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",
    "* [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)"
   ]
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   "cell_type": "markdown",
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   "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"
   ]
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   "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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   "cell_type": "markdown",
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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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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Code"
   ]
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  {
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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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   "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.*"
   ]
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     "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",
    "    \n",
    "    # TODO: It would be better if we had unit tests for each\n",
    "    # HashTable 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()"
   ]
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