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https://github.com/donnemartin/interactive-coding-challenges
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Appended _solution to current notebooks to prepare for challenge and solution notebooks.
This commit is contained in:
Donne Martin
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arrays_strings/replace_char/replace_char_solution.ipynb
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210
arrays_strings/replace_char/replace_char_solution.ipynb
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{
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"cells": [
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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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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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"## Problem: Given a string, replace in-place all spaces with '%20'\n",
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"\n",
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"* [Constraints](#Constraints)\n",
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"* [Test Cases](#Test-Cases)\n",
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"* [Algorithm](#Algorithm)\n",
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"* [Code](#Code)\n",
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"* [Pythonic-Code: Not In-Place](#Pythonic-Code:-Not-In-Place)\n",
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"* [Unit Test](#Unit-Test)"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"## Constraints\n",
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"\n",
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"*Problem statements are often intentionally ambiguous. Identifying constraints and stating assumptions can help to ensure you code the intended solution.*\n",
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"\n",
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"* Can I assume the string is ASCII?\n",
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" * Yes\n",
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" * Note: Unicode strings could require special handling depending on your language\n",
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"* Is there enough space in the data structure for this operation?\n",
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" * Yes"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"## Test Cases\n",
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"\n",
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"* NULL->NULL\n",
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"* ' ' -> '%20'\n",
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"* ' foo bar ' -> '%20foo%20bar%20'\n",
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"* 'foo' -> 'foo'"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"## Algorithm\n",
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"\n",
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"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",
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"\n",
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"\n",
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"\n",
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"* Count the number of spaces in the bytearray\n",
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"* Determine the new bytearray length\n",
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"* For each character code in the bytearray, starting from the end of the string\n",
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" * If the character code is a space\n",
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" * bytearray[new length] = '0',\n",
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" * bytearray[new length - 1] = '2',\n",
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" * bytearray[new length - 2] = '%',\n",
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" * new length -= 3\n",
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" * Else\n",
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" * bytearray[new length] = character code,\n",
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" * new length -= 1\n",
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"\n",
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"Complexity:\n",
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"* Time: O(n)\n",
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"* Space: In-place"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"## Code"
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]
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},
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{
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"cell_type": "code",
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"execution_count": 1,
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"metadata": {
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"collapsed": false
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},
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"outputs": [],
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"source": [
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"def encode_spaces(string, length):\n",
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" if string is None:\n",
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" return\n",
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" num_spaces = string.count(' ')\n",
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" new_length = length + 2 * num_spaces - 1\n",
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" for i in xrange(length-1, -1, -1):\n",
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" if chr(string[i]) == ' ':\n",
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" string[new_length] = '0'\n",
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" string[new_length-1] = '2'\n",
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" string[new_length-2] = '%'\n",
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" new_length -= 3\n",
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" else:\n",
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" string[new_length] = string[i]\n",
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" new_length -= 1"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"## Pythonic-Code: Not In-Place\n",
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"\n",
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"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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]
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},
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{
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"cell_type": "code",
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"execution_count": 2,
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"metadata": {
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"collapsed": false
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},
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"outputs": [],
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"source": [
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"import re\n",
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"\n",
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"def encode_spaces_alt(string):\n",
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" return re.sub(' ', '%20', string)\n",
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"\n",
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"def encode_spaces_alt2(string):\n",
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" return string.replace(' ', '%20')"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"## Unit Test"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"*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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]
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},
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{
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"cell_type": "code",
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"execution_count": 3,
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"metadata": {
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"collapsed": false
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},
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"outputs": [
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{
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"name": "stdout",
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"output_type": "stream",
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"text": [
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"Success: test_replace_char\n"
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]
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}
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],
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"source": [
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"from nose.tools import assert_equal\n",
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"\n",
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"class Test(object):\n",
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" def test_replace_char(self, func):\n",
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" str0 = None\n",
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" str1 = bytearray(' ||')\n",
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" str2 = bytearray(' foo bar ||||||')\n",
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" str3 = bytearray('foo')\n",
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" func(str0, 0)\n",
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" func(str1, 1)\n",
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" func(str2, 9)\n",
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" func(str3, 3)\n",
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" assert_equal(str0, None)\n",
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" assert_equal(str1, '%20')\n",
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" assert_equal(str2, '%20foo%20bar%20')\n",
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" assert_equal(str3, 'foo')\n",
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" print('Success: test_replace_char')\n",
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"\n",
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"if __name__ == '__main__':\n",
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" test = Test()\n",
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" test.test_replace_char(encode_spaces)"
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]
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}
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],
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"metadata": {
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"kernelspec": {
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"display_name": "Python 2",
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"language": "python",
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"name": "python2"
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},
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"language_info": {
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"codemirror_mode": {
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"name": "ipython",
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"version": 2
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},
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"file_extension": ".py",
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"mimetype": "text/x-python",
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"name": "python",
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"nbconvert_exporter": "python",
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"pygments_lexer": "ipython2",
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"version": "2.7.10"
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}
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},
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"nbformat": 4,
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"nbformat_minor": 0
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}
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