interactive-coding-challenges/arrays_strings/hash_map/hash_map_solution.ipynb

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   "source": [
    "This notebook was prepared by [Donne Martin](http://donnemartin.com). Source and license info is on [GitHub](https://github.com/donnemartin/interactive-coding-challenges)."
   ]
  },
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# Solution Notebook"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Problem: Implement a hash table with set, get, and remove methods.\n",
    "\n",
    "* [Constraints](#Constraints)\n",
    "* [Test Cases](#Test-Cases)\n",
    "* [Algorithm](#Algorithm)\n",
    "* [Code](#Code)\n",
    "* [Unit Test](#Unit-Test)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Constraints\n",
    "\n",
    "* For simplicity, are the keys integers only?\n",
    "    * Yes\n",
    "* For collision resolution, can we use chaining?\n",
    "    * Yes\n",
    "* Do we have to worry about load factors?\n",
    "    * No\n",
    "* Do we have to validate inputs?\n",
    "    * No\n",
    "* Can we assume this fits memory?\n",
    "    * Yes"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Test Cases\n",
    "\n",
    "* `get` no matching key -> KeyError exception\n",
    "* `get` matching key -> value\n",
    "* `set` no matching key -> new key, value\n",
    "* `set` matching key -> update value\n",
    "* `remove` no matching key -> KeyError exception\n",
    "* `remove` matching key -> remove key, value"
   ]
  },
  {
   "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, raise KeyError\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",
    "* Else, raise KeyError\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
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   "outputs": [],
   "source": [
    "class Item(object):\n",
    "\n",
    "    def __init__(self, key, value):\n",
    "        self.key = key\n",
    "        self.value = value\n",
    "\n",
    "\n",
    "class HashTable(object):\n",
    "\n",
    "    def __init__(self, size):\n",
    "        self.size = size\n",
    "        self.table = [[] for _ in range(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",
    "        raise KeyError('Key not found')\n",
    "\n",
    "    def remove(self, key):\n",
    "        hash_index = self._hash_function(key)\n",
    "        for index, item in enumerate(self.table[hash_index]):\n",
    "            if item.key == key:\n",
    "                del self.table[hash_index][index]\n",
    "                return\n",
    "        raise KeyError('Key not found')"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Unit Test"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {
    "collapsed": false
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   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Overwriting test_hash_map.py\n"
     ]
    }
   ],
   "source": [
    "%%writefile test_hash_map.py\n",
    "from nose.tools import assert_equal, assert_raises\n",
    "\n",
    "\n",
    "class TestHashMap(object):\n",
    "\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_raises(KeyError, hash_table.get, 0)\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_raises(KeyError, hash_table.get, 10)\n",
    "\n",
    "        print(\"Test: remove on a key that doesn't exist\")\n",
    "        assert_raises(KeyError, hash_table.remove, -1)\n",
    "\n",
    "        print('Success: test_end_to_end')\n",
    "\n",
    "\n",
    "def main():\n",
    "    test = TestHashMap()\n",
    "    test.test_end_to_end()\n",
    "\n",
    "\n",
    "if __name__ == '__main__':\n",
    "    main()"
   ]
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  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {
    "collapsed": false
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    {
     "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": [
    "run -i test_hash_map.py"
   ]
  }
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