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.. _sec_machine_translation:
Machine Translation and the Dataset
===================================
We have used RNNs to design language models, which are key to natural
language processing. Another flagship benchmark is *machine
translation*, a central problem domain for *sequence transduction*
models that transform input sequences into output sequences. Playing a
crucial role in various modern AI applications, sequence transduction
models will form the focus of the remainder of this chapter and
:numref:`chap_attention`. To this end, this section introduces the
machine translation problem and its dataset that will be used later.
*Machine translation* refers to the automatic translation of a sequence
from one language to another. In fact, this field may date back to 1940s
soon after digital computers were invented, especially by considering
the use of computers for cracking language codes in World War II. For
decades, statistical approaches had been dominant in this field
:cite:`Brown.Cocke.Della-Pietra.ea.1988,Brown.Cocke.Della-Pietra.ea.1990`
before the rise of end-to-end learning using neural networks. The latter
is often called *neural machine translation* to distinguish itself from
*statistical machine translation* that involves statistical analysis in
components such as the translation model and the language model.
Emphasizing end-to-end learning, this book will focus on neural machine
translation methods. Different from our language model problem in
:numref:`sec_language_model` whose corpus is in one single language,
machine translation datasets are composed of pairs of text sequences
that are in the source language and the target language, respectively.
Thus, instead of reusing the preprocessing routine for language
modeling, we need a different way to preprocess machine translation
datasets. In the following, we show how to load the preprocessed data
into minibatches for training.
.. code:: java
%load ../utils/djl-imports
%load ../utils/plot-utils
%load ../utils/Functions.java
%load ../utils/timemachine/Vocab.java
%load ../utils/timemachine/RNNModel.java
%load ../utils/timemachine/RNNModelScratch.java
%load ../utils/timemachine/TimeMachine.java
%load ../utils/timemachine/TimeMachineDataset.java
.. code:: java
import java.nio.charset.*;
import java.util.zip.*;
import java.util.stream.*;
.. code:: java
NDManager manager = NDManager.newBaseManager();
Downloading and Preprocessing the Dataset
-----------------------------------------
To begin with, we download an English-French dataset that consists of
`bilingual sentence pairs from the Tatoeba
Project `__. Each line in the dataset
is a tab-delimited pair of an English text sequence and the translated
French text sequence. Note that each text sequence can be just one
sentence or a paragraph of multiple sentences. In this machine
translation problem where English is translated into French, English is
the *source language* and French is the *target language*.
.. code:: java
public static String readDataNMT() throws IOException {
DownloadUtils.download(
"http://d2l-data.s3-accelerate.amazonaws.com/fra-eng.zip", "fra-eng.zip");
ZipFile zipFile = new ZipFile(new File("fra-eng.zip"));
Enumeration extends ZipEntry> entries = zipFile.entries();
while (entries.hasMoreElements()) {
ZipEntry entry = entries.nextElement();
if (entry.getName().contains("fra.txt")) {
InputStream stream = zipFile.getInputStream(entry);
return new String(stream.readAllBytes(), StandardCharsets.UTF_8);
}
}
return null;
}
String rawText = readDataNMT();
System.out.println(rawText.substring(0, 75));
.. parsed-literal::
:class: output
Go. Va !
Hi. Salut !
Run! Cours !
Run! Courez !
Who? Qui ?
Wow! Ça alors !
After downloading the dataset, we proceed with several preprocessing
steps for the raw text data. For instance, we replace non-breaking space
with space, convert uppercase letters to lowercase ones, and insert
space between words and punctuation marks.
.. code:: java
public static String preprocessNMT(String text) {
// Replace non-breaking space with space, and convert uppercase letters to
// lowercase ones
text = text.replace('\u202f', ' ').replaceAll("\\xa0", " ").toLowerCase();
// Insert space between words and punctuation marks
StringBuilder out = new StringBuilder();
Character currChar;
for (int i = 0; i < text.length(); i++) {
currChar = text.charAt(i);
if (i > 0 && noSpace(currChar, text.charAt(i - 1))) {
out.append(' ');
}
out.append(currChar);
}
return out.toString();
}
public static boolean noSpace(Character currChar, Character prevChar) {
/* Preprocess the English-French dataset. */
return new HashSet<>(Arrays.asList(',', '.', '!', '?')).contains(currChar)
&& prevChar != ' ';
}
String text = preprocessNMT(rawText);
System.out.println(text.substring(0, 80));
.. parsed-literal::
:class: output
go . va !
hi . salut !
run ! cours !
run ! courez !
who ? qui ?
wow ! ça alors !
Tokenization
------------
Different from character-level tokenization in
:numref:`sec_language_model`, for machine translation we prefer
word-level tokenization here (state-of-the-art models may use more
advanced tokenization techniques). The following ``tokenizeNMT``
function tokenizes the the first ``numExamples`` text sequence pairs,
where each token is either a word or a punctuation mark. This function
returns two lists of token lists: ``source`` and ``target``.
Specifically, ``source.get(i)`` is a list of tokens from the
:math:`i^\mathrm{th}` text sequence in the source language (English
here) and ``target.get(i)`` is that in the target language (French
here).
.. code:: java
public static Pair, ArrayList> tokenizeNMT(
String text, Integer numExamples) {
ArrayList source = new ArrayList<>();
ArrayList target = new ArrayList<>();
int i = 0;
for (String line : text.split("\n")) {
if (numExamples != null && i > numExamples) {
break;
}
String[] parts = line.split("\t");
if (parts.length == 2) {
source.add(parts[0].split(" "));
target.add(parts[1].split(" "));
}
i += 1;
}
return new Pair<>(source, target);
}
Pair, ArrayList> pair = tokenizeNMT(text.toString(), null);
ArrayList source = pair.getKey();
ArrayList target = pair.getValue();
for (String[] subArr : source.subList(0, 6)) {
System.out.println(Arrays.toString(subArr));
}
.. parsed-literal::
:class: output
[go, .]
[hi, .]
[run, !]
[run, !]
[who, ?]
[wow, !]
.. code:: java
for (String[] subArr : target.subList(0, 6)) {
System.out.println(Arrays.toString(subArr));
}
.. parsed-literal::
:class: output
[va, !]
[salut, !]
[cours, !]
[courez, !]
[qui, ?]
[ça, alors, !]
Let us plot the histogram of the number of tokens per text sequence. In
this simple English-French dataset, most of the text sequences have
fewer than 20 tokens.
.. code:: java
double[] y1 = new double[source.size()];
for (int i = 0; i < source.size(); i++) y1[i] = source.get(i).length;
double[] y2 = new double[target.size()];
for (int i = 0; i < target.size(); i++) y2[i] = target.get(i).length;
HistogramTrace trace1 =
HistogramTrace.builder(y1).opacity(.75).name("source").nBinsX(20).build();
HistogramTrace trace2 =
HistogramTrace.builder(y2).opacity(.75).name("target").nBinsX(20).build();
Layout layout = Layout.builder().barMode(Layout.BarMode.GROUP).build();
new Figure(layout, trace1, trace2);
.. raw:: html
Vocabulary
----------
Since the machine translation dataset consists of pairs of languages, we
can build two vocabularies for both the source language and the target
language separately. With word-level tokenization, the vocabulary size
will be significantly larger than that using character-level
tokenization. To alleviate this, here we treat infrequent tokens that
appear less than 2 times as the same unknown ("") token. Besides
that, we specify additional special tokens such as for padding ("")
sequences to the same length in minibatches, and for marking the
beginning ("") or end ("") of sequences. Such special tokens
are commonly used in natural language processing tasks.
.. code:: java
Vocab srcVocab =
new Vocab(
source.stream().toArray(String[][]::new),
2,
new String[] {"", "", ""});
System.out.println(srcVocab.length());
.. parsed-literal::
:class: output
10012
.. _subsec_mt_data_loading:
Loading the Dataset
-------------------
Recall that in language modeling each sequence example, either a segment
of one sentence or a span over multiple sentences, has a fixed length.
This was specified by the ``numSteps`` (number of time steps or tokens)
argument in :numref:`sec_language_model`. In machine translation, each
example is a pair of source and target text sequences, where each text
sequence may have different lengths.
For computational efficiency, we can still process a minibatch of text
sequences at one time by *truncation* and *padding*. Suppose that every
sequence in the same minibatch should have the same length ``numSteps``.
If a text sequence has fewer than ``numSteps`` tokens, we will keep
appending the special "" token to its end until its length reaches
``numSteps``. Otherwise, we will truncate the text sequence by only
taking its first ``numSteps`` tokens and discarding the remaining. In
this way, every text sequence will have the same length to be loaded in
minibatches of the same shape.
The following ``truncatePad`` function truncates or pads text sequences
as described before.
.. code:: java
public static int[] truncatePad(Integer[] integerLine, int numSteps, int paddingToken) {
/* Truncate or pad sequences */
int[] line = Arrays.stream(integerLine).mapToInt(i -> i).toArray();
if (line.length > numSteps) {
return Arrays.copyOfRange(line, 0, numSteps);
}
int[] paddingTokenArr = new int[numSteps - line.length]; // Pad
Arrays.fill(paddingTokenArr, paddingToken);
return IntStream.concat(Arrays.stream(line), Arrays.stream(paddingTokenArr)).toArray();
}
int[] result = truncatePad(srcVocab.getIdxs(source.get(0)), 10, srcVocab.getIdx(""));
System.out.println(Arrays.toString(result));
.. parsed-literal::
:class: output
[47, 4, 1, 1, 1, 1, 1, 1, 1, 1]
Now we define a function to transform text sequences into minibatches
for training. We append the special “” token to the end of every
sequence to indicate the end of the sequence. When a model is predicting
by generating a sequence token after token, the generation of the
“” token can suggest that the output sequence is complete. Besides,
we also record the length of each text sequence excluding the padding
tokens. This information will be needed by some models that we will
cover later.
.. code:: java
public static Pair buildArrayNMT(
List lines, Vocab vocab, int numSteps) {
/* Transform text sequences of machine translation into minibatches. */
List linesIntArr = new ArrayList<>();
for (String[] strings : lines) {
linesIntArr.add(vocab.getIdxs(strings));
}
for (int i = 0; i < linesIntArr.size(); i++) {
List temp = new ArrayList<>(Arrays.asList(linesIntArr.get(i)));
temp.add(vocab.getIdx(""));
linesIntArr.set(i, temp.toArray(new Integer[0]));
}
NDManager manager = NDManager.newBaseManager();
NDArray arr = manager.create(new Shape(linesIntArr.size(), numSteps), DataType.INT32);
int row = 0;
for (Integer[] line : linesIntArr) {
NDArray rowArr = manager.create(truncatePad(line, numSteps, vocab.getIdx("")));
arr.set(new NDIndex("{}:", row), rowArr);
row += 1;
}
NDArray validLen = arr.neq(vocab.getIdx("")).sum(new int[] {1});
return new Pair<>(arr, validLen);
}
Putting All Things Together
---------------------------
Finally, we define the ``loadDataNMT`` function to return the data
iterator, together with the vocabularies for both the source language
and the target language.
.. code:: java
public static Pair> loadDataNMT(
int batchSize, int numSteps, int numExamples) throws IOException {
/* Return the iterator and the vocabularies of the translation dataset. */
String text = preprocessNMT(readDataNMT());
Pair, ArrayList> pair = tokenizeNMT(text, numExamples);
ArrayList source = pair.getKey();
ArrayList target = pair.getValue();
Vocab srcVocab =
new Vocab(
source.toArray(String[][]::new),
2,
new String[] {"", "", ""});
Vocab tgtVocab =
new Vocab(
target.toArray(String[][]::new),
2,
new String[] {"", "", ""});
Pair pairArr = buildArrayNMT(source, srcVocab, numSteps);
NDArray srcArr = pairArr.getKey();
NDArray srcValidLen = pairArr.getValue();
pairArr = buildArrayNMT(target, tgtVocab, numSteps);
NDArray tgtArr = pairArr.getKey();
NDArray tgtValidLen = pairArr.getValue();
ArrayDataset dataset =
new ArrayDataset.Builder()
.setData(srcArr, srcValidLen)
.optLabels(tgtArr, tgtValidLen)
.setSampling(batchSize, true)
.build();
return new Pair<>(dataset, new Pair<>(srcVocab, tgtVocab));
}
Let us read the first minibatch from the English-French dataset.
.. code:: java
Pair> output = loadDataNMT(2, 8, 600);
ArrayDataset dataset = output.getKey();
srcVocab = output.getValue().getKey();
Vocab tgtVocab = output.getValue().getValue();
Batch batch = dataset.getData(manager).iterator().next();
NDArray X = batch.getData().get(0);
NDArray xValidLen = batch.getData().get(1);
NDArray Y = batch.getLabels().get(0);
NDArray yValidLen = batch.getLabels().get(1);
System.out.println(X);
System.out.println(xValidLen);
System.out.println(Y);
System.out.println(yValidLen);
.. parsed-literal::
:class: output
ND: (2, 8) gpu(0) int32
[[71, 5, 3, 1, 1, 1, 1, 1],
[ 9, 56, 4, 3, 1, 1, 1, 1],
]
ND: (2) gpu(0) int64
[ 3, 4]
ND: (2, 8) gpu(0) int32
[[ 0, 5, 3, 1, 1, 1, 1, 1],
[126, 15, 29, 74, 4, 3, 1, 1],
]
ND: (2) gpu(0) int64
[ 3, 6]
Summary
-------
- Machine translation refers to the automatic translation of a sequence
from one language to another.
- Using word-level tokenization, the vocabulary size will be
significantly larger than that using character-level tokenization. To
alleviate this, we can treat infrequent tokens as the same unknown
token.
- We can truncate and pad text sequences so that all of them will have
the same length to be loaded in minibatches.
Exercises
---------
1. Try different values of the ``numExamples`` argument in the
``loadDataNMT`` function. How does this affect the vocabulary sizes
of the source language and the target language?
2. Text in some languages such as Chinese and Japanese does not have
word boundary indicators (e.g., space). Is word-level tokenization
still a good idea for such cases? Why or why not?