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I have list of Integer like this:

val aRowcol: List[List[Int]]] = List(List(0, 0), List(0, 1), List(0, 2)), List(List(1, 0), List(1, 1), List(1, 2)), List(List(2, 0), List(2, 1), List(2, 2)), List(List(0, 0), List(1, 1), List(2, 2)), List(List(2, 0), List(1, 1), List(0, 2)), List(List(1, 0), List(0, 1), List(0, 2)), List(List(1, 0), List(2, 1), List(2, 2)) val aAlpha: List[List[String]] = List( List("a","b","c","d"), List("e","f","g","h"), List("i","j","k","l","m")) val i = 4 val resNum:List[List[Int,String]] = (0 to i) { _map => List( aRowcol.take(i).head.head, aRowcol.take(i).head(1), aAlpha(aRowcol.take(i).head.head)(aRowcol.take(i).head(1))} .toList

But the result I want for val resNum is:

List( List(0,0,"a"), List(1,0,"e"), List(2,0,"i"), List(0,0,"a"), List(2,0,"i"))

(0,0) means first row first column, we have "a" on that possition, so i will define how many aAlpha we will have. I think it will be much easier if we do i++, but you know that we couldn't do i++ in scala.

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    can you better clarify what you're trying to achieve? I can't figure out the logic here. Plus - list.take(i).head is equivalent to list.head, because take(n) returns the first n items and head returns the first - so "first of first n" is the same as just "first" Commented Oct 31, 2016 at 16:03
  • 3
    And should aRowcol be List[List[List[Int]]]? Your sample code doesn't compile... Commented Oct 31, 2016 at 16:09
  • I don't see aNumber list in the code sample Commented Oct 31, 2016 at 16:12
  • Please explain your use case and what are you trying to do here ? Commented Oct 31, 2016 at 16:13

1 Answer 1

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I'm guessing that you want to treat the first element in each "list of lists" in aRowcol as the "coordinates" of a letter in aAlpha, and want to append that letter to each of these "first elements".

If so:

val result: List[List[Any]] = aRowcol.take(5) // only 5 first rows .map(_.head) // first List(i, j) only, the rest is ignored .map { case List(i, j) => List(i, j, aAlpha(i)(j)) } // append the right letter to list result.foreach(println) // List(0, 0, a) // List(1, 0, e) // List(2, 0, i) // List(0, 0, a) // List(2, 0, i)

If that's not what you meant - please clarify.

EDIT: as for your version - it can work (and achieve the same goal) with a few fixes:

  • list.take(i) doesn't return the i-th element, it returns a list with the first i elements, I think you're trying to use list.apply(i) which returns the i-th element, or it's shorthand version: list(i)
  • If you want to map the numbers 0..4 - call map and then name the argument of the anonymous function you pass i - don't use a var declared outside of the method and expect it to increment

With these corrections (and some more), your version becomes:

val resNum: List[List[Any]] = (0 to 4).map { i => List( aRowcol(i).head.head, aRowcol(i).head(1), aAlpha(aRowcol(i).head.head)(aRowcol(i).head(1))) } .toList

Which works as you expect; But above is a similar yet simpler version.

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5 Comments

Thanks @Tzach let me try.
do you think the i will return if we use for (i <- 1 to 4) ??
Also take 5 means the list number (0 to 4) isn't it?
Not sure I understand the question... Why would you need it? It's much easier (and more efficient!) to map existing collections (i.e. calling map on the first 5 items in aRowcol) then creating these index variables. aRowcol.take(5) does not create any index or list of numbers, it simply creates a sublist of the original one, and mapping over it gives you what you need.
For sure your answer is the most efficient way. I just want to know if for also usable for this case. Now I understand why we didn't use for in many scala case.

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