last element of array matching scala

Question

Good afternoon! I'm using Scala and I want to match first three element of a list and the last one, no matter how much of them are in the list.

val myList:List[List[Int]] = List(List(3,1,2,3,4),List(23,45,6,7,2),List(3,3,2,1,5,34,43,2),List(8,5,3,34,4,5,3,2),List(3,2,45,56))

def parse(lists: List[Int]): List[Int] = lists.toArray match{
  case Array(item, site, buyer, _*, date) => List(item, site, buyer, date)}

myList.map(parse _)

But I get : error: bad use of _* (a sequence pattern must be the last pattern) I understand why I get it, but how can I avoid?

My use case is that I'm reading from hdfs, and every file has exact N (N is constant and equal for all files) columns, so I want to match only some of them, without writing something like case Array(item1, item2 , ..., itemN) => List(item1, item2, itemK, itemN)

Thank you!

Community · Accepted Answer · 2017-05-23 11:50:47Z

3

You do not need to convert lists to Arrays, because lists are designed for pattern matching.

scala> myList match { 
  case item :: site :: buyer :: tail if tail.nonEmpty => 
    item :: site :: buyer :: List(tail.last)
}
res3: List[List[Int]] = List(List(3, 1, 2, 3, 4), List(23, 45, 6, 7, 2), 
  List(3, 3, 2, 1, 5, 34, 43, 2), List(3, 2, 45, 56))

Or even more concise solution suggested by Kolmar

scala> myList match { 
  case item :: site :: buyer :: (_ :+ date) => List(item, site, buyer, date) 
}

edited May 23, 2017 at 11:50

CommunityBot

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answered Apr 29, 2015 at 15:44

Nikita

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3 Comments

Michael Zajac Over a year ago

You'll want to add an if(tail.nonEmpty), otherwise this will match List(1, 2, 3), but throw a NoSuchElementException when you call tail.last.

Kolmar Over a year ago

Or just use myList match { case item :: site :: buyer :: (_ :+ date) => List(item, site, buyer, date) }

Nikita Over a year ago

@Kolmar Wow, that's even better then my original suggestion.

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