Using DataFrame with MLlib - apache-spark

Question

Let's say I have a DataFrame (that I read in from a csv on HDFS) and I want to train some algorithms on it via MLlib. How do I convert the rows into LabeledPoints or otherwise utilize MLlib on this dataset?

Answer 1

Assuming you're using Scala:
Let's say your obtain the DataFrame as follows:
val results : DataFrame = sqlContext.sql(...)
Step 1: call results.printSchema() -- this will show you not only the columns in the DataFrame and (this is important) their order, but also what Spark SQL thinks are their types. Once you see this output things get a lot less mysterious.
Step 2: Get an RDD[Row] out of the DataFrame:
val rows: RDD[Row] = results.rdd
Step 3: Now it's just a matter of pulling whatever fields interest you out of the individual rows. For this you need to know the 0-based position of each field and it's type, and luckily you obtained all that in Step 1 above. For example,
let's say you did a SELECT x, y, z, w FROM ... and printing the schema yielded
root
|-- x double (nullable = ...)
|-- y string (nullable = ...)
|-- z integer (nullable = ...)
|-- w binary (nullable = ...)
And let's say all you wanted to use x and z. You can pull them out into an RDD[(Double, Integer)] as follows:
rows.map(row => {
// x has position 0 and type double
// z has position 2 and type integer
(row.getDouble(0), row.getInt(2))
})
From here you just use Core Spark to create the relevant MLlib objects. Things could get a little more complicated if your SQL returns columns of array type, in which case you'll have to call getList(...) for that column.

Answer 2

Assuming you're using JAVA (Spark version 1.6.2):
Here is a simple example of JAVA code using DataFrame for machine learning.
It loads a JSON with the following structure,
[{"label":1,"att2":5.037089672359123,"att1":2.4100883023159456}, ... ]
splits the data into training and testing,
train the model using the train data,
apply the model to the test data and
stores the results.
Moreover according to the official documentation the "DataFrame-based API is primary API" for MLlib since the current version 2.0.0. So you can find several examples using DataFrame.
The code:
SparkConf conf = new SparkConf().setAppName("MyApp").setMaster("local[2]");
SparkContext sc = new SparkContext(conf);
String path = "F:\\SparkApp\\test.json";
String outputPath = "F:\\SparkApp\\justTest";
System.setProperty("hadoop.home.dir", "C:\\winutils\\");
SQLContext sqlContext = new org.apache.spark.sql.SQLContext(sc);
DataFrame df = sqlContext.read().json(path);
df.registerTempTable("tmp");
DataFrame newDF = df.sqlContext().sql("SELECT att1, att2, label FROM tmp");
DataFrame dataFixed = newDF.withColumn("label", newDF.col("label").cast("Double"));
VectorAssembler assembler = new VectorAssembler().setInputCols(new String[]{"att1", "att2"}).setOutputCol("features");
StringIndexer indexer = new StringIndexer().setInputCol("label").setOutputCol("labelIndexed");
// Split the data into training and test
DataFrame[] splits = dataFixed.randomSplit(new double[] {0.7, 0.3});
DataFrame trainingData = splits[0];
DataFrame testData = splits[1];
DecisionTreeClassifier dt = new DecisionTreeClassifier().setLabelCol("labelIndexed").setFeaturesCol("features");
Pipeline pipeline = new Pipeline().setStages(new PipelineStage[] {assembler, indexer, dt});
// Train model
PipelineModel model = pipeline.fit(trainingData);
// Make predictions
DataFrame predictions = model.transform(testData);
predictions.rdd().coalesce(1,true,null).saveAsTextFile("justPlay.txt" +System.currentTimeMillis());

Answer 3

RDD based Mllib is on its way to be deprecated, so you should rather use DataFrame based Mllib.
Generally the input to these MLlib apis is a DataFrame containing 2 columns - label and feature. There are various methods to build this DataFrame - low level apis like org.apache.spark.mllib.linalg.{Vector, Vectors}, org.apache.spark.mllib.regression.LabeledPoint, org.apache.spark.mllib.linalg.{Matrix, Matrices} etc. They all take numeric values for feature and label.
Words can be converted to vectors using - org.apache.spark.ml.feature.{Word2Vec, Word2VecModel}. This documentation explains more - https://spark.apache.org/docs/latest/mllib-data-types.html
Once input dataframe with label and feature is created, instantiate the MLlib api and pass in the DataFrame to 'fit' function to get the model and then call 'transform' or 'predict' function on the model to get the results.
Example -
training file looks like -
<numeric label> <a string separated by space>
//Build word vector
val trainingData = spark.read.parquet(<path to training file>)
val sampleDataDf = trainingData
.map { r =>
val s = r.getAs[String]("value").split(" ")
val label = s.head.toDouble
val feature = s.tail
(label, feature)
}.toDF("lable","feature_words")
val word2Vec = new Word2Vec()
.setInputCol("feature_words")
.setOutputCol("feature_vectors")
.setMinCount(0)
.setMaxIter(10)
//build word2Vector model
val model = word2Vec.fit(sampleDataDf)
//convert training text data to vector and labels
val wVectors = model.transform(sampleDataDf)
//train LinearSVM model
val svmAlgorithm = new LinearSVC()
.setFeaturesCol("feature_vectors")
.setMaxIter(100)
.setLabelCol("lable")
.setRegParam(0.01)
.setThreshold(0.5)
.fit(wVectors) //use word vectors created
//Predict new data, same format as training data containing words
val predictionData = spark.read.parquet(<path to prediction file>)
val pDataDf = predictionData
.map { r =>
val s = r.getAs[String]("value").split(" ")
val label = s.head.toDouble
val feature = s.tail
(label, feature)
}.toDF("lable","feature_words")
val pVectors = model.transform(pDataDf)
val predictionlResult = pVectors.map{ r =>
val s = r.getAs[Seq[String]]("feature_words")
val v = r.getAs[Vector]("feature_vectors")
val c = svmAlgorithm.predict(v) // predict using trained SVM
s"$c ${s.mkString(" ")}"
}