F#:编写函数以递归方式建立元组列表，语法错误 [英] F# : Writing a function that builds a list of tuples recursively, a syntax error

问题描述

我正在尝试编写一个返回List<的递归函数. int * int>，但是我在语法上遇到了麻烦.

I'm trying to write a recursive function that return a List < int * int>, but I'm having some trouble with the syntax.

该函数应在递归结束时返回一个空列表，否则元组(int * int)与递归调用返回的List合并:

The function should return an empty list when the recursione ends, otherwise a tuple (int * int) merged with a List returned by a recursive call to itself:

let rec foobar () : List<int * int> =
    if (recursionIsEnded) then
        []
    else 
        foobar () :: (10, 10) // this is wrong
        // (10,10) works, but I need to concatenate it with the elements returned by foobar recursive call

有人可以向我解释我在做什么错吗?

Could someone explain to me what I'm doing wrong?

我将尝试提供更多细节. 其实我的功能有点复杂.我正在遍历2d数组，并使用满足一定条件的数组索引元素构建元组列表.其实那是我的代码:

I'll try to give more details . Actually my function is a bit more complicated. I'm iterating through a 2d array and build a list of tuples with the array index elements that satisfy a certain condition. Actually that's my code:

let rec GetSameColorNeighs(grid : Option<Ball> [,], row : int , col : int, color : Microsoft.Xna.Framework.Color) : List<int * int> =
    if (row < 0 || col < 0 || row > MaxLineNumber - 1 || col > BallsPerLine - 1) then
        []
    else
        let ball = grid.[row,col]
        match ball with 
            |Some(ball) -> 

                if (!ball.visited = false || not <| ball.color.Equals(color)) then
                    [row , col]
                else
                     ball.visited := true
                     (row,col) ::GetSameColorNeighs(grid, row + 1, col + 1, color) ::  GetSameColorNeighs(grid, row - 1, col - 1, color) 

            |None  -> []

所以这里还有2个问题:):

So here's 2 more question :):

1. 如何修改下一行以使其编译?

1. How modify the following row in order to make it compile?

(row,col) ::GetSameColorNeighs(grid, row + 1, col + 1, color) ::  GetSameColorNeighs(grid, row - 1, col - 1, color) 

还有更好的方法吗?

Is there any better way to do that?

我不在乎列表的元素顺序.

I don't care about the list's elements order.

推荐答案

将内部函数与累加器一起使用可能是最简单的方法(此外，它具有尾部递归功能).

Using an inner function with an accumulator is probably the easiest way to do it (as a bonus it's tail recursive).

let foobar() =
  let rec foobar acc =
    if (recursionIsEnded) then
        List.rev acc
    else 
        foobar ((10, 10)::acc)
  foobar []

根据您的更新，我希望使用一种可变的解决方案.像

Based on your update, I'd prefer a mutable solution. Something like

let GetSameColorNeighs(grid : Option<Ball> [,], row : int , col : int, color : Microsoft.Xna.Framework.Color) : List<int * int> =
    let results = ResizeArray()
    let rec loop (row, col) =
      if (row < 0 || col < 0 || row > MaxLineNumber - 1 || col > BallsPerLine - 1) then ()
      else
          let ball = grid.[row,col]
          match ball with 
              |Some(ball) -> 

                  if (!ball.visited = false || not <| ball.color.Equals(color)) then
                      [row , col] //doesn't compile, not sure what to do here
                  else
                       ball.visited := true
                       results.Add(row, col)
                       loop(row + 1, col + 1) 
                       loop(row - 1, col - 1)

              |None  -> ()
    loop(row, col)
    results |> Seq.toList

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