I'm currently trying to learn Symfony and a big part of it is Doctrine. I've been reading the official documentation for Doctrine and in the part about Collections library I stumbled upon this thing called "ordered map". I tried to search it on google, but I wasn't able to find any satisfying answer. There were just answers for specific languages (mostly Java and C++), but I want to understand it in general. How it works and what it is, because in the Doctrine documentation they are comparing it to the ArrayCollection, so I hope if I can understand what it is, it will be easier for me to understand ArrayCollection as well.
I tried to search for things like "what is an ordered map" or "ordered map explained", but as I said earlier, I didn't find what I was looking for.
A map is sometimes called ordered when the entries remain in the same sequence in which they were inserted.
For example, arrays in PHP are ordered (preserve insertion order). So creating/modifying an array like this:
$array = [2 => 'a', 1 => 'b'];
$array[0] = 'c';
will indeed result in the PHP array [2 => 'a', 1 => 'b', 0 => 'c'] - it preserves the insertion order - while in some other languages it will be turned into [0 => 'c', 1 => 'b', 2 => 'a'].
This affects a few operations. Iterating over an array with foreach will return the entries in insertion order. You can do key-wise or value-wise sorting on PHP arrays, the default sorting function sort will drop the original keys and reindex numerically. Serialization and deserialization with numeric keys may have unintended consequences. And some other effects that sometimes are beneficial and sometimes are surprising or annoying (or both). You can read lots of it on PHP's array doc page and the array function pages.
In the context of Doctrine (since it's written in PHP) this means, that a collection where values are the entity objects can be sorted in any manner you want (including id of course), and if you iterate over that collection, you get the entity objects in the order they were added by doctrine (the order of the SQL/DQL query). Doctrine also allows to set the keys to the entities' ids, while still preserving the SQL/DQL query order. This can simplify code since Doctrine's Collection implements PHP's ArrayAccess.
As a counter example, maps can also be unordered or sorted, where the first means when you retrieve the pairs the order can be random (in golang, the starting index used to be random when iterating over maps, don't know if this is still true) or automatically sorted (like SortedMap in Java).