On: How PHP's foreach works

[unconed]

> perhaps especially for people who've never worked in it

At my last job, one of our interview questions for an experienced PHP programmer was "What makes PHP arrays different or unique, from a computer science point of view?" Not a single one ever got anywhere close to the right answer (that they're ordered hash tables, not arrays).

Pretty sure this applies to by-value and by-reference semantics too.

[Confusion → unconed]

That doesn't make PHP unique. AWK already did that.

By extension, a pitfall of interview questions: you can't always assume you actually know the correct answer or even that there is a correct answer. Which may cause interviewees to start secondguessing themselves and make them appear worse than they are. You can basically send them into an infinite loop by an illformed question. "Does he mean that they aren't actually arrays? Well, he can't mean that, because there are other languages whose arrays are actually hash tables. And they aren't called 'associative arrays' for nothing. So what does he mean?"

[chime → Confusion]

If the candidate said all of the above instead of just thinking it within their head, I would award them additional points. When I'm interviewing someone, I care about how they think and process instead of what they know.

[1SaltwaterC → unconed]

Most of the time, getting the right answer is a matter of asking the right question. If most experienced people can't ask a fairly simple question, then maybe the question is to blame. That bit about the "computer science point of view" may be a little bit vague. Asking for implementation specifics may be more appropriate. I do know that arrays in PHP are implemented as hash tables as this is a common rant about it. But from your question I did not understand what you mean.

I'm still curious what do you mean by "ordered" because the PHP arrays don't order their keys. The common rant previously mentioned aka having to use asort() for getting a proper array:

  php > $arr = array();
  php > $arr[2] = 2;
  php > $arr[1] = 1;
  php > var_dump($arr);
  array(2) {
    [2]=>
    int(2)
    [1]=>
    int(1)
  }

[stormbrew → 1SaltwaterC]

Most hash tables have a semi-random order dictated by the hash algorithm in combination with the bucket count. PHP Arrays are ordered by insertion order (each slot in the hash table has a next pointer, the last of which is appended to on insertion).

The order may be unusual or even non-obvious, but it is predictable.

[jeltz → stormbrew]

Ruby 1.9 hash tables are also ordered by insertion order.

[smsm42 → 1SaltwaterC]

"Ordered" means "having order", not "having order that I wanted it to have". The order, unless you have sorted it, is the order of creation of elements. Which may not be what you want if you want numeric key order and you insert keys in different order. So what it means is that PHP array/hash structure supports the concept of order - so you can ask questions like "what is the first element? what is the next one?" Not for all hash structures this is true - for many hashes, asking "what is the next element after this one" is meaningless since there's no order defined on elements.

[1SaltwaterC → smsm42]

I got it now. The issue is the language barrier. And by language I mean my native language where "ordered" is sometimes used as synonym for "sorted" (at least in CS and Math fields), hence the confusion.

[stormbrew → unconed]

Insert-ordered hash tables are a little less unique now. Ruby 1.9+ has them (with the built-in Hash), though it was introduced quietly and not a lot of people seem to know that.

It's a surprisingly useful data structure sometimes. I'm not sure if conflating three major kinds of data structures into one was a great decision, though. Honestly the OP's weirdness is much less likely to be important than the mixed-integer/string key weirdnesses you're more likely to run into.

[objectified → unconed]

But why would you ask a question like that in an interview? What does it tell you about the person you're interviewing? Or does it tell you something about yourself?

[maratd → objectified]

> But why would you ask a question like that in an interview?

Simple. You want to find out if they spend their time on HN. There hasn't been a single discussion on HN about PHP that doesn't mention that arrays in PHP are hash tables. Not one.

[KMag → objectified]

It tests the candidate's ability to work on a vaguely specified problem and also gives them insight on how the candidate feels about working on a lot of poorly specified projects.

If the candidate is not smart enough to get up and leave the interview after being asked such a terrible question, you don't want to hire them. The company's actual business model is to sell the resumes of people who are smart enough to reject working there to companies looking for good coders.

Edit: confusing use of "their".

[cpleppert → unconed]

I am absolutely certain that you are correct. I have never programmed in PHP so I was confused trying to understand exactly what was meant by references in the stack overflow question. I was struck by how every discussion of the topic was completely imprecise.

There was no clear explanation of references and their relationship with variables and how they differ from what are usually called 'pointers' that clearly distinguished PHP's view of variables from every other object oriented programming language that primarily used reference semantics for objects.

It was only in the context of a discussion of how zvals work and the is_ref and the refcount fields that I could understand exactly what semantics were being used by PHP.