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Hash Table
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What does a hash table do ?
A hash table is a data structure that uses hashing to
implement mappings of key / value pairs.
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What does a hash table do ?
A simplistic diagram
1. Takes a key /
value pair as
input
2. Key goes through
the hash function
and returns an index
3. The pair is stored
at that index
hash
function
1 { key: value }
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What makes the hash table great ?
• A hash function is irreversible. One cannot take the
index and find the key. It only works one way.
• A hash table is deterministic system. One gets the
same result from the same input every time.
• A hash table is very efficient. The time complexity
in big O notation is O(1).
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Notes
• However it is possible for two different inputs to
return the same result. This is called a collision.
• Collisions are stored in a nested array. This method
is called seperate chaining.
• This is also why the time complexity worst case is
O(n), n being the number of inputs.
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Before we get to the code
• Hash tables are included in most programming
languages. This article is for a better understanding
on how they work than a tutorial on how to
implement one yourself.
• In the following code example, we will implement
an simple hash function. There are multiple ways to
write them.
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code review
Hash Table
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First comes the hash
function. It takes a key
of type string and an
integer size as
parameter.
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The hashedkey is the
sum of each character's
code.
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The size is the length of
the array storing the
key/value pairs. Thus
hashedkey % size returns
the index.
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Then the hash table as an
IIFE to immediately
create the instance.
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Fill the buckets array
with Map objects to
handle collisions.
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The insert function
invoking the hash
function to get the index.
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Then use the set method
from Map object to
store the key/value pair.
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The remove function will
save the value of the
element to be removed
with the get method of
the Map object.
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The delete method of
Map will remove the
element.
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The search function
returns the value with
get method of Map with
the appropriate index
returned by the hash
function.
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The myBuckets function
simply logs the buckets
array.
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All that is left to do is
return the closures in an
object.
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Since we referenced
hashTable to an
Immediatly Invoked
Function Expression, we
can just invoke our
methods.
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Play around with the code on
codepen by clicking the link below