Part 2 of series Detecting cyclic arrays in PHP.
In Part 1 of this series I explained the problem of detecting cyclic arrays, and why it cannot be done in pure PHP. I also promised to present a solution that is both simple and correct for all possible arrays, which is what this post is about. But before that, let me restate the definition of cyclic arrays from the previous post: An array is cyclic if iterating all of its items and subitems does not terminate.
Here is an example of a cyclic array:
Continue reading “Detecting cyclic arrays in PHP – A new approach”
$array = [1, [2, 3]];
$array = &$array;
Part 1 of series Detecting cyclic arrays in PHP.
This post is about a problem that might only be relevant to a small minority of PHP programmers who maybe want to write a custom serializer, or something else that requires recursively iterating an array. But, in trying to solve it, I found some interesting PHP weirdness that I think is worth highlighting.
As an aside:
If you are just looking for a reliable solution for detecting cyclic arrays – and you want to miss out on learning about some cool (but also complicated) PHP quirks – then skip to Part 2 of this series.
First of all, let’s define our terms. For the purpose of this post, we can espouse a very pragmatic definition: An array is cyclic if iterating all of its items and subitems does not terminate. Cyclic arrays are also sometimes called circular or recursive. Let’s look at some examples:
// Contains a reference to itself
$v = [1, 2, 3];
$v = &$v;
// Contains a nested array that contains a reference to $x
$x = [1, [2, 3]];
$x = &$x;
// Contains a nested array that contains a reference to an ancestor
$y = [1, [2, [3, 4]]];
$y = &$y;
// Contains a nested array that contains a reference to itself
$z = [1, [2, 3]];
$z = &$z;
I think you get the idea. All of these arrays contain a cycle at some level.
Continue reading “Why it is impossible to detect cyclic arrays in pure PHP”