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From sethah <...@git.apache.org>
Subject [GitHub] spark pull request #16715: [Spark-18080][ML] Python API & Examples for Local...
Date Tue, 07 Feb 2017 21:11:58 GMT
Github user sethah commented on a diff in the pull request:

    https://github.com/apache/spark/pull/16715#discussion_r99923657
  
    --- Diff: python/pyspark/ml/feature.py ---
    @@ -120,6 +122,200 @@ def getThreshold(self):
             return self.getOrDefault(self.threshold)
     
     
    +class LSHParams(Params):
    +    """
    +    Mixin for Locality Sensitive Hashing(LSH) algorithm parameters.
    +    """
    +
    +    numHashTables = Param(Params._dummy(), "numHashTables", "number of hash tables, where
" +
    +                          "increasing number of hash tables lowers the false negative
rate, " +
    +                          "and decreasing it improves the running performance.",
    +                          typeConverter=TypeConverters.toInt)
    +
    +    def __init__(self):
    +        super(LSHParams, self).__init__()
    +
    +    @since("2.2.0")
    +    def setNumHashTables(self, value):
    +        """
    +        Sets the value of :py:attr:`numHashTables`.
    +        """
    +        return self._set(numHashTables=value)
    +
    +    @since("2.2.0")
    +    def getNumHashTables(self):
    +        """
    +        Gets the value of numHashTables or its default value.
    +        """
    +        return self.getOrDefault(self.numHashTables)
    +
    +
    +class LSHModel():
    +    """
    +    Mixin for Locality Sensitive Hashing(LSH) models.
    +    """
    +
    +    @since("2.2.0")
    +    def approxNearestNeighbors(self, dataset, key, numNearestNeighbors, singleProbing=True,
    +                               distCol="distCol"):
    +        """
    +        Given a large dataset and an item, approximately find at most k items which have
the
    +        closest distance to the item. If the :py:attr:`outputCol` is missing, the method
will
    +        transform the data; if the :py:attr:`outputCol` exists, it will use that. This
allows
    +        caching of the transformed data when necessary.
    +
    +        :param dataset: The dataset to search for nearest neighbors of the key.
    +        :param key: Feature vector representing the item to search for.
    +        :param numNearestNeighbors: The maximum number of nearest neighbors.
    +        :param distCol: Output column for storing the distance between each result row
and the key.
    +                        Use "distCol" as default value if it's not specified.
    +        :return: A dataset containing at most k items closest to the key. A distCol is
added
    +                 to show the distance between each row and the key.
    +        """
    +        return self._call_java("approxNearestNeighbors", dataset, key, numNearestNeighbors,
    +                               distCol)
    +
    +    @since("2.2.0")
    +    def approxSimilarityJoin(self, datasetA, datasetB, threshold, distCol="distCol"):
    +        """
    +        Join two dataset to approximately find all pairs of rows whose distance are smaller
than
    +        the threshold. If the :py:attr:`outputCol` is missing, the method will transform
the data;
    +        if the :py:attr:`outputCol` exists, it will use that. This allows caching of
the
    +        transformed data when necessary.
    +
    +        :param datasetA: One of the datasets to join.
    +        :param datasetB: Another dataset to join.
    +        :param threshold: The threshold for the distance of row pairs.
    +        :param distCol: Output column for storing the distance between each result row
and the key.
    +                        Use "distCol" as default value if it's not specified.
    +        :return: A joined dataset containing pairs of rows. The original rows are in
columns
    +                "datasetA" and "datasetB", and a distCol is added to show the distance
of
    +                each pair.
    +        """
    +        return self._call_java("approxSimilarityJoin", datasetA, datasetB, threshold,
distCol)
    +
    +
    +@inherit_doc
    +class BucketedRandomProjectionLSH(JavaEstimator, LSHParams, HasInputCol, HasOutputCol,
HasSeed,
    +                                  JavaMLReadable, JavaMLWritable):
    +    """
    +    .. note:: Experimental
    +
    +    LSH class for Euclidean distance metrics.
    +    The input is dense or sparse vectors, each of which represents a point in the Euclidean
    +    distance space. The output will be vectors of configurable dimension. Hash value
in the
    +    same dimension is calculated by the same hash function.
    +
    +    .. seealso:: `Stable Distributions \
    +    <https://en.wikipedia.org/wiki/Locality-sensitive_hashing#Stable_distributions>`_
    +    .. seealso:: `Hashing for Similarity Search: A Survey <https://arxiv.org/abs/1408.2927>`_
    +
    +    >>> from pyspark.ml.linalg import Vectors
    +    >>> data = [(Vectors.dense([-1.0, -1.0 ]),),
    +    ...         (Vectors.dense([-1.0, 1.0 ]),),
    +    ...         (Vectors.dense([1.0, -1.0 ]),),
    +    ...         (Vectors.dense([1.0, 1.0]),)]
    +    >>> df = spark.createDataFrame(data, ["keys"])
    +    >>> rp = BucketedRandomProjectionLSH(inputCol="keys", outputCol="values",
    +    ...                                  seed=12345, bucketLength=1.0)
    +    >>> model = rp.fit(df)
    +    >>> model.randUnitVectors
    +    [DenseVector([-0.3041, 0.9527])]
    +    >>> model.transform(df).head()
    +    Row(keys=DenseVector([-1.0, -1.0]), values=[DenseVector([-1.0])])
    +    >>> data2 = [(Vectors.dense([2.0, 2.0 ]),),
    +    ...          (Vectors.dense([2.0, 3.0 ]),),
    +    ...          (Vectors.dense([3.0, 2.0 ]),),
    +    ...          (Vectors.dense([3.0, 3.0]),)]
    +    >>> df2 = spark.createDataFrame(data2, ["keys"])
    +    >>> model.approxNearestNeighbors(df2, Vectors.dense([1.0, 2.0]), 1).collect()
    +    [Row(keys=DenseVector([2.0, 2.0]), values=[DenseVector([1.0])], distCol=1.0)]
    +    >>> model.approxSimilarityJoin(df, df2, 3.0).select("distCol").head()[0]
    --- End diff --
    
    Since doctests are also partly used to demonstrate the usage of the algorithm, I don't
think this line is particularly useful. It is quite hard to interpret. I think it might be
nicer to add an "id" column to the dataframes and then do a "show" here to see the joined
dataframes, as in the Scala example. Then again, you end up with:
    
    ````
    +--------------------+--------------------+----------------+
    |            datasetA|            datasetB|         distCol|
    +--------------------+--------------------+----------------+
    |[[1.0,1.0],Wrappe...|[[3.0,2.0],Wrappe...|2.23606797749979|
    +--------------------+--------------------+----------------+
    ````
    Which is also confusing! Thoughts on which option is better?


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