How to split a string by delimiter from the right?

Question

How to split a string by a delimiter from the right?

e.g.

scala> "hello there how are you?".rightSplit(" ", 1)
res0: Array[java.lang.String] = Array(hello there how are, you?)

Python has a .rsplit() method which is what I'm after in Scala:

In [1]: "hello there how are you?".rsplit(" ", 1)
Out[1]: ['hello there how are', 'you?']

Ladylove

Answer 1

I think the simplest solution is to search for the index position and then split based on that. For example:

scala> val msg = "hello there how are you?"
msg: String = hello there how are you?

scala> msg splitAt (msg lastIndexOf ' ')
res1: (String, String) = (hello there how are," you?")

And since someone remarked on lastIndexOf returning -1, that's perfectly fine with the solution:

scala> val msg = "AstringWithoutSpaces"
msg: String = AstringWithoutSpaces

scala> msg splitAt (msg lastIndexOf ' ')
res0: (String, String) = ("",AstringWithoutSpaces)

Interstratify

lastIndexOf can return -1. – Inca

@Inca In which case splitAt will return an empty string first, and the original string second. – Interstratify

Dang, you have thought of everything! You are right it works. – Inca

unlike the .rsplit() example above, this contains the delimiter, though – Prevision

Answer 2

You could use plain old regular expressions:

scala> val LastSpace = " (?=[^ ]+$)"
LastSpace: String = " (?=[^ ]+$)"

scala> "hello there how are you?".split(LastSpace)
res0: Array[String] = Array(hello there how are, you?)

(?=[^ ]+$) says that we'll look ahead (?=) for a group of non-space ([^ ]) characters with at least 1 character length. Finally this space followed by such sequence has to be at the end of the string: $.

This solution wont break if there is only one token:

scala> "hello".split(LastSpace)
res1: Array[String] = Array(hello)

Karisakarissa

I thought of suggesting a regular expression, but it's way less efficient and more difficult to understand than the simpler approach I ended up suggesting. Not to cast aspersions on your solution -- which is perfectly workable and different than the others presented --, but I wonder what it is about complex solutions that make people prefer them? – Interstratify

Answer 3

scala> val sl = "hello there how are you?".split(" ").reverse.toList
sl: List[String] = List(you?, are, how, there, hello)

scala> val sr = (sl.head :: (sl.tail.reverse.mkString(" ") :: Nil)).reverse
sr: List[String] = List(hello there how are, you?)

Empathize

Answer 4

I implemented a custom rsplit function in Scala to imitate Python's rsplit.

The concept behind it involves iteratively using lastIndexOf and substring to perform rsplit.

object Main {
  def rsplit(string: String, delimiter: String, maxSplits: Int): Array[String] = {
    var remainingSplits = maxSplits
    var str = string
    var result = List[String]()

    while (remainingSplits > 0) {
      val pos = str.lastIndexOf(delimiter)
      if (pos == -1) {
        result = str :: result
        remainingSplits = 0
        str=""
      } else {
        result = str.substring(pos + delimiter.length) :: result
        str = str.substring(0, pos)
        remainingSplits -= 1
      }
    }

    if (str.nonEmpty || maxSplits == 0) {
      result = str :: result
    }

    result.toArray
  }

  def myPrint(stringArray: Array[String]): Unit= {
    println("["++stringArray.map("\'"++_++"\'").mkString(", ")++"]");
  }

  def main(args: Array[String]): Unit = {
    // Test cases
    myPrint(rsplit("bor tor tor tor", "tor", 2))  // More than maxSplits
    myPrint(rsplit("bor tor tor", "tor", 2))      // Exactly equal to maxSplits
    myPrint(rsplit("bor tor", "tor", 2))      // Less than maxSplits
  }
}

Attempt This Scala code Online!

print("bor tor tor tor".rsplit("tor", 2)) # ['bor tor ', ' ', '']
print("bor tor tor".rsplit("tor", 2))      # ['bor ', ' ', '']
print("bor tor".rsplit("tor", 2))           # ['bor ', '']

Run the reference Python code online!

Hendrik