C#•2y ago

❔ Best way to create a unique key made out of x y integer coordonates for a dictionary

Hello, i want to create a unique key to access values in a Dictionary, this key has to be unique and able to be created using just simple x y integer type coordonates. Any help is appreciated. Thank you

47 Replies

sibber•2y ago

why not make TKey the coordinate

BirdmanOP•2y ago

not sure i follow

sibber•2y ago

Dictionary<Coordinate, Whatever>

BirdmanOP•2y ago

but let's say i make a Coordonate(1, 2) and then create a new Coordonate(1, 2) object to access that value, would it work?

sibber•2y ago

if they return the same hash code then yes

BirdmanOP•2y ago

but would they retun the same hash code? like if i do Coordonate(1, 2) Coordonate(1, 2) would both have same hash?

sibber•2y ago

i mean you should override GetHashCode

ero•2y ago

If you implement the method, sure

sibber•2y ago

or preferably make it a record

ero•2y ago

Definitely make it a record A ValueTuple might just be enough But less concrete

BirdmanOP•2y ago

but don't i have to manually generate the hash myself to always get a unique one based on the coordonates?

sibber•2y ago

GetHashCode is a method from object you just override it in the class/struct or use a record which does it for you, as well as implementing value equality and other things

BirdmanOP•2y ago

okay sure, i get that, but my issue is how do i generate that hash so it's unique but also recreatable from the coordonates

D.Mentia•2y ago

also if it's a struct you're already done; as long as the values match it's equal

sibber•2y ago

you dont, Dictionary does

ero•2y ago

The whole point is that it is recreatable You implement the creation

D.Mentia•2y ago

Hashcodes are kindof a whole separate thing that you don't really need to know anymore because of records Just use a struct, or a record, and it'll do the job

sibber•2y ago

not exactly

D.Mentia•2y ago

? That's how value types work, right? And structs are always value types?

ero•2y ago

Only if they're unmanaged As in, fit the unmanaged constraint If the struct contains a reference type, it won't work

D.Mentia•2y ago

Oh yeah, that's fair

MODiX•2y ago

Cyberrex#8052

REPL Result: Success

S s1 = new() { Y = "a" };
S s2 = new() { Y = "b" };
Console.WriteLine(s1.GetHashCode() == s2.GetHashCode());

struct S
{
    public int X {get;set;}
    public string Y { get; set; }
}

S s1 = new() { Y = "a" };
S s2 = new() { Y = "b" };
Console.WriteLine(s1.GetHashCode() == s2.GetHashCode());

struct S
{
    public int X {get;set;}
    public string Y { get; set; }
}

Console Output

True

True

Compile: 462.573ms | Execution: 68.553ms | React with ❌ to remove this embed.

ero•2y ago

String is the exception of course

D.Mentia•2y ago

Though I think the real answer to this question is, don't use a Dictionary, use a 2d array. There's no advantage to using a Dictionary for this, I don't think? Since there is a known lower and upper bound, and it's already integers, the 2d array lookup would be just as fast as a dictionary (or faster, since you're not making a Coordinate)

sibber•2y ago

not really, it depends on what theyre trying to do

D.Mentia•2y ago

(a 2d array also means you could do things like, binary search to find the entity that is closest to a given coordinate...? Maybe. I've been thinking about a similar implementation)

BirdmanOP•2y ago

hmm, how much slower would be iterating through a 100000x100000 array vs a dictionary with a much smaller list of coordonates?

sibber•2y ago

how are they an exception in this case?

BirdmanOP•2y ago

cause i have to iterate through the whole thing

sibber•2y ago

probably faster than a dictionary

D.Mentia•2y ago

I've been told you should never iterate a dictionary, but I don't know how bad it is if you do

sibber•2y ago

oh wait smaller? depends, benchmark it i mean yeah a dictionary isnt meant to be iterated

D.Mentia•2y ago

If you're iterating it, why do you need to be able to lookup the value?

sibber•2y ago

iterating it will be relatively slow

BirdmanOP•2y ago

huge mostly empty array vs a dictionary with lets say 20% of the values in the 2d array to remove values from it and also add

sibber•2y ago

i think having a coordinate type and a dictionary makes more sense

BirdmanOP•2y ago

and also build something from it, that's why i have to iterate through all of it

sibber•2y ago

still why do you need to iterate it?

ero•2y ago

String already implements Equals and GetHashCode

sibber•2y ago

yeah but the default struct.GetHashCode doesnt use that

D.Mentia•2y ago

Then a HashSet is probably your best bet; constant time add/remove/contains, iteration is fine. That's the point where you need to make sure what you're storing has good Equals and GetHashCode implementations for a HashSet

sibber•2y ago

this might be an xy problem

D.Mentia•2y ago

... well, idk. Dictionary is fine. The other answers are fine. Just pointing out other options, but it does depend on what you're doing with it and all that I would go with a simple record (struct?) for the coordinates, if Coordinate doesn't already implement value comparisons

sibber•2y ago

yeah id make coordinate a record struct

D.Mentia•2y ago

Also just btw, dictionaries and hashsets don't use the hash for a real equality check; looking up something with the same hash (but different Equals) won't give you a result. The hashes are just the fast way it can look things up, if there aren't many collisions in the hashes, but it still checks the Equals too

BirdmanOP•2y ago

thanks guys, i'll look into this further to see that's optimal, your support is appreciated 👍

Accord•2y ago

Was this issue resolved? If so, run /close - otherwise I will mark this as stale and this post will be archived until there is new activity.

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❔ Best way to create a unique key made out of x y integer coordonates for a dictionary