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> I hate that many other languages, including C#, Ruby and Python, force me to choose between either an unordered map or a list of (key, value) tuples Another

> I hate that many other languages, including C#, Ruby and Python, force me to choose between either an unordered map or a list of (key, value) tuples Another commenter mentioned Python's OrderedDict, and Ruby's hashtables are ordered (from 1.9+): https://www.igvita.com/2009/02/04/ruby-19-internals-ordered-hash/ https://www.igvita.com/2009/02/04/ruby-19-internals-ordered-... It looks like C# also has an OrderedDictionary class: http://msdn.microsoft.com/en-us/library/system.collections.specialized.ordereddictionary.aspx http://msdn.microsoft.com/en-us/library/system.collections.s... I never had to use any of these classes even if I do use hashmaps all the time, so IMHO the Python way is best (default to a 'normal' unordered hashmap, offer an ordered alternative)

mduerksen 12y ago
Thanks for the pointers! I find myself needing this surprisingly often: An array of elements in a certain order which I also want to lookup by Id. Sometimes the order is defined by configuration, sometimes by some other criteria that is not accessible for this particular component, so I can't resort to a SortedHashMap. Still, I need a 1) fast and 2) convenient way of lookup by some key. Convenience is often more important to me than performance in those cases, since the data size is not huge, but I hate it when I have to write array.find(e => e.Key == myKey) instead of orderedLookup[myKey].
breuleux 12y ago
I think there is one reason to default to something ordered, which is that iterating through unordered hashmaps can be non-deterministic from one run to the other. For instance, if you hash by id, the same objects may be assigned different ids from a run to the next, meaning that the map will be iterated in a different order. If there is a bug that's sensitive to iteration order, it will not be reproducible and that can be very annoying to debug.
- aidos 12y ago
  If you've got bugs due to iteration order you have a whole other issue. If you know that iteration order is important, it should be explicit in the way you prepare/store/handle the data. If your code only works when you iterate in a specific order and you didn't deliberately define that order, it's only really working by accident. Don't get me wrong, it's actually a bug I've run into in the past, and you're right, the non-deterministic issue makes it harder to debug. But when I discovered it I blamed myself for not being explicit with my constraints.
- deleted 12y ago
  [deleted]

mduerksen 12y ago

Thanks for the pointers! I find myself needing this surprisingly often: An array of elements in a certain order which I also want to lookup by Id. Sometimes the order is defined by configuration, sometimes by some other criteria that is not accessible for this particular component, so I can't resort to a SortedHashMap. Still, I need a 1) fast and 2) convenient way of lookup by some key. Convenience is often more important to me than performance in those cases, since the data size is not huge, but I hate it when I have to write array.find(e => e.Key == myKey) instead of orderedLookup[myKey].

breuleux 12y ago

I think there is one reason to default to something ordered, which is that iterating through unordered hashmaps can be non-deterministic from one run to the other. For instance, if you hash by id, the same objects may be assigned different ids from a run to the next, meaning that the map will be iterated in a different order. If there is a bug that's sensitive to iteration order, it will not be reproducible and that can be very annoying to debug.

aidos 12y ago
If you've got bugs due to iteration order you have a whole other issue. If you know that iteration order is important, it should be explicit in the way you prepare/store/handle the data. If your code only works when you iterate in a specific order and you didn't deliberately define that order, it's only really working by accident. Don't get me wrong, it's actually a bug I've run into in the past, and you're right, the non-deterministic issue makes it harder to debug. But when I discovered it I blamed myself for not being explicit with my constraints.
deleted 12y ago
[deleted]