interactive-coding-challenges/arrays_strings/replace_char/replace_char.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: Given a string, replace in-place all spaces with '%20'\n",
    "\n",
    "* [Constraints and Assumptions](#Constraints-and-Assumptions)\n",
    "* [Test Cases](#Test-Cases)\n",
    "* [Algorithm](#Algorithm)\n",
    "* [Code](#Code)\n",
    "* [Pythonic-Code: Not In-Place](#Pythonic-Code:-Not-In-Place)\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",
    "* Can I assume the string is ASCII?\n",
    "    * Yes\n",
    "    * Note: Unicode strings could require special handling depending on your language\n",
    "* Is there enough space in the data structure for this operation?\n",
    "    * Yes"
   ]
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Test Cases\n",
    "\n",
    "* NULL->NULL\n",
    "* ' ' -> '%20'\n",
    "* ' foo bar ' -> '%20foo%20bar%20'\n",
    "* 'foo' -> 'foo'"
   ]
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   "cell_type": "markdown",
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   "source": [
    "## Algorithm\n",
    "\n",
    "Since Python strings are immutable, we'll use a bytearray instead to exercise in-place string manipulation as you would get with a C string (which is null terminated, as seen in the diagram below).  Python does not use a null-terminator.\n",
    "\n",
    "![alt text](https://raw.githubusercontent.com/donnemartin/algorithms-data-structures/master/images/replace_string.jpg)\n",
    "\n",
    "* Count the number of spaces in the bytearray\n",
    "* Determine the new bytearray length\n",
    "* For each character code in the bytearray, starting from the end of the string\n",
    "    * If the character code is a space\n",
    "        * bytearray[new length] = '0',\n",
    "        * bytearray[new length - 1] = '2',\n",
    "        * bytearray[new length - 2] = '%',\n",
    "        * new length -= 3\n",
    "    * Else\n",
    "        * bytearray[new length] = character code,\n",
    "        * new length -= 1\n",
    "\n",
    "Complexity:\n",
    "* Time: O(n)\n",
    "* Space: In-place"
   ]
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Code"
   ]
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  {
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   "source": [
    "def encode_spaces(string, length):\n",
    "    if string is None:\n",
    "        return\n",
    "    num_spaces = string.count(' ')\n",
    "    new_length = length + 2 * num_spaces - 1\n",
    "    for i in xrange(length-1, -1, -1):\n",
    "        if chr(string[i]) == ' ':\n",
    "            string[new_length] = '0'\n",
    "            string[new_length-1] = '2'\n",
    "            string[new_length-2] = '%'\n",
    "            new_length -= 3\n",
    "        else:\n",
    "            string[new_length] = string[i]\n",
    "            new_length -= 1"
   ]
  },
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Pythonic-Code: Not In-Place\n",
    "\n",
    "The following code is Pythonic, but requires using additional data structures as Python strings are immutable.  You could use a bytearray or a list instead of a string to simulate manipulating an array of characters."
   ]
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  {
   "cell_type": "code",
   "execution_count": 2,
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   "source": [
    "import re\n",
    "\n",
    "def encode_spaces_alt(string):\n",
    "    return re.sub(' ', '%20', string)\n",
    "\n",
    "def encode_spaces_alt2(string):\n",
    "    return string.replace(' ', '%20')"
   ]
  },
  {
   "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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   "cell_type": "code",
   "execution_count": 3,
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   "outputs": [
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     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Success: test_replace_char\n"
     ]
    }
   ],
   "source": [
    "from nose.tools import assert_equal\n",
    "\n",
    "class Test(object):\n",
    "    def test_replace_char(self, func):\n",
    "        str0 = None\n",
    "        str1 = bytearray(' ||')\n",
    "        str2 = bytearray(' foo bar ||||||')\n",
    "        str3 = bytearray('foo')\n",
    "        func(str0, 0)\n",
    "        func(str1, 1)\n",
    "        func(str2, 9)\n",
    "        func(str3, 3)\n",
    "        assert_equal(str0, None)\n",
    "        assert_equal(str1, '%20')\n",
    "        assert_equal(str2, '%20foo%20bar%20')\n",
    "        assert_equal(str3, 'foo')\n",
    "        print('Success: test_replace_char')\n",
    "\n",
    "if __name__ == '__main__':\n",
    "    test = Test()\n",
    "    test.test_replace_char(encode_spaces)"
   ]
  }
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