Session 07 text data.pptx

Handling Text Data
INAFU6513 Lecture 7b

Lab 7: your 5-7 things
Get familiar with text processing
Get familiar with text data
Read text data
Classify text data
Analyse text data

Text processing
● Information retrieval
○ Search
○ Named entity recognition
● Learning
○ Classification
○ Clustering
○ Topic identification/ topic following
○ Sentiment analysis
○ Network analysis (words, people etc)

Text Data Sources
● Messages (tweets, emails, sms messages...)
● Document text (reports, blogposts, website text…)
● Audio (via speech-to-text processing)
● Images (via OCR)

Get your raw text data
fsipa = open('sipatext.txt', 'r')
sipatext = fsipa.read()
fsipa.close()
print(sipatext)

Counting: Bags of Words
from sklearn.feature_extraction.text import CountVectorizer
count_vect = CountVectorizer()
word_counts = count_vect.fit_transform([sipatext])
print('{}'.format(word_counts))
print('{}'.format(count_vect.vocabulary_))

Counting sets of words: N-Grams
● Pairs (or triples, 4s etc) of words
● Also: pairs etc of characters, e.g. [‘mor’, ‘ore’, ‘re ‘,
‘e t’, ‘ th’, ‘tha’, ‘han’]
● Know your Ns:
○ ‘Unigram’ == 1-gram
○ ‘Bigram’ == 2-gram
○ ‘Trigram’ == 3-gram
count_vectn = CountVectorizer(ngram_range =(2, 2))

Stopwords
count_vect2 =
CountVectorizer(stop_words='english')
word_counts2 =
count_vect2.fit_transform([sipatext])

Term Frequencies
● TF: Term Frequency:
○ word count / (number of words in this document)
○ “How important (0 to 1) is this word to this document”?
● IDF: Inverse Document Frequency
○ 1 / (number of documents this word appears in)
○ “How common is this word in this corpus”?
● TFIDF:
○ TF * IDF

Machine Learning with Text Data

Classifying Text
Words are a valid input to machine learning algorithms
In this example, we’re using:
● Newsgroup emails as samples (‘rows’ in our input)
● Words in each email as features (‘columns’)
● Newsgroup ids as targets

The 20newsgroups dataset
from sklearn.datasets import fetch_20newsgroups
cats = ['alt.atheism', 'soc.religion.christian', 'comp.graphics', 'sci.med']
twenty_train = fetch_20newsgroups( subset='train', categories=cats)
twenty_test = fetch_20newsgroups(subset='test', categories=cats)

Convert words to TFIDF scores
from sklearn.feature_extraction.text import CountVectorizer
from sklearn.feature_extraction.text import TfidfTransformer
count_vect = CountVectorizer()
X_train_counts = count_vect.fit_transform(twenty_train.data)
tfidf_transformer = TfidfTransformer(use_idf=True)
X_train_tfidf = tfidf_transformer.fit_transform(X_train_counts)

Fit your model to the data
from sklearn.naive_bayes import MultinomialNB
nb_classifier = MultinomialNB().fit(X_train_tfidf, twenty_train.target)

Test your model
docs_test = ['God is love', 'OpenGL on the GPU is fast']
X_new_counts = count_vect.transform(docs_test)
X_new_tfidf = tfidf_transformer.transform(X_new_counts)
predicted = nb_classifier.predict(X_new_tfidf)
for doc, category in zip(docs_test, predicted):
print('{} => {}'.format(doc, twenty_train.target_names[category]))

Text Clustering
We can also ‘cluster’ documents
● The ‘distance’ function is based on the words they have in common
Common machine learning algorithms for text clustering include:
● Latent Semantic Analysis
● Latent Dirichlet Allocation

Word colocation
● Create a graph (network visualisation) of words that appear together in
documents
● Use network analysis (later session) to show which pairs of words are
important in your documents

Sentiment analysis
● Mark documents (e.g. tweets) as having positive or negative sentiment
● Using machine learning
○ Training set: sentences, with ‘positive’/’negative’ for each sentence
● Using a sentiment dictionary
○ Positive or negative ‘score’ for each emotive word
○ Sentiment dictionaries can be used as machine learning algorithms
‘seeds’

Named Entity Recognition
● Find the names of people, organisations, locations etc in text
● Can use these to create social graphs (networks showing how people etc
connect to each other) and find ‘hubs’, ‘connectors’ etc

Natural Language Processing
● Understanding the grammar and meaning of text
● Useful for, e.g. translation between languages
● Python library: NLTK

Getting started with NLTK
import nltk
nltk.download()

Get text ready for NLTK processing
from nltk import word_tokenize
from nltk.text import Text
fsipa = open('example_data/sipatext.txt', 'r')
sipatext = fsipa.read()
fsipa.close()
sipawords = word_tokenize(sipatext)
textlist = Text(sipawords)

NLTK: concordance
textlist.concordance(‘school’)
textlist.similar('school')
textlist.common_contexts(['school', 'university'])

NLTK: word dispersion plots
from nltk.book import *
text2.dispersion_plot(['Elinor', 'Willoughby', 'Sophia'])

NLTK: Word Meanings
from nltk.corpus import wordnet as wn
word = 'class'
synset = wn.synsets(word)
print('Synset: {}n'.format(synset))
for i in range(len(synset)):
print('Meaning {}: {} {}'.format(i, synset[i].lemma_names(), synset[i].definition()))

NLTK: converting words into logic
from nltk import load_parser
parser = load_parser('grammars/book_grammars/simple-sem.fcfg', trace=0)
sentence = 'Angus gives a bone to every dog'
tokens = sentence.split()
for tree in parser.parse(tokens):
print(tree.label()['SEM'])

Exercises
Try the code in the 7.x series notebooks

Session 07 text data.pptx

More Related Content

What's hot

Viewers also liked

Similar to Session 07 text data.pptx

More from Sara-Jayne Terp

Recently uploaded

Session 07 text data.pptx

Editor's Notes