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From m...@apache.org
Subject git commit: [SPARK-3143][MLLIB] add tf-idf user guide
Date Thu, 21 Aug 2014 00:41:40 GMT
Repository: spark
Updated Branches:

Moved TF-IDF before Word2Vec because the former is more basic. I also added a link for Word2Vec.
atalwalkar

Author: Xiangrui Meng <meng@databricks.com>

Closes #2061 from mengxr/tfidf-doc and squashes the following commits:

a5ea4b4 [Xiangrui Meng] add tf-idf user guide

Project: http://git-wip-us.apache.org/repos/asf/spark/repo
Commit: http://git-wip-us.apache.org/repos/asf/spark/commit/e1571874
Tree: http://git-wip-us.apache.org/repos/asf/spark/tree/e1571874
Diff: http://git-wip-us.apache.org/repos/asf/spark/diff/e1571874

Commit: e1571874f26c1df2dfd5ac2959612372716cd2d8
Parents: ba3c730
Author: Xiangrui Meng <meng@databricks.com>
Authored: Wed Aug 20 17:41:36 2014 -0700
Committer: Xiangrui Meng <meng@databricks.com>
Committed: Wed Aug 20 17:41:36 2014 -0700

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docs/mllib-feature-extraction.md | 83 +++++++++++++++++++++++++++++++++--
1 file changed, 80 insertions(+), 3 deletions(-)
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http://git-wip-us.apache.org/repos/asf/spark/blob/e1571874/docs/mllib-feature-extraction.md
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diff --git a/docs/mllib-feature-extraction.md b/docs/mllib-feature-extraction.md
index 4b3cb71..2031b96 100644
--- a/docs/mllib-feature-extraction.md
+++ b/docs/mllib-feature-extraction.md
@@ -7,9 +7,88 @@ displayTitle: <a href="mllib-guide.html">MLlib</a> - Feature
Extraction
{:toc}

+
+## TF-IDF
+
+[Term frequency-inverse document frequency (TF-IDF)](http://en.wikipedia.org/wiki/Tf%E2%80%93idf)
is a feature
+vectorization method widely used in text mining to reflect the importance of a term to a
document in the corpus.
+Denote a term by $t$, a document by $d$, and the corpus by $D$.
+Term frequency $TF(t, d)$ is the number of times that term $t$ appears in document $d$,
+while document frequency $DF(t, D)$ is the number of documents that contains term $t$.
+If we only use term frequency to measure the importance, it is very easy to over-emphasize
terms that
+appear very often but carry little information about the document, e.g., "a", "the", and
"of".
+If a term appears very often across the corpus, it means it doesn't carry special information
+a particular document.
+Inverse document frequency is a numerical measure of how much information a term provides:
+$+IDF(t, D) = \log \frac{|D| + 1}{DF(t, D) + 1}, +$
+where $|D|$ is the total number of documents in the corpus.
+Since logarithm is used, if a term appears in all documents, its IDF value becomes 0.
+Note that a smoothing term is applied to avoid dividing by zero for terms outside the corpus.
+The TF-IDF measure is simply the product of TF and IDF:
+$+TFIDF(t, d, D) = TF(t, d) \cdot IDF(t, D). +$
+There are several variants on the definition of term frequency and document frequency.
+In MLlib, we separate TF and IDF to make them flexible.
+
+Our implementation of term frequency utilizes the
+[hashing trick](http://en.wikipedia.org/wiki/Feature_hashing).
+A raw feature is mapped into an index (term) by applying a hash function.
+Then term frequencies are calculated based on the mapped indices.
+This approach avoids the need to compute a global term-to-index map,
+which can be expensive for a large corpus, but it suffers from potential hash collisions,
+where different raw features may become the same term after hashing.
+To reduce the chance of collision, we can increase the target feature dimension, i.e.,
+the number of buckets of the hash table.
+The default feature dimension is $2^{20} = 1,048,576$.
+
+**Note:** MLlib doesn't provide tools for text segmentation.
+We refer users to the [Stanford NLP Group](http://nlp.stanford.edu/) and
+[scalanlp/chalk](https://github.com/scalanlp/chalk).
+
+<div class="codetabs">
+<div data-lang="scala" markdown="1">
+
+TF and IDF are implemented in [HashingTF](api/scala/index.html#org.apache.spark.mllib.feature.HashingTF)
+and [IDF](api/scala/index.html#org.apache.spark.mllib.feature.IDF).
+HashingTF takes an RDD[Iterable[_]] as the input.
+Each record could be an iterable of strings or other types.
+
+{% highlight scala %}
+import org.apache.spark.rdd.RDD
+import org.apache.spark.SparkContext
+import org.apache.spark.mllib.feature.HashingTF
+import org.apache.spark.mllib.linalg.Vector
+
+val sc: SparkContext = ...
+
+// Load documents (one per line).
+val documents: RDD[Seq[String]] = sc.textFile("...").map(_.split(" ").toSeq)
+
+val hashingTF = new HashingTF()
+val tf: RDD[Vector] = hasingTF.transform(documents)
+{% endhighlight %}
+
+While applying HashingTF only needs a single pass to the data, applying IDF needs two
passes:
+first to compute the IDF vector and second to scale the term frequencies by IDF.
+
+{% highlight scala %}
+import org.apache.spark.mllib.feature.IDF
+
+// ... continue from the previous example
+tf.cache()
+val idf = new IDF().fit(tf)
+val tfidf: RDD[Vector] = idf.transform(tf)
+{% endhighlight %}
+</div>
+</div>
+
## Word2Vec

-Word2Vec computes distributed vector representation of words. The main advantage of the distributed
of words.
+The main advantage of the distributed
representations is that similar words are close in the vector space, which makes generalization
to
novel patterns easier and model estimation more robust. Distributed vector representation
is
showed to be useful in many natural language processing applications such as named entity

@@ -69,5 +148,3 @@ for((synonym, cosineSimilarity) <- synonyms) {
{% endhighlight %}
</div>
</div>
-
-## TFIDF
\ No newline at end of file

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