interactive-coding-challenges/arrays-strings/hash-map.ipynb

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    "<small><i>This notebook was prepared by [Donne Martin](http://donnemartin.com). Source and license info is on [GitHub](https://bit.ly/code-notes).</i></small>"
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    "## Problem: Implement a hash table with set, get, and remove methods.\n",
    "\n",
    "* [Clarifying Questions](#Clarifying-Questions)\n",
    "* [Test Cases](#Test-Cases)\n",
    "* [Algorithm](#Algorithm)\n",
    "* [Code](#Code)\n",
    "* [Pythonic-Code](#Pythonic-Code)"
   ]
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   "cell_type": "markdown",
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   "source": [
    "## Clarifying Questions\n",
    "\n",
    "* Are the keys integers only?\n",
    "    * Yes"
   ]
  },
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   "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"
   ]
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   "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)"
   ]
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   "metadata": {},
   "source": [
    "## Code"
   ]
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   "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]"
   ]
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    "table_size = 10\n",
    "hash_table = HashTable(table_size)\n",
    "print(\"get on an empty hash table index\")\n",
    "print(hash_table.get(0))\n",
    "print(\"set on an empty hash table index\")\n",
    "hash_table.set(0, 'foo')\n",
    "print(hash_table.get(0))\n",
    "hash_table.set(1, 'bar')\n",
    "print(hash_table.get(1))\n",
    "print(\"set on a non empty hash table index\")\n",
    "hash_table.set(10, 'foo2')\n",
    "print(hash_table.get(0))\n",
    "print(hash_table.get(10))\n",
    "print(\"set on a key that already exists\")\n",
    "hash_table.set(10, 'foo3')\n",
    "print(hash_table.get(0))\n",
    "print(hash_table.get(10))\n",
    "print(\"remove on a key that already exists\")\n",
    "hash_table.remove(10)\n",
    "print(hash_table.get(0))\n",
    "print(hash_table.get(10))\n",
    "print(\"remove on a key that doesn't exist\")\n",
    "hash_table.remove(-1)"
   ]
  }
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