TF – IDF for Bigrams & Trigrams

TF-IDF in NLP stands for Term Frequency – Inverse document frequency. It is a very popular topic in Natural Language Processing which generally deals with human languages. During any text processing, cleaning the text (preprocessing) is vital. Further, the cleaned data needs to be converted into a numerical format where each word is represented by a matrix (word vectors). This is also known as word embedding
Term Frequency (TF) = (Frequency of a term in the document)/(Total number of terms in documents)
Inverse Document Frequency(IDF) = log( (total number of documents)/(number of documents with term t))
TF.IDF = (TF).(IDF)

Bigrams: Bigram is 2 consecutive words in a sentence. E.g. “The boy is playing football”. The bigrams here are:

The boy
Boy is
Is playing
Playing football

Trigrams: Trigram is 3 consecutive words in a sentence. For the above example trigrams will be:

The boy is
Boy is playing
Is playing football

From the above bigrams and trigram, some are relevant while others are discarded which do not contribute value for further processing.
Let us say from a document we want to find out the skills required to be a “Data Scientist”. Here, if we consider only unigrams, then the single word cannot convey the details properly. If we have a word like ‘Machine learning developer’, then the word extracted should be ‘Machine learning’ or ‘Machine learning developer’. The words simply ‘Machine’, ‘learning’ or ‘developer’ will not give the expected result.

Code – Illustrating the detailed explanation for trigrams

# Importing libraries
import nltk
import re
from sklearn.feature_extraction.text import CountVectorizer, TfidfVectorizer
from nltk.corpus import stopwords
from nltk.tokenize import word_tokenize
import pandas as pd

# Input the file
txt1 = []
with open('C:\\Users\\DELL\\Desktop\\MachineLearning1.txt') as file:
    txt1 = file.readlines()

# Preprocessing
def remove_string_special_characters(s):

    # removes special characters with ' '
    stripped = re.sub('[^a-zA-z\s]', '', s)
    stripped = re.sub('_', '', stripped)

    # Change any white space to one space
    stripped = re.sub('\s+', ' ', stripped)

    # Remove start and end white spaces
    stripped = stripped.strip()
    if stripped != '':
            return stripped.lower()

# Stopword removal
stop_words = set(stopwords.words('english'))
your_list = ['skills', 'ability', 'job', 'description']
for i, line in enumerate(txt1):
    txt1[i] = ' '.join([x for
        x in nltk.word_tokenize(line) if
        ( x not in stop_words ) and ( x not in your_list )])

# Getting trigrams
vectorizer = CountVectorizer(ngram_range = (3,3))
X1 = vectorizer.fit_transform(txt1)
features = (vectorizer.get_feature_names())
print("\n\nFeatures : \n", features)
print("\n\nX1 : \n", X1.toarray())

# Applying TFIDF
vectorizer = TfidfVectorizer(ngram_range = (3,3))
X2 = vectorizer.fit_transform(txt1)
scores = (X2.toarray())
print("\n\nScores : \n", scores)

# Getting top ranking features
sums = X2.sum(axis = 0)
data1 = []
for col, term in enumerate(features):
    data1.append( (term, sums[0,col] ))
ranking = pd.DataFrame(data1, columns = ['term','rank'])
words = (ranking.sort_values('rank', ascending = False))
print ("\n\nWords head : \n", words.head(7))

Output:

Features : 
 ['10 experience working', '11 exposure implementing', 'able work minimal',
 'accounts commerce added', 'analysis recognition face', 'analytics contextual image',
 'analytics nlp ensemble', 'applying data science', 'bagging boosting text',
 'beyond existing learn', 'boosting text analytics', 'building using logistics',
 'building using supervised', 'classification facial expression', 
 'classifier deep learning', 'commerce added advantage',
 'complex engineering analysis', 'contextual image processing',
 'creative projects work', 'data science problem', 'data science solutions',
 'decisions report progress', 'deep learning analytics', 'deep learning framework',
 'deep learning neural', 'demonstrated development role', 'demonstrated leadership role',
 'description machine learning', 'detection tracking classification',
 'development role machine', 'direction project less', 'domains essential position',
 'domains like healthcare', 'ensemble classifier deep', 'existing learn quickly',
 'experience object detection', 'experience working multiple',
 'experienced technical personnel', 'expertise visualizing manipulating',
 'exposure implementing data', 'expression analysis recognition',
 'extensively worked python', 'face iris finger', 'facial expression analysis',
 'finance accounts commerce', 'forest bagging boosting', 'framework tensorflow keras',
 'good oral written', 'guidance direction project', 'guidance make decisions',
 'healthcare finance accounts', 'implementing data science', 'including provide guidance',
 'innovative creative projects', 'iris finger gesture', 'job description machine',
 'keras or pytorch', 'leadership role projects', 'learn quickly new',
 'learning analytics contextual', 'learning framework tensorflow',
 'learning neural networks', 'learning projects including', 'less experienced technical',
 'like healthcare finance', 'linear regression svm', 'logistics regression linear',
 'machine learning developer', 'machine learning projects', 'make decisions report',
 'manipulating big datasets', 'minimal guidance make', 'model building using',
 'motivated able work', 'multiple domains like', 'must self motivated',
 'new domains essential', 'nlp ensemble classifier', 'object detection tracking',
 'oral written communication', 'perform complex engineering', 'problem solving proven',
 'problem statements bring', 'proficiency deep learning', 'proficiency problem solving',
 'project less experienced', 'projects including provide', 'projects work spare',
 'proven perform complex', 'proven record working', 'provide guidance direction',
 'quickly new domains', 'random forest bagging', 'recognition face iris',
 'record working innovative', 'regression linear regression', 'regression svm random',
 'role machine learning', 'role projects including', 'science problem statements',
 'science solutions production', 'self motivated able', 'solutions production environments',
 'solving proven perform', 'spare time plus', 'statements bring insights',
 'supervised unsupervised algorithms', 'svm random forest', 'tensorflow keras or',
 'text analytics nlp', 'tracking classification facial', 'using logistics regression',
 'using supervised unsupervised', 'visualizing manipulating big', 'work minimal guidance',
 'work spare time', 'working innovative creative', 'working multiple domains']


X1 : 
 [[0 0 0 ... 0 0 0]
 [0 0 0 ... 0 0 0]
 [0 0 0 ... 0 0 0]
 ...
 [0 0 0 ... 0 0 0]
 [0 0 0 ... 0 0 0]
 [0 0 0 ... 0 0 0]]


Scores : 
 [[0. 0. 0. ... 0. 0. 0.]
 [0. 0. 0. ... 0. 0. 0.]
 [0. 0. 0. ... 0. 0. 0.]
 ...
 [0. 0. 0. ... 0. 0. 0.]
 [0. 0. 0. ... 0. 0. 0.]
 [0. 0. 0. ... 0. 0. 0.]]


Words head : 
                             term      rank
41     extensively worked python  1.000000
79    oral written communication  0.707107
47             good oral written  0.707107
72          model building using  0.673502
27  description machine learning  0.577350
70     manipulating big datasets  0.577350
67    machine learning developer  0.577350

Now, if w do it for bigrams then the initial part of code will remain the same. Only the bigram formation part will change.
Code : Python code for implementing bigrams

# Getting bigrams
vectorizer = CountVectorizer(ngram_range =(2, 2))
X1 = vectorizer.fit_transform(txt1)
features = (vectorizer.get_feature_names())
print("\n\nX1 : \n", X1.toarray())

# Applying TFIDF
# You can still get n-grams here
vectorizer = TfidfVectorizer(ngram_range = (2, 2))
X2 = vectorizer.fit_transform(txt1)
scores = (X2.toarray())
print("\n\nScores : \n", scores)

# Getting top ranking features
sums = X2.sum(axis = 0)
data1 = []
for col, term in enumerate(features):
    data1.append( (term, sums[0, col] ))
ranking = pd.DataFrame(data1, columns = ['term', 'rank'])
words = (ranking.sort_values('rank', ascending = False))
print ("\n\nWords : \n", words.head(7))

Output:

X1 : 
 [[0 0 0 ... 0 0 0]
 [0 0 0 ... 0 0 0]
 [0 0 0 ... 0 0 0]
 ...
 [0 0 0 ... 0 0 0]
 [0 0 0 ... 0 0 0]
 [0 0 0 ... 0 0 0]]


Scores : 
 [[0. 0. 0. ... 0. 0. 0.]
 [0. 0. 0. ... 0. 0. 0.]
 [0. 0. 0. ... 0. 0. 0.]
 ...
 [0. 0. 0. ... 0. 0. 0.]
 [0. 0. 0. ... 0. 0. 0.]
 [0. 0. 0. ... 0. 0. 0.]]


Words : 
                     term      rank
50   great interpersonal  1.000000
110     skills abilities  1.000000
23         deep learning  0.904954
72      machine learning  0.723725
21          data science  0.723724
128        worked python  0.707107
42    extensively worked  0.707107

Likewise, we can obtain the TF IDF scores for bigrams and trigrams as per our use. These can help us get a better outcome without having to process more on data.