In this recipe, we show how to handle text data with scikit-learn. Working with text requires careful preprocessing and feature extraction. It is also quite common to deal with highly sparse matrices.

We will learn to recognize whether a comment posted during a public discussion is considered insulting to one of the participants. We will use a labeled dataset from Impermium, released during a Kaggle competition.

You need to download the *troll* dataset on the book's website. (https://ipython-books.github.io)

- Let's import our libraries.

In [ ]:

```
import numpy as np
import pandas as pd
import sklearn
import sklearn.model_selection as ms
import sklearn.feature_extraction.text as text
import sklearn.naive_bayes as nb
import matplotlib.pyplot as plt
%matplotlib inline
```

- Let's open the csv file with Pandas.

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```
df = pd.read_csv("data/troll.csv")
```

- Each row is a comment. There are three columns: whether the comment is insulting (1) or not (0), the data, and the unicode-encoded contents of the comment.

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```
df[['Insult', 'Comment']].tail()
```

- Now, we are going to define the feature matrix $\mathbf{X}$ and the labels $\mathbf{y}$.

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```
y = df['Insult']
```

Obtaining the feature matrix from the text is not trivial. Scikit-learn can only work with numerical matrices. How to convert text into a matrix of numbers? A classical solution is to first extract a **vocabulary**: a list of words used throughout the corpus. Then, we can count, for each sample, the frequency of each word. We end up with a **sparse matrix**: a huge matrix containiny mostly zeros. Here, we do this in two lines. We will give more explanations in *How it works...*.

In [ ]:

```
tf = text.TfidfVectorizer()
X = tf.fit_transform(df['Comment'])
print(X.shape)
```

- There are 3947 comments and 16469 different words. Let's estimate the sparsity of this feature matrix.

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```
print("Each sample has ~{0:.2f}% non-zero features.".format(
100 * X.nnz / float(X.shape[0] * X.shape[1])))
```

- Now, we are going to train a classifier as usual. We first split the data into a train and test set.

In [ ]:

```
(X_train, X_test,
y_train, y_test) = ms.train_test_split(X, y,
test_size=.2)
```

- We use a
**Bernoulli Naive Bayes classifier**with a grid search on the parameter $\alpha$.

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```
bnb = ms.GridSearchCV(nb.BernoulliNB(), param_grid={'alpha':np.logspace(-2., 2., 50)})
bnb.fit(X_train, y_train);
```

- What is the performance of this classifier on the test dataset?

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```
bnb.score(X_test, y_test)
```

- Let's take a look at the words corresponding to the largest coefficients (the words we find frequently in insulting comments).

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```
# We first get the words corresponding to each feature.
names = np.asarray(tf.get_feature_names())
# Next, we display the 50 words with the largest
# coefficients.
print(','.join(names[np.argsort(
bnb.best_estimator_.coef_[0,:])[::-1][:50]]))
```

- Finally, let's test our estimator on a few test sentences.

In [ ]:

```
print(bnb.predict(tf.transform([
"I totally agree with you.",
"You are so stupid.",
"I love you."
])))
```

You'll find all the explanations, figures, references, and much more in the book (to be released later this summer).

IPython Cookbook, by Cyrille Rossant, Packt Publishing, 2014 (500 pages).