Text Generation With RNN + TensorFlow

July 9, 2020

The potential of artificial intelligence to emulate human thought goes from passive tasks such as object recognition to self-driving cars, it also extends to creative tasks such as text-generation, music generation, art generation, etc. In this article/tutorial, we will see how neural networks can be used to generate text data, the same can be used for music generation.

Text Generation

Text is a form of sequence data, to a neural network it is but a sequence of digits. Sequences like text and music can be generated by training a deep learning model to predict the next word (for text) or note (for music) given a sequence of words or notes. For example, given an input “the cat is on the ma,” the network is trained to predict the letter t, the next character.

Pre-Requisites:

  • Knowledge of Python, Tensorflow, Machine Learning concepts. (Check out the resources.)

Development Environment

RNN- Recurrent Neural Network

Recurrent neural networks (RNN) are a class of artificial neural networks that is powerful for modelling sequence data such as time series or natural language. Vanilla neural networks have one shortcoming when compared to RNNs, they cannot solve machine learning problems which need to remember information about the past inputs. When processing sequential data, it is key that we remember the relationships in the data, and plain CNNs are not good at length-varying input and output. Hence, we are using RNNs for the task of text generation.

rnn

We will use a special type of RNN called LSTM, which are equipped to handle very large sequences of data. Simple RNNs have a problem called the vanishing gradient problem, because of which they cannot handle large sequences. LSTMs are designed to handle long-term dependencies.

lstm

Text Generation Using RNN

When working with text data tokens are words or characters and any network that can model the probability of the next token is called language model. A language model captures the statistical structure of the text. If we are training the neural network to predict the next character, it is called Character Level Model. Similarly, we can train the model to predict the next word, given a sequence of words called Word Level Models. We are implementing character level model.

model

Implementing in Tensorflow

The Dataset

For this tutorial, we will use a dataset which contains the works of Shakespeare.

import tensorflow as tf
import numpy as np
#Download the dataset
path = tf.keras.utils.get_file('shakespeare.txt', 'https://storage.googleapis.com/download.tensorflow.org/data/shakespeare.txt')
#Explore the data
text = open(path, "r").read()
print(text[:200])

Data Pre-Processing

Before training we need to map strings to numbers, extract partially overlapping sequences and pack them in a 3D numpy array of shape (sequences, maxlen, unique_characters). We one-hot encode the data.

maxlen = 60 #extract sequences of length 60
step = 3
sentences = []	#holds extracted sequences
next_chars = [] #holds the targets
for i in range(0, len(text)-maxlen, step):
	sentences.append(text[i:i+maxlen])
	next_chars.append(text[i+maxlen])
#VECTORIZATION
chars = sorted(set(text))
char_indices = dict((char, chars.index(char)) for char in chars)
x = np.zeros((len(sentences), maxlen, len(chars)), dtype=np.bool)
y = np.zeros((len(sentences), len(chars)), dtype=np.bool)
for i, sentence in enumerate(sentences):
	for t, char in enumerate(sentence):
		x[i, t, char_indices[char]] = 1
	y[i, char_indices[next_chars[i]]] = 1

Building the Network

The network is a single LSTM layer followed by a Dense classifier and softmax over all possible characters.

from tensorflow.keras import layers
model = tf.keras.models.Sequential()
model.add(layers.LSTM(128, input_shape=(maxlen, len(chars))))
model.add(layers.Dense(len(chars), activation="softmax"))

Compile and Train the model

Use the categorical_crossentropy loss to train the model as the targets are one-hot encoded.

model.compile(loss="categorical_crossentropy", optimizer="adam")

Given a trained model and a seed snippet, you can generate text by doing the following repeatedly:

  • Draw from the model a probability distribution for the next character
  • Reweight the distribution to a certain temperature. (The temperature can be used to control the randomness of the output. Higher temperatures result in sampling distributions of higher entropy that will generate more surprising and unstructured generated data, whereas a lower temperature will result in less randomness and much more predictable generated data)
  • Sample the next character according to the reweighted distribution.
  • Add the character to the seed text
def sample(preds, temperature=1.0):
	preds = np.asarray(preds).astype('float64')
	preds = np.log(preds) / temperature
	exp_preds = np.exp(preds)
	preds = exp_preds / np.sum(exp_preds)
	probas = np.random.multinomial(1, preds, 1)
	return np.argmax(probas)

The following loop repeatedly trains and generates the text.

import random
for epoch in range(1, 60):
	model.fit(x, y, batch_size=128)
	start_index = random.randint(0, len(text) - maxlen - 1)
	generated_text = x[start_index: start_index + maxlen]
	for i in range(400): #generates 400 length string
		preds = model.predict(generated_text)[0]
		   next_index = sample(preds, temperature)
		   next_char = chars[next_index]
		   generated_text += next_char
		   generated_text = generated_text[1:]

The above-mentioned loop is used for training if you want to look at the results at each epoch or at different temperatures, print or append to a file in between the epochs. You can even give your own starting string instead of randomly choosing one and observe how the model evolves through the epochs.

After 10 epochs and giving the output string length = 2000 the output is as follows:

ROMEO: Depose
The nostest have aldeghtenly
Unsuep a disposinging trebs: which least
Be it not to be, to sign our awherit to him.

Nurse: good speed!
I protest, up her countening and virtuous to the world:
Who is't it in,
and I will tet: these corn, Hastings, then I'ld
Cambio and trustly 'Hell my power from the tappe of her tafe:
Upon my rememinance, if he other gracious word,
Was not upon madch'd teach a dear ladgeness, that every faces
Proteck me for joyour of little henrer: tell me,
Engur King Edward's curse?
Within this angry ANTIGO:
His lady as you
Wast thou wilt bid the king!
Was a joy o'erthe moved, here forth, be with a pip-r,
To death alighture I have the mine
For this all when RINCE Turst, how mein yourselves for Rose, if thou dost i' thy peace.

CURTIS:
In the good Romeo Lord Shall Henry, py free-det, and then the king
Is chains so setch him, he will live with my pridoner.

LEONTES:
Madam, modrows give me leh on that mercy most we,
Command to tell him sweet as thou reported to ope?
Ah, Richard, wisely makes my twope:
I pray thee I am sups from Fersuing ignorance,
We merry they wouldiops by the world good to the servica, With ablected
Many's gross leed all heavy in meant to the flatteret?
Wasted along to speak. We thanks inhapp'd,
Is touch again to me, takes, he appeal with complaint
Against thy backs: then to-morrow,
Shall I trust to draze our patrim.
Will they be that last were nothing better thine;
Some mightyou tapet in feart, that or my resignner'd arre
Against whet have ever you to play and give my charter't.

PAULINA:
Let them boy!

KING EDWARD IV:
Why, then they shall be confect it again;
Thou still-stable shake! for he has been,
Till the king's bloody state, that she's my tell well
The search o' your obedience, hath he chairs
To the I did not go:
Rath being the first shall be a metted gone.

CLAOW OF GONTIS:
Why, and yet, I pray you shatter, gone to Rome,
Ere hard his king's master.

GRUMIO:
I prith earth, hath their
pride, sir, by the fresh perfects to o

Conclusion

As we can see the sentences that the model is generating are not perfect and some are not exactly english words, but we have to keep in mind that we are simply inferring from a statistical model, which can be improved. To improve the model we can use stacked LSTM layers instead of a single LSTM layer, use Bidirectional LSTM or Dropout to avoid over-fitting or clean the data to remove puntuation.

Full code here.

References and Resources


About the author

Rohan Reddy

Rohan Reddy is an undergraduate student at the University of Hyderabad pursuing a degree in Computer Science. Rohan is particularly interested in machine learning and web development.

This article was contributed by a student member of Section's Engineering Education Program. Please report any errors or innaccuracies to enged@section.io.