This notebook was prepared by [Donne Martin](http://donnemartin.com). Source and license info is on [GitHub](https://github.com/donnemartin/interactive-coding-challenges).

Solution Notebook

Problem: Implement a hash table with set, get, and remove methods.

Constraints
Test Cases
Algorithm
Code
Unit Test

Constraints

For simplicity, are the keys integers only?
- Yes
For collision resolution, can we use linked lists?
- Yes
Do we have to worry about load factors?
- No

Test Cases

get on an empty hash table index
set on an empty hash table index
set on a non empty hash table index
set on a key that already exists
remove on a key with an entry
remove on a key without an entry

Algorithm

Hash Function

Return key % table size

Complexity:

Time: O(1)
Space: O(1)

Set

Get hash index for lookup
If key exists, replace
Else, add

Complexity:

Time: O(1) average and best, O(n) worst
Space: O(1) space for newly added element

Get

Get hash index for lookup
If key exists, return value
Else, return None

Complexity:

Time: O(1) average and best, O(n) worst
Space: O(1)

Remove

Get hash index for lookup
If key exists, delete the item

Complexity:

Time: O(1) average and best, O(n) worst
Space: O(1)

Code



In [1]:

    
class Item(object):

    def __init__(self, key, value):
        self.key = key
        self.value = value


class HashTable(object):

    def __init__(self, size):
        self.size = size
        self.table = [[] for _ in range(self.size)]

    def hash_function(self, key):
        return key % self.size

    def set(self, key, value):
        hash_index = self.hash_function(key)
        for item in self.table[hash_index]:
            if item.key == key:
                item.value = value
                return
        self.table[hash_index].append(Item(key, value))

    def get(self, key):
        hash_index = self.hash_function(key)
        for item in self.table[hash_index]:
            if item.key == key:
                return item.value
        return None

    def remove(self, key):
        hash_index = self.hash_function(key)
        for i, item in enumerate(self.table[hash_index]):
            if item.key == key:
                del self.table[hash_index][i]

Unit Test



In [2]:

    
%%writefile test_hash_map.py
from nose.tools import assert_equal


class TestHashMap(object):

    # TODO: It would be better if we had unit tests for each
    # method in addition to the following end-to-end test
    def test_end_to_end(self):
        hash_table = HashTable(10)

        print("Test: get on an empty hash table index")
        assert_equal(hash_table.get(0), None)

        print("Test: set on an empty hash table index")
        hash_table.set(0, 'foo')
        assert_equal(hash_table.get(0), 'foo')
        hash_table.set(1, 'bar')
        assert_equal(hash_table.get(1), 'bar')

        print("Test: set on a non empty hash table index")
        hash_table.set(10, 'foo2')
        assert_equal(hash_table.get(0), 'foo')
        assert_equal(hash_table.get(10), 'foo2')

        print("Test: set on a key that already exists")
        hash_table.set(10, 'foo3')
        assert_equal(hash_table.get(0), 'foo')
        assert_equal(hash_table.get(10), 'foo3')

        print("Test: remove on a key that already exists")
        hash_table.remove(10)
        assert_equal(hash_table.get(0), 'foo')
        assert_equal(hash_table.get(10), None)

        print("Test: remove on a key that doesn't exist")
        hash_table.remove(-1)

        print('Success: test_end_to_end')


def main():
    test = TestHashMap()
    test.test_end_to_end()


if __name__ == '__main__':
    main()









    



Overwriting test_hash_map.py



In [3]:

    
run -i test_hash_map.py









    



Test: get on an empty hash table index
Test: set on an empty hash table index
Test: set on a non empty hash table index
Test: set on a key that already exists
Test: remove on a key that already exists
Test: remove on a key that doesn't exist
Success: test_end_to_end