假设我有一个如下数据框:
import org.apache.spark.sql.{Row, DataFrame, SparkSession}
import org.apache.spark.sql.types.{StructType, StructField, IntegerType, StringType, DoubleType, NumericType}
import org.apache.spark.sql.functions.{udf, col, skewness}
val someData = Seq(
Row(8, "bat"),
Row(64, "mouse"),
Row(-27, "horse"),
Row(null, "mouse"),
Row(27, null)
)
val someSchema = List(
StructField("number", IntegerType, true),
StructField("word", StringType, true)
)
val someDF = spark.createDataFrame(
spark.sparkContext.parallelize(someData),
StructType(someSchema)
)
val df = someDF.withColumn("constantColumn", lit(1))
我想计算每个具有类似NumericType类型的列的偏度。然后,如果列的偏斜度高于某个阈值,我想通过f(x) = log(x + 1)
对其进行转换。 (我知道对负数据执行对数转换会产生NaN,但我最终希望编写代码来考虑这种可能性)。
到目前为止,我已经尝试过:
我已经找到了一种方法,但是它需要一个可变的数据帧df
。据我有限的理解,这是不可取的。
val log1p = scala.math.log1p(_)
val log1pUDF = udf(scala.math.log1p(_: Double))
val transformThreshold = 0.04
// filter those columns which have a type that inherits from NumericType
val numericColumns = df.columns.filter(column => df.select(column).schema(0).dataType.isInstanceOf[NumericType])
// for columns having NumericType, filter those that are sufficiently skewed
val columnsToTransform = numericColumns.filter(numericColumn => df.select(skewness(df(numericColumn))).head.getDouble(0) > transformThreshold)
// for all columns that are sufficiently skewed, perform log1p transform and add it to df
for(column <- columnsToTransform) {
// df should be mutable here!
df = df.withColumn(column + "_log1p", log1pUDF(df(column)))
}
我的问题:
(在Spark 2.4.0,Scala 2.11.12上运行。)
答案 0 :(得分:4)
您可以使用递归函数代替for()
结构:
def rec(df: DataFrame, columns: List[String]): DataFrame = columns match {
case Nil => df
case h :: xs => rec(df.withColumn(s"${h}_log1p", log1pUDF(col(h))), xs)
}