scala的递归算术运算

Question

I have a scala List object with a recursive definition of all operations I have to do with columns of a spark dataframe. For example, the operations

(C1 - C2) + ( (C3 - C4)- (C5 -C6) )

are defined by the next scala List:

List("addition", List("substraction",List("C1","C2")),
                 List("substraction",
                                  List("substraction",List("C3","C4")),
                                  List("substraction"), List("C5","C6"))
)

where "C1",...,"C5" are the names of the spark dataframes columns.

I would like to define a recursive scala function that gives me the final column result.

Does anyone know a way to do it?

Answer 1

The way you define the operation is quite strange. You encapsulate column name operands in a list, but not complex operands. Therefore your lists can either have 2 or three elements. How would you define something like (A + (BC)) ? I would start by fixing that and write your operation either like this (3 elements per list):

val list = List("addition",
    List("substraction","C1","C2"),
    List("substraction",
        List("substraction","C3","C4"),
        List("substraction", "C5","C6")
    )
)

or like this (2 elements per list):

val list = List("addition", List(
    List("substraction", List("C1","C2")),
    List("substraction", List(
        List("substraction", List("C3","C4")),
        List("substraction", List("C5","C6"))
    )))
)

The second version being much more verbose, let's pick the first one and write the recursive function:

def operation_to_col(operation : Any) : Column = {
    operation match {
        case x : String => col(x)
        case List("addition", s1 : Any, s2 : Any) =>
             operation_to_col(s1) + operation_to_col(s2)
        case List("substraction", s1 : Any, s2 : Any) =>
             operation_to_col(s1) + operation_to_col(s2)
    }
}

Answer 2

First, I am going to change the definition of the operations. For example, the operations

(C1 - C2) + ( (C3 - C4)- (C5 -C6) )

are defined by the next scala List:

val list = List("addition",
        List("substraction","C1","C2"),
        List("substraction",
              List("substraction","C3","C4"),
              List("substraction", "C5","C6")

) )

I am going to create a dataframe for an example:

val data = Seq((1000, 1, 2,3,4,5), (2000,1,2,3,4,5), (3000,1,2,3,4,5))
val rdd = spark.sparkContext.parallelize(data)
val df = rdd.toDF("C1","C2","C3","C4","C5","C6")

The List of permitted operations is:

val operations=List("addition","subtraction","multiplication","division")

I created the next Map object to associate the operations and their symbols:

val oprSimbols:Map[String,String] = Map("addition"->"+", "substraction"-> "-", "multiplication"->"*","division"->"/")

Finally, I define the function that solves the problem:

def operation_to_col(df: DataFrame,oprSimbols: Map[String,String], 
     operations:List[String], list : Any) : DataFrame = {
     list match {
        case x if operations.contains(x.toString) => df.select(col(x.toString))

        case List(oprName:String,x:String, y:String) =>{
           val sym = oprSimbols(oprName)
           val exprOpr = List(x,sym,y).mkString(" ")
           df.selectExpr(exprOpr)}

        case List(oprName:String, s1 : Any, s2 : Any) =>{
           val df1 = operation_to_col(df,oprSimbols,operations,s1)
           val df2 = operation_to_col(df,oprSimbols,operations,s2)
           val sym = oprSimbols(oprName)
           val exprOpr = List(df1.columns(0),sym,df2.columns(0)).mkString(" ")
           df.selectExpr(exprOpr)
}

} }

We can check it:

operation_to_col(df,oprSimbols, operations, list )