Natural language processing (NLP) is an area of computer science and artificial intelligence concerned with the interactions between computers and human (natural) languages, in particular how to program computers to process and analyze large amounts of natural language data. It is the branch of machine learning which is about analyzing any text and handling predictive analysis.
Scikit-learn is a free software machine learning library for Python programming language. Scikit-learn is largely written in Python, with some core algorithms written in Cython to achieve performance. Cython is a superset of the Python programming language, designed to give C-like performance with code that is written mostly in Python.
Let’s understand the various steps involved in text processing and the flow of NLP.
This algorithm can be easily applied to any other kind of text like classify book into like Romance, Friction, but for now, let’s use a restaurant review dataset to review negative or positive feedback.
Step 1: Import dataset with setting delimiter as ‘\t’ as columns are separated as tab space. Reviews and their category(0 or 1) are not separated by any other symbol but with tab space as most of the other symbols are is the review (like $ for price, ….!, etc) and the algorithm might use them as delimiter, which will lead to strange behavior (like errors, weird output) in output.
To download the Restaurant_Reviews.tsv dataset used, click here.
Step 2: Text Cleaning or Preprocessing
- Remove Punctuations, Numbers: Punctuations, Numbers doesn’t help much in processong the given text, if included, they will just increase the size of bag of words that we will create as last step and decrase the efficency of algorithm.
- Stemming: Take roots of the word
- Convert each word into its lower case: For example, it useless to have same words in different cases (eg ‘good’ and ‘GOOD’).
Examples: Before and after applying above code (reviews = > before, corpus => after)
Step 3: Tokenization, involves splitting sentences and words from the body of the text.
Step 4: Making the bag of words via sparse matrix
- Take all the different words of reviews in the dataset without repeating of words.
- One column for each word, therefore there are going to be many columns.
- Rows are reviews
- If word is there in row of dataset of reviews, then the count of word will be there in row of bag of words under the column of the word.
Examples: Let’s take a dataset of reviews of only two reviews
Input : "dam good steak", "good food good servic" Output :
For this purpose we need
CountVectorizer class from sklearn.feature_extraction.text.
We can also set a max number of features (max no. features which help the most via attribute “max_features”). Do the training on the corpus and then apply the same transformation to the corpus “.fit_transform(corpus)” and then convert it into an array. If review is positive or negative that answer is in the second column of the dataset[:, 1] : all rows and 1st column (indexing from zero).
Description of the dataset to be used:
- Columns seperated by \t (tab space)
- First column is about reviews of people
- In second column, 0 is for negative review and 1 is for positive review
Step 5 : Splitting Corpus into Training and Test set. For this, we need class train_test_split from sklearn.cross_validation. Split can be made 70/30 or 80/20 or 85/15 or 75/25, here I choose 75/25 via “test_size”.
X is the bag of words, y is 0 or 1 (positive or negative).
Step 6: Fitting a Predictive Model (here random forest)
- Since Random fored is ensemble model (made of many trees) from sklearn.ensemble, import RandomForestClassifier class
- With 501 tree or “n_estimators” and criterion as ‘entropy’
- Fit the model via .fit() method with attributes X_train and y_train
Step 7: Pridicting Final Results via using .predict() method with attribute X_test
Note: Accuracy with random forest was 72%.(It may be different when performed experiment with different test size, here = 0.25).
Step 8: To know the accuracy, confusion matrix is needed.
Confusion Matrix is a 2X2 Matrix.
TRUE POSITIVE : measures the proportion of actual positives that are correctly identified.
TRUE NEGATIVE : measures the proportion of actual positives that are not correctly identified.
FALSE POSITIVE : measures the proportion of actual negatives that are correctly identified.
FALSE NEGATIVE : measures the proportion of actual negatives that are not correctly identified.
Note : True or False refers to the assigned classification being Correct or Incorrect, while Positive or Negative refers to assignment to the Positive or the Negative Category