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I've been coding in c++ and java entirety of my life but on C#, I feel like it's a totally different animal.
In case of hash collision in Dictionary container in c#, what does it do? or does it even detect the collision?
In case of collisions in similar containers in SDL, some would make a key value section link data to key value section like linked list, or some would attempt to find different hash method.
[Update 10:56 A.M. 6/4/2010]
I am trying to make a counter per user. And set user # is not defined, it can both increase or decrease. And I'm expecting the size of data to be over 1000.
So, I want :
Hashmap was my solution, and it seems Dictionary is what is similar to hashmap in c#...
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A collision, or more specifically, a hash code collision in a HashMap, is a situation where two or more key objects produce the same final hash value and hence point to the same bucket location or array index.
Python dict uses open addressing to resolve hash collisions (see dictobject. c:296-297). In open addressing, hash collisions are resolved by probing (explained below) . The hash table is just a contiguous block of memory (like an array, so you can do O(1) lookup by index).
Chain hashing avoids collision. The idea is to make each cell of hash table point to a linked list of records that have same hash function value. Note: In Linear Probing, whenever a collision occurs, we probe to the next empty slot.
One of the main things you want to avoid in a hashed collection is collisions. This is when two or more keys map to the same bucket. These collisions mean you have to do more work to check the key is the one you expected as there is now multiple keys in the same bucket. Ideally there is at most 1 key in each bucket.
Otherwise, it will simply create a whole new key-value pair. Collision happens when multiple keys hash to the same bucket or, say when two or more objects have the same hashcode but are different.
Hash collisions are correctly handled by Dictionary<> - in that so long as an object implements GetHashCode () and Equals () correctly, the appropriate instance will be returned from the dictionary. First, you shouldn't make any assumptions about how Dictionary<> works internally - that's an implementation detail that is likely to change over time.
Collision happens when multiple keys hash to the same bucket or, say when two or more objects have the same hashcode but are different. When two keys get hashed to the same value, a linked list is formed at the bucket location, where all the information is stored as an entry of the map, which contains the key-value pair.
Whenever two different object instances yield the same hash code, they are stored in the same internal bucket of the dictionary. The result of this, is that a linear scan must be performed, calling Equals () on each instance until a match is found.
Hash collisions are correctly handled by Dictionary<> - in that so long as an object implements GetHashCode() and Equals() correctly, the appropriate instance will be returned from the dictionary.
First, you shouldn't make any assumptions about how Dictionary<> works internally - that's an implementation detail that is likely to change over time. Having said that....
What you should be concerned with is whether the types you are using for keys implement GetHashCode() and Equals() correctly. The basic rules are that GetHashCode() must return the same value for the lifetime of the object, and that Equals() must return true when two instances represent the same object. Unless you override it, Equals() uses reference equality - which means it only returns true if two objects are actually the same instance. You may override how Equals() works, but then you must ensure that two objects that are 'equal' also produce the same hash code.
From a performance standpoint, you may also want to provide an implementation of GetHashCode() that generates a good spread of values to reduce the frequency of hashcode collision. The primarily downside of hashcode collisions, is that it reduces the dictionary into a list in terms of performance. Whenever two different object instances yield the same hash code, they are stored in the same internal bucket of the dictionary. The result of this, is that a linear scan must be performed, calling Equals() on each instance until a match is found.
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