently because they aren’t based on locations but on delivery routes and sequencing.

This was news for me. I know the few zip+4 I memorize never change.

I think the source for the parent is AI slop. See [1].

> Due to an increase in population or to the improve postal operations, the US Postal Service® will occasionally add a new ZIP Code or change ZIP Code boundaries.

The plus four digits encode:

> [67] : Sector or Several Blocks

> [89] : Segment or One Side of a Street

Note that this contradicts the parent.

[1]: https://faq.usps.com/s/article/ZIP-Code-The-Basics

The census bureau (very) periodically publishes zip code data (which is where some places get their geolocation info). If you work with enough addresses you’ll find some zip+4s that are wildly far away from where they used to be. There are paid services that have better accuracy, but I’m not sure how they acquire their data.

Some people don't realize just how much you can "customize" deliverability with the post office, especially if you're big (like a school or large business) - you can have something that looks like your physical address, but is actually really a maildrop/PO Box at the nearby post office.

You can do relatively complex forwarding that would only appear to the end users if they can decode the barcode.

ZIP+4? I think that's literally enough digits to give every house in the US (about 150,000,000 apparently) its own identifier.

Only if there's never more than 10,000 addresses in a single zip code, which means that if you enforce that, you can force a zip code to appear by building enough house

That happens, and worse. I've lived in multiple areas that have had their zip code changed. Area codes, too, sometime more than once.

I think my point is that if you're going to make people learn a 9-digit identifier for their house you might as well make that identifier unique and then that's the only information they need to fill in. Having non-unique 9-digit identifiers feels wasteful.

There are far more addresses than there are houses

Zip isn't uniformly distributed numbers though so you dont have the equivalent of that many digits of decimal numbers. Other comments have more detail but just for the top level example the first number is the zone and goes from 0 on the east coast to 9 on the west.

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.

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