Question

我有这个问题需要解决

我已经设法解决了它但没有该函数execute_ops。这是我的解决方案：

datatype list_op = insert of int*int | delete of int | setsize of int;

exception wrong_case;

fun insert(x,pos,nil) = [x]
  | insert(x,pos,h::t) = if (pos=h) then x::h::t else h::insert(x,pos,t);

fun setsize(0) = nil
  | setsize(1) = [0]
  | setsize(n) = 0 :: setsize(n-1);

fun delete (l,0) = l
  | delete (nil,_) = raise wrong_case
  | delete (h::t,n) = if n = h then h::t else delete(t,n-1);

insert(3, 4, delete(setsize(5), 3));

首先我不确定我的解决方案是否正确，因为我不明白setize是什么。

如何以正确的方式编写代码？

谢谢

Answer 1

因此，对于这三个操作中的每一个，您似乎已经定义了 helper 函数，这些函数执行操作背后的逻辑。除了修复这个逻辑以便工作之外，剩下的就是将这些函数拼凑在一起形成函数execute_ops。

如果这些值同时是数据类型的值构造函数，则不得将函数命名为insert，delete和setsize。简单地说，一旦定义了数据类型，然后定义了同一范围内的函数，函数就会遮蔽值构造函数，并使您无法表达 list_op 类型的值。

重要的，我怀疑令人困惑的一点是，insert，delete和setsize不是在列表上执行操作的函数。它们是值构造函数，很像leaf和node用于创建二叉树：
```
datatype tree = leaf | node of tree*int*tree
```
比较pos=h中的insert没有意义。列表中的元素与其位置不同。您希望在位置 pos处插入x，而不是在等于 pos的第一个元素处插入x。值不一定等于它在列表中的位置。
遗憾的是delete完全没有意义;您的代码所说的是＆＃34;如果数字n等于列表中的元素，则返回整个列表。否则，删除元素并继续删除元素，直到这种情况，或者列表为空。＆＃34;

根据定义，也不需要在delete中提出异常。
我不明白你不懂什么是什么意思。它应该是一个设置列表大小的操作。重复分配文本，如果大小小于当前长度，则删除多余的元素，否则用零扩展列表的末尾。

以下是解决作业的模板：

datatype list_op = insert of int*int | delete of int | setsize of int

fun insert_h (0, elem, x::xs) = ...   (* insert elem instead of x *)
  | insert_h (pos, elem, []) = ...    (* list was too short *)
  | insert_h (pos, elem, x::xs) = ... (* not there yet *)

fun setsize_h (0, xs) = ...           (* wonder what list has size 0 *)
  | setsize_h (n, []) = ...           (* extend list n times more *)
  | setsize_h (n, x::xs) = ...        (* not there yet *)

fun delete_h (0, x::xs) = ...         (* delete element at this position *)
  | delete_h (n, []) = ...            (* list was too short *)
  | delete_h (n, x::xs) = ...         (* not there yet *)

fun execute_ops [] xs = ...           (* no more operations to execute *)
  | execute_ops (list_op::list_ops) xs =
    let val new_xs = (case list_op of
                          insert (pos, elem) => insert_h (pos, elem, xs)
                        | delete pos         => delete_h (pos, xs)
                        | setsize size       => setsize_h (size, xs))
    in execute_ops list_ops new_xs
    end

您可能希望使用execute_ops单独或组合测试这些功能：

val test_insert_h_1 = insert_h (2, 7, [1,2,3,4]) = [1,2,7,4]
val test_insert_h_2 = insert_h (9, 5, [1,2,3,4]) = [1,2,3,4,5]

val test_setsize_h_1 = setsize_h (2, [5,6,7,8]) = [5,6]
val test_setsize_h_2 = setsize_h (5, [1,2,3]) = [1,2,3,0,0]

val test_delete_h_1 = delete_h (3, [4,5,6,7,8]) = [4,5,6,8]
val test_delete_h_2 = delete_h (9, [1,2,3]) = [1,2,3]

val test_execute_ops_1 =
    execute_ops [insert (0, 5), insert (1, 6), insert (2, 7)] [2,3,5] = [5,6,7]

val test_execute_ops_2 =
    execute_ops [setsize 5, insert (4, 9)] [] = [0,0,0,0,9]

val test_execute_ops_3 =
    execute_ops [setsize 6, insert (1, 5), delete 3] [8,8,8,8,9] = [8,5,8,9,0]

列表操作的数据类型

1 个答案: