Meaning of Open hashing and Closed hashing

Question

Open Hashing  Separate Chaining       In open hashing  keys are stored in linked lists attached to cells of a hash table    Closed Hashing  Open Addressing       In closed hashing  all keys are stored in the hash table itself without the use of linked lists    I am unable to understand why they are called open  closed and Separate  Can some one explain it

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You have an array that is the  hash table    In Open Hashing each cell in the array points to a list containg the collisions  The hashing has produced the same index for all items in the linked list   In Closed Hashing you use only one array for everything  You store the collisions in the same array  The trick is to use some smart way to jump from collision to collision unitl you find what you want  And do this in a reproducible   deterministic way

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The name open addressing refers to the fact that the location   address   of the element is not determined by its hash value   This method is also called closed hashing    In separate chaining  each bucket is independent  and has some sort of ADT  list  binary search trees  etc  of entries with the same index  In a good hash table  each bucket has zero or one entries  because we need operations of order O 1   for insert  search  etc   This is a example of separate chaining using C   with a simple hash function using mod operator  clearly  a bad hash function

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The use of  closed  vs   open  reflects whether or not we are locked in to using a certain position or data structure  this is an extremely vague description  but hopefully the rest helps    For instance  the  open  in  open addressing  tells us the index  aka  address  at which an object will be stored in the hash table is not completely determined by its hash code  Instead  the index may vary depending on what s already in the hash table      The  closed  in  closed hashing  refers to the fact that we never leave the hash table  every object is stored directly at an index in the hash table s internal array  Note that this is only possible by using some sort of open addressing strategy  This explains why  closed hashing  and  open addressing  are synonyms   Contrast this with open hashing - in this strategy  none of the objects are actually stored in the hash table s array  instead once an object is hashed  it is stored in a list which is separate from the hash table s internal array   open  refers to the freedom we get by leaving the hash table  and using a separate list  By the way   separate list  hints at why open hashing is also known as  separate chaining    In short   closed  always refers to some sort of strict guarantee  like when we guarantee that objects are always stored directly within the hash table  closed hashing   Then  the opposite of  closed  is  open   so if you don t have such guarantees  the strategy is considered  open

[hash] Meaning of Open hashing and Closed hashing

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