OCAML中使用FOLD的列表函数中按3个元素分组的说明 [英] Explanation for group by 3 elements from a list function using FOLD in OCAML

问题描述

我有一段代码可以执行以下操作:将n个元素的列表中的3个元素分组.主要功能称为group_by_3.例如，执行group_by_3 [1;2;3;4;5;6;7]将给我([1;2;3],[4;5;6],[7]).

I have a piece of code that does the following: group 3 elements of a list of n elements. The main function is called group_by_3. For example, executing group_by_3 [1;2;3;4;5;6;7] will give me ([1;2;3],[4;5;6],[7]).

let group_by_3 lst =
  let accum = ( [], [], 0 )
  in
  let f (all_groups, current_group, size) x =
    if size = 3
    then ( (List.rev current_group) :: all_groups, 
           [x], 1 )
    else ( all_groups, x::current_group, size+1)
  in
  let (groups, last, _) = List.fold_left f accum lst
  in List.rev ( List.rev last :: groups)

我不太明白为什么这样做(在课堂上提供).

I don't really understand why this works (it is provided in class).

• 什么是all_groups，current_group，大小?

• 这是做什么用的?

• What are all_groups, current_group, size?

• What does this do?

if size = 3
then ( (List.rev current_group) :: all_groups, 
       [x], 1 )
else ( all_groups, x::current_group, size+1)

谢谢！

推荐答案

什么是all_groups，current_group，size?

这是通过访问输入序列中的每个项目来产生分组所需的三个状态.这些单独的状态组合成3个元组，以形成折叠的单个状态.最终，我们只关心all_groups，其他两个只是构造它的必要中间状态.

These are the three pieces of state required to produce the groupings by visiting each item in the input sequence. These individual states are combined in a 3-tuple to form a single state for the fold. Ultimately we'll only care about all_groups, and the other two are just intermediate state necessary to construct that.

• all_groups:这是增加完成的分组的列表值.每当我们看到足以满足组大小的新元素时，我们就会创建一个新组并将其添加到此列表中.
• current_group:这也是一个列表值，但更多的是用于建立分组直到到达size的临时缓冲区.当它足够大时，它将被添加到all_groups，并使用当前项目[x]重置为新的组/列表.
• size这只是一个计数器，用于跟踪current_group中有多少个项目.

• all_groups: This is a list value that accretes completed groupings. Whenever we've seen enough new elements to satisfy the group size, we make a new group and add it to this list.
• current_group: This is also a list value, but more of a temporary buffer to build up a grouping until it reaches size. When it's big enough, it gets added to all_groups and is reset to a new group/list with the current item [x].
• size this is just a counter to track how many items are in current_group.

这是做什么的?

What does this do?

if size = 3只是确定我们是要继续积累元素还是要有足够的分组能力.

if size = 3 simply decides whether we want to keep accumulating elements or if we've got enough for a grouping.

then ( (List.rev current_group) :: all_groups, [x], 1 )正在建立/返回新的累加器值，该值是all_groups，current_group和size的三元组.由于 else 分支中列表的生成方式，因此必须使用List.rev调用；将项目添加到列表的 front 最快，但这是输入序列的 reverse ，因此我们将它们反向. x是当前项目，它将是新的，正在增长的组中的第一项. 1当然是该新组的大小.

then ( (List.rev current_group) :: all_groups, [x], 1 ) is building/returning the new accumulator value, which is a 3-tuple of all_groups, current_group, and size. The List.rev call is necessary because of the way the list is being grown, in the else branch; it's fastest to add items to the front of a list, but this is the reverse of the input sequence thus we reverse them. x is the current item, which will be the first item in the new, growing group. 1 is of course the size of that new group.

else ( all_groups, x::current_group, size+1)将当前项目x弹出到current_group列表的前面，并增加size计数器.

else ( all_groups, x::current_group, size+1) is popping the current item x onto the front of the current_group list and incrementing the size counter.

该部分下面的逻辑是处理所有不能整齐地归为三类的散乱项目.

Below that section is the logic that takes care of any straggler items that don't fit neatly into groupings of three.

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