ETM {topicmodels.etm}

Topic Modelling in Semantic Embedding Spaces

Description

ETM is a generative topic model combining traditional topic models (LDA) with word embeddings (word2vec).

• It models each word with a categorical distribution whose natural parameter is the inner product between a word embedding and an embedding of its assigned topic.

• The model is fitted using an amortized variational inference algorithm on top of libtorch.

Usage

ETM(
  k = 20,
  embeddings,
  dim = 800,
  activation = c("relu", "tanh", "softplus", "rrelu", "leakyrelu", "elu", "selu",
    "glu"),
  dropout = 0.5,
  vocab = rownames(embeddings)
)

Arguments

k

the number of topics to extract

embeddings

either a matrix with pretrained word embeddings or an integer with the dimension of the word embeddings. Defaults to 50 if not provided.

dim

dimension of the variational inference hyperparameter theta (passed on to nn_linear). Defaults to 800.

activation

character string with the activation function of theta. Either one of 'relu', 'tanh', 'softplus', 'rrelu', 'leakyrelu', 'elu', 'selu', 'glu'. Defaults to 'relu'.

dropout

dropout percentage on the variational distribution for theta (passed on to nn_dropout). Defaults to 0.5.

vocab

a character vector with the words from the vocabulary. Defaults to the rownames of the embeddings argument.

Value

an object of class ETM which is a torch nn_module containing o.a.

• num_topics: the number of topics

• vocab: character vector with the terminology used in the model

• vocab_size: the number of words in vocab

• rho: The word embeddings

• alphas: The topic embeddings

Methods

fit(data, optimizer, epoch, batch_size, normalize = TRUE, clip = 0, lr_anneal_factor = 4, lr_anneal_nonmono = 10)

Fit the model on a document term matrix by splitting the data in 70/30 training/test set and updating the model weights.

Arguments

data

bag of words document term matrix in dgCMatrix format

optimizer

object of class torch_Optimizer

epoch

integer with the number of iterations to train

batch_size

integer with the size of the batch

normalize

logical indicating to normalize the bag of words data

clip

number between 0 and 1 indicating to do gradient clipping - passed on to nn_utils_clip_grad_norm_

lr_anneal_factor

divide the learning rate by this factor when the loss on the test set is monotonic for at least lr_anneal_nonmono training iterations

lr_anneal_nonmono

number of iterations after which learning rate annealing is executed if the loss does not decreases

References

https://arxiv.org/pdf/1907.04907.pdf

Examples

library(torch)
library(topicmodels.etm)
library(word2vec)
library(udpipe)
data(brussels_reviews_anno, package = "udpipe")
##
## Toy example with pretrained embeddings
##

## a. build word2vec model
x          <- subset(brussels_reviews_anno, language %in% "nl")
x          <- paste.data.frame(x, term = "lemma", group = "doc_id") 
set.seed(4321)
w2v        <- word2vec(x = x$lemma, dim = 15, iter = 20, type = "cbow", min_count = 5)
embeddings <- as.matrix(w2v)

## b. build document term matrix on nouns + adjectives, align with the embedding terms
dtm <- subset(brussels_reviews_anno, language %in% "nl" & upos %in% c("NOUN", "ADJ"))
dtm <- document_term_frequencies(dtm, document = "doc_id", term = "lemma")
dtm <- document_term_matrix(dtm)
dtm <- dtm_conform(dtm, columns = rownames(embeddings))
dtm <- dtm[dtm_rowsums(dtm) > 0, ]

## create and fit an embedding topic model - 8 topics, theta 100-dimensional
if (torch::torch_is_installed()) {

set.seed(4321)
torch_manual_seed(4321)
model       <- ETM(k = 8, dim = 100, embeddings = embeddings, dropout = 0.5)
optimizer   <- optim_adam(params = model$parameters, lr = 0.005, weight_decay = 0.0000012)
overview    <- model$fit(data = dtm, optimizer = optimizer, epoch = 40, batch_size = 1000)
scores      <- predict(model, dtm, type = "topics")

lastbatch   <- subset(overview$loss, overview$loss$batch_is_last == TRUE)
plot(lastbatch$epoch, lastbatch$loss)
plot(overview$loss_test)

## show top words in each topic
terminology <- predict(model, type = "terms", top_n = 7)
terminology

##
## Toy example without pretrained word embeddings
##
set.seed(4321)
torch_manual_seed(4321)
model       <- ETM(k = 8, dim = 100, embeddings = 15, dropout = 0.5, vocab = colnames(dtm))
optimizer   <- optim_adam(params = model$parameters, lr = 0.005, weight_decay = 0.0000012)
overview    <- model$fit(data = dtm, optimizer = optimizer, epoch = 40, batch_size = 1000)
terminology <- predict(model, type = "terms", top_n = 7)
terminology





}