Question

is there a commonly used data structure for mult-key data? e.g. (key1, key2, …, keyN) -> value. I used to use dictionaries of dictionaries (in c#), and then wrote my own wrapper on top of this to make the syntax look a bit nicer. but it seems like I still have to write a wrapper for each N-dictionary, where N is the number of keys, since I have to define the nested dictionary structure within the code.

assuming I’m using c#, is there a data structure that better encapsulates this sort of usage, and could contain an arbitrary number of keys with hashtable-like lookup performance? I can’t simply combine all the keys into a single unique key, because I need to be able to do something like

foreach key2 in data[key1]
    foreach key3 in data[key1][key2]
        foreach key4 in data[key1][key2][key3]

Answer 1

No, it isn’t.

Without breaking type-safety, I think there are two solutions.

• Dictionaries of Dictionaries – Dictionary<T1, Dictionary<T2, TRes>>
• Dictionaries of tuples – Dictionary<Tuple3<T1, T2, T3>, Res>. Note that – unlike F# where you could write Map<T1 * T2 * T3, Res> – C# doesn’t have a builtin tuple type – you’d have to implement this separately as a generic class or struct.

But regarding your example code, jagged dictionaries (Dictionaries of Dictionaries) are the only alternative.