sitc/nlp/4_4_Classification.ipynb

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    "![](images/EscUpmPolit_p.gif \"UPM\")"
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   "source": [
    "# Course Notes for Learning Intelligent Systems"
   ]
  },
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   "metadata": {},
   "source": [
    "Department of Telematic Engineering Systems, Universidad Politécnica de Madrid, © 2016 Carlos A. Iglesias"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# Text Classification"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# Table of Contents\n",
    "* [Objectives](#Objectives)\n",
    "* [Corpus](#Corpus)\n",
    "* [Classifier](#Classifier)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# Objectives"
   ]
  },
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   "source": [
    "In this session we provide a quick overview of how the vector models we have presented previously can be used for applying machine learning techniques, such as classification.\n",
    "\n",
    "The main objectives of this session are:\n",
    "* Understand how to apply machine learning techniques on textual sources\n",
    "* Learn the facilities provided by scikit-learn"
   ]
  },
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# Corpus"
   ]
  },
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   "source": [
    "We are going to use on of the corpus that come prepackaged with Scikit-learn: the [20 newsgroup datase](http://qwone.com/~jason/20Newsgroups/). The 20  newsgroup dataset contains 20k documents that belong to 20 topics.\n",
    "\n",
    "We inspect now the corpus using the facilities from Scikit-learn, as explain in [scikit-learn](http://scikit-learn.org/stable/datasets/twenty_newsgroups.html#newsgroups)"
   ]
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  {
   "cell_type": "code",
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   "source": [
    "from sklearn.datasets import fetch_20newsgroups\n",
    "\n",
    "# We remove metadata to avoid bias in the classification\n",
    "newsgroups_train = fetch_20newsgroups(subset='train', remove=('headers', 'footers', 'quotes'))\n",
    "newsgroups_test = fetch_20newsgroups(subset='test', remove=('headers', 'footers', 'quotes'))\n",
    "\n",
    "# print categories\n",
    "print(list(newsgroups_train.target_names))"
   ]
  },
  {
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   "source": [
    "#Number of categories\n",
    "print(len(newsgroups_train.target_names))"
   ]
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   "source": [
    "# Show a document\n",
    "docid = 1\n",
    "doc = newsgroups_train.data[docid]\n",
    "cat = newsgroups_train.target[docid]\n",
    "\n",
    "print(\"Category id \" +  str(cat) + \" \" + newsgroups_train.target_names[cat])\n",
    "print(\"Doc \" + doc)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
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   "outputs": [],
   "source": [
    "#Number of files\n",
    "newsgroups_train.filenames.shape"
   ]
  },
  {
   "cell_type": "code",
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   "source": [
    "# Obtain a vector\n",
    "\n",
    "from sklearn.feature_extraction.text import TfidfVectorizer\n",
    "\n",
    "vectorizer = TfidfVectorizer(analyzer='word', stop_words='english')\n",
    "\n",
    "vectors_train = vectorizer.fit_transform(newsgroups_train.data)\n",
    "vectors_train.shape"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": false
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   "outputs": [],
   "source": [
    "# The tf-idf vectors are very sparse with an average of 66 non zero components in 101.323 dimensions (.06%)\n",
    "vectors_train.nnz / float(vectors_train.shape[0])"
   ]
  },
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# Classifier"
   ]
  },
  {
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   "metadata": {},
   "source": [
    "Once we have vectors, we can create classifiers (or other machine learning algorithms such as clustering) as we saw previously in the notebooks of machine learning with scikit-learn."
   ]
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   "source": [
    "from sklearn.naive_bayes import MultinomialNB\n",
    "\n",
    "from sklearn import metrics\n",
    "\n",
    "\n",
    "# We learn the vocabulary (fit) with the train dataset and transform into vectors (fit_transform)\n",
    "# Nevertheless, we only transform the test dataset into vectors  (transform, not fit_transform)\n",
    "\n",
    "model = MultinomialNB(alpha=.01)\n",
    "model.fit(vectors_train, newsgroups_train.target)\n",
    "\n",
    "vectors_test = vectorizer.transform(newsgroups_test.data)\n",
    "pred = model.predict(vectors_test)\n",
    "\n",
    "metrics.f1_score(newsgroups_test.target, pred, average='weighted')\n"
   ]
  },
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "We are getting F1 of 0.69 for 20 categories this could be improved (optimization, preprocessing, etc.)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
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   "source": [
    "from sklearn.utils.extmath import density\n",
    "\n",
    "print(\"dimensionality: %d\" % model.coef_.shape[1])\n",
    "print(\"density: %f\" % density(model.coef_))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
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   "source": [
    "# We can review the top features per topic in Bayes (attribute coef_)\n",
    "import numpy as np\n",
    "\n",
    "def show_top10(classifier, vectorizer, categories):\n",
    "    feature_names = np.asarray(vectorizer.get_feature_names())\n",
    "    for i, category in enumerate(categories):\n",
    "        top10 = np.argsort(classifier.coef_[i])[-10:]\n",
    "        print(\"%s: %s\" % (category, \" \".join(feature_names[top10])))\n",
    "\n",
    "        \n",
    "show_top10(model, vectorizer, newsgroups_train.target_names)"
   ]
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  {
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   "source": [
    "# We try the classifier in two new docs\n",
    "\n",
    "new_docs = ['This is a survey of PC computers', 'God is love']\n",
    "new_vectors = vectorizer.transform(new_docs)\n",
    "\n",
    "pred_docs = model.predict(new_vectors)\n",
    "print(pred_docs)\n",
    "print([newsgroups_train.target_names[i] for i in pred_docs])"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## References\n",
    "\n"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "* [NLTK Book. Natural Language Processing with Python. Steven Bird, Ewan Klein, and Edward Loper. O'Reilly Media, 2009 ](http://www.nltk.org/book_1ed/)\n",
    "* [NLTK Essentials, Nitin Hardeniya, Packt Publishing, 2015](http://proquest.safaribooksonline.com/search?q=NLTK%20Essentials)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Licence"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "The notebook is freely licensed under under the [Creative Commons Attribution Share-Alike license](https://creativecommons.org/licenses/by/2.0/).  \n",
    "\n",
    "© 2016 Carlos A. Iglesias, Universidad Politécnica de Madrid."
   ]
  }
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