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Date: Sat, 6 May 2017 16:12:11 +0100
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To: Ryan Pallas <derokorian@gmail.com>
Cc: Jesse Schalken <me@jesseschalken.com>, PHP internals <internals@lists.php.net>
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Subject: Re: [PHP-DEV] Add is_vectorlike($array) function
From: aidantwoods@gmail.com (Aidan Woods)

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> Also, given the implementation above, why does the array have to be
ksorted
> correctly to count? Specifically, why isn't this array considered vector
> like?
> $arr = [
>     1=> 1,
>     0=> 0,
> ];

I suppose the question would be, given that array (or a similar
out-of-order array), should is_vectorlike afford you the expectation that
in a foreach loop:

foreach ($arr as $i => $v) {
  // do something
}

that (within the length of the array) the value $arr[$i + 1] has not yet
been processed by the code in the loop, and is also the next item to be
processed.

Or maybe more succinctly, should is_vectorlike guarantee that items are
iterated over in order?

On 5 May 2017 at 22:19, Ryan Pallas <derokorian@gmail.com> wrote:

> On Tue, May 2, 2017 at 3:13 AM, Jesse Schalken <me@jesseschalken.com>
> wrote:
>
> > Related to the optimisation made by Sara Golemon here:
> > https://github.com/php/php-src/commit/c74bc87c74f48bc55541b3bf2fc67d
> > 595f58a3b5
> >
> > I often define a function like this, which checks if an array is "vector
> > like" i.e. has keys 0,1,2..N:
> >
> > function is_vectorlike(array $a): bool {
> >   $i = 0;
> >   foreach ($a as $k => $v) {
> >     if ($k !== $i++) {
> >       return false;
> >     }
> >   }
> >   return true;
> > }
> >
> > The problem is that this function is O(n), but in PHP7 an array that is
> > vector-like is likely to be packed and without holes (HT_IS_PACKED(x) &&
> > HT_IS_WITHOUT_HOLES(x)), in which case it is known to be vector-like
> > without needing to iterate over it, but PHP code can't check for this
> (nor
> > should it be able to, since it's really an implementation detail).
> >
> > Would it be a good idea to define this is_vectorlike() function in the
> PHP
> > runtime, so it can short circuit to return true on packed arrays without
> > holes? The above code would be a suitable polyfill.
> >
>
>
> I just read this thread and am wondering what exactly is the use case? Like
> are you going to do something if it is vector-like, and do something
> different (or not do anything at all) if it's not vector-like? I mean, if
> you have to crawl it, and need a vector, why not just call array_values and
> guarantee you have a vector?
>
> Also, given the implementation above, why does the array have to be ksorted
> correctly to count? Specifically, why isn't this array considered vector
> like?
> $arr = [
>     1=> 1,
>     0=> 0,
> ];
>
> Wouldn't the following be a better test?
> function is_vectorlike(array $a): bool {
>   $l = count($a);
>   for (i=0; $i<$l, $i++) {
>     if (!array_key_exists($i, $a)) {
>       return false;
>     }
>   }
>   return true;
> }
>

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