{ "cells": [ { "cell_type": "markdown", "metadata": {}, "source": [ "This notebook was prepared by [Donne Martin](http://donnemartin.com). Source and license info is on [GitHub](https://bit.ly/code-notes)." ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Problem: Implement a hash table with set, get, and remove methods.\n", "\n", "* [Constraints and Assumptions](#Constraints-and-Assumptions)\n", "* [Test Cases](#Test-Cases)\n", "* [Algorithm](#Algorithm)\n", "* [Code](#Code)\n", "* [Pythonic-Code](#Pythonic-Code)\n", "* [Unit Test](#Unit-Test)" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Constraints and Assumptions\n", "\n", "*Problem statements are often intentionally ambiguous. Identifying constraints and stating assumptions can help to ensure you code the intended solution.*\n", "\n", "* For simplicity, are the keys integers only?\n", " * Yes\n", "* For collision resolution, can we use linked lists?\n", " * Yes\n", "* Do we have to worry about load factors?\n", " * No" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Test Cases\n", "\n", "* get on an empty hash table index\n", "* set on an empty hash table index\n", "* set on a non empty hash table index\n", "* set on a key that already exists\n", "* remove on a key with an entry\n", "* remove on a key without an entry" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Algorithm\n", "\n", "### Hash Function\n", "\n", "* Return key % table size\n", "\n", "Complexity:\n", "* Time: O(1)\n", "* Space: O(1)\n", "\n", "### Set\n", "\n", "* Get hash index for lookup\n", "* If key exists, replace\n", "* Else, add\n", "\n", "Complexity:\n", "* Time: O(1) average and best, O(n) worst\n", "* Space: O(1) space for newly added element\n", "\n", "### Get\n", "\n", "* Get hash index for lookup\n", "* If key exists, return value\n", "* Else, return NULL\n", "\n", "Complexity:\n", "* Time: O(1) average and best, O(n) worst\n", "* Space: O(1)\n", "\n", "### Remove\n", "\n", "* Get hash index for lookup\n", "* If key exists, delete the item\n", "\n", "Complexity:\n", "* Time: O(1) average and best, O(n) worst\n", "* Space: O(1)" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Code" ] }, { "cell_type": "code", "execution_count": 1, "metadata": { "collapsed": false }, "outputs": [], "source": [ "class Item(object):\n", " def __init__(self, key, value):\n", " self.key = key\n", " self.value = value\n", "\n", "class HashTable(object):\n", " def __init__(self, size):\n", " self.size = size\n", " self.table = [[] for _ in xrange(self.size)]\n", "\n", " def hash_function(self, key):\n", " return key % self.size\n", "\n", " def set(self, key, value):\n", " hash_index = self.hash_function(key)\n", " for item in self.table[hash_index]:\n", " if item.key == key:\n", " item.value = value\n", " return\n", " self.table[hash_index].append(Item(key, value))\n", "\n", " def get(self, key):\n", " hash_index = self.hash_function(key)\n", " for item in self.table[hash_index]:\n", " if item.key == key:\n", " return item.value\n", " return None\n", "\n", " def remove(self, key):\n", " hash_index = self.hash_function(key)\n", " for i, item in enumerate(self.table[hash_index]):\n", " if item.key == key:\n", " del self.table[hash_index][i]" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Unit Test" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "*It is important to identify and run through general and edge cases from the [Test Cases](#Test-Cases) section by hand. You generally will not be asked to write a unit test like what is shown below.*" ] }, { "cell_type": "code", "execution_count": 2, "metadata": { "collapsed": false }, "outputs": [ { "name": "stdout", "output_type": "stream", "text": [ "Test: get on an empty hash table index\n", "Test: set on an empty hash table index\n", "Test: set on a non empty hash table index\n", "Test: set on a key that already exists\n", "Test: remove on a key that already exists\n", "Test: remove on a key that doesn't exist\n", "Success: test_end_to_end\n" ] } ], "source": [ "from nose.tools import assert_equal\n", "\n", "class Test(object):\n", " # TODO: It would be better if we had unit tests for each\n", " # method in addition to the following end-to-end test\n", " def test_end_to_end(self):\n", " hash_table = HashTable(10)\n", " \n", " print(\"Test: get on an empty hash table index\")\n", " assert_equal(hash_table.get(0), None)\n", " \n", " print(\"Test: set on an empty hash table index\")\n", " hash_table.set(0, 'foo')\n", " assert_equal(hash_table.get(0), 'foo')\n", " hash_table.set(1, 'bar')\n", " assert_equal(hash_table.get(1), 'bar')\n", " \n", " print(\"Test: set on a non empty hash table index\")\n", " hash_table.set(10, 'foo2')\n", " assert_equal(hash_table.get(0), 'foo')\n", " assert_equal(hash_table.get(10), 'foo2')\n", " \n", " print(\"Test: set on a key that already exists\")\n", " hash_table.set(10, 'foo3')\n", " assert_equal(hash_table.get(0), 'foo')\n", " assert_equal(hash_table.get(10), 'foo3')\n", " \n", " print(\"Test: remove on a key that already exists\")\n", " hash_table.remove(10)\n", " assert_equal(hash_table.get(0), 'foo')\n", " assert_equal(hash_table.get(10), None)\n", " \n", " print(\"Test: remove on a key that doesn't exist\")\n", " hash_table.remove(-1)\n", " \n", " print('Success: test_end_to_end')\n", "\n", "if __name__ == '__main__':\n", " test = Test()\n", " test.test_end_to_end()" ] } ], "metadata": { "kernelspec": { "display_name": "Python 2", "language": "python", "name": "python2" }, "language_info": { "codemirror_mode": { "name": "ipython", "version": 2 }, "file_extension": ".py", "mimetype": "text/x-python", "name": "python", "nbconvert_exporter": "python", "pygments_lexer": "ipython2", "version": "2.7.10" } }, "nbformat": 4, "nbformat_minor": 0 }