foreach 不能对变量进行操作，不能获取枚举数，链表
foreach cannot operate on variables, cannot get enumerator, linkedlists
我有一个简单的程序，我们在我的编程课程中忙于单向链表，我正在尝试计算不同类型对象的数量，我有一个 class 单向链表，App , Onceoff : App, Distrubuted : App, Cell
我想要实现的是获取名为 Developed 的单链表中每个项目的对象类型，并尝试使用 "is" 语句确定它是哪种类型并增加所需的计数器，但是 foreach带有下划线的是以下错误
"foreach cannot operate on variables of type 'PRr05.SinglyLinkedList' because 'PRr05.SinglyLinkedList' does not contain a public definition for 'GetEnumarator'"，我不太确定这是什么意思，但如果我是正确的，Enumarator returns 类型，我该如何解决这个问题，SinglyLinkedList class 下面
(ps, 是的，这很实用，但我更进一步 ;) )
FOREACH 方法
public int countApps(char Type)
{
int countO = 0, countD = 0, countR = 0;
foreach (App item in Developed)
{
if (item is Onceoff)
{
countO++;
}
else
if (item is Distributed)
{
countD++;
}
else countR++;
}
if (Type == 'O')
{
return countO;
}
if (Type == 'D')
{
return countD;
}
if (Type == 'R')
{
return countR;
}
}
单链表Class
使用系统；
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace Pr05
{
public class SinglyLinkedList // NO CHANGES PERMITTED TO EXISTING METHODS IN THIS CLASS
{
// Sentinel cell
private int counter;
private Cell head; // pointer to first cell in the list
public SinglyLinkedList()
{
counter = 0;
head = null;
}
System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() { get {return Current;}}
object System.Collections.IEnumerator.Current { get { return Current; } }
public int Count()
{
return counter;
}
public Cell getFirst()
{
return head;
}
public void addFirst(Object newItem) // Section Adding Cells at the Beginning pp 59 - 60
/* pre: Have object to be added to calling singly linked list object, which may be empty.
* post: newItem is the element of the FIRST cell of the singly linked list. All other existing cells of the
* singly linked list retain their ordering AFTER the new first cell.
* The counter is modified to reflect the addition of a new cell to the singly linked list. */
{
// ADD CODE FOR addFirst HERE
Cell newCell = new Cell(newItem);
newCell.setNext(head);
head = newCell;
counter++;
}
public void addLast(Object newItem) // Section Adding Cells at the End pp 60 - 61
/* pre: Have object to be added to calling singly linked list object, which may be empty.
* post: newItem is the element of the LAST cell of the singly linked list. All other existing cells of the
* singly linked list retain their ordering BEFORE the new last cell.
* The counter is modified to reflect the addition of a new cell to the singly linked list.
* CAREFUL: C# has certain restrictions which do not allow direct implemention of the code as specified in the
* prescribed text. Find a way around the restriction. */
{
// ADD CODE FOR addLast HERE
if (this.Count() == 0)
{
this.addFirst(newItem);
return;
}
Cell newCell = new Cell(newItem);
Cell cur = head;
while (cur.next() != null)
cur = cur.next();
cur.setNext(newCell);
counter++;
}
public Cell removeFirst()
/* pre: Have at least one cell in calling singly linked list object.
* post: Return the cell removed, which is the first cell in the list.
* The counter is modified to reflect the removal of the first cell from the singly linked list. */
{
// ADD CODE FOR removeFirst HERE
Cell cur = head;
head = cur.next();
cur.setNext(null);
counter--;
return cur;
}
public Cell removeLast()
/* pre: Have at least one cell in calling singly linked list object.
* post: Return the cell removed, which is the last cell in the list.
* The counter is modified to reflect the removal of the last cell from the singly linked list.
* CAREFUL: C# has certain restrictions - find a way around the restriction. */
{
// ADD CODE FOR removeLast HERE
Cell cur;
if (this.Count() == 1)
{
cur = head;
this.Clear();
return cur;
}
cur = head;
Cell prev = head;
while (cur.next() != null)
{
prev = cur;
cur = cur.next();
}
prev.setNext(null);
counter--;
return cur;
}
public void addBefore(Object newItem, Cell link)
/* pre: Have object to be added to calling singly linked list object, and a link in the singly linked list BEFORE
* which the newItem's cell must be added.
* post: newItem is the element of the added cell of the singly linked list. All other existing cells of the
* singly linked list retain their ordering relevant to the position of the newly added cell.
* The counter is modified to reflect the addition of a new cell to the singly linked list. */
{
// ADD CODE FOR addBefore HERE
if (link == null) // list either empty or must be added at end of list
{
this.addLast(newItem);
return;
}
Cell newCell = new Cell(newItem);
Cell cur = head;
if (cur == link) // must be added as first cell
{
this.addFirst(newItem);
return;
}
while (cur.next() != link)
cur = cur.next();
cur.setNext(newCell);
newCell.setNext(link);
counter++;
}
public void addAfter(Object newItem, Cell link) // Section Inserting Cells After Other Cells pp 61 - 62
/* pre: Have object to be added to calling singly linked list object, and a link in the singly linked list AFTER
* which the newItem's cell must be added.
* post: newItem is the element of the added cell of the singly linked list. All other existing cells of the
* singly linked list retain their ordering relevant to the position of the newly added cell.
* The counter is modified to reflect the addition of a new cell to the singly linked list. */
{
// ADD CODE FOR addAfter HERE
if (link == null)
{
this.addLast(newItem);
return;
}
Cell newCell = new Cell(newItem);
newCell.setNext(link.next());
link.setNext(newCell);
counter++;
}
public void Clear()
/* pre: Have calling singly linked list object, which could be empty.
* post: EFFICIENTLY clear the singly linked list. */
{
// ADD CODE FOR Clear HERE
counter = 0;
head = null;
}
// OPTIONAL TASKS
public Cell removeBefore(Cell link)
/* pre: Have at least one cell in calling singly linked list object. Have a link in the singly linked list.
* post: Return the cell removed, which is the cell BEFORE the given link.
* The counter is modified to reflect the removal of the cell from the singly linked list. */
{
// ADD CODE FOR removeBefore HERE
if (link == head) // nothing to remove in front of link
return null;
if (link == null) // then remove last cell
return removeLast();
Cell cur = head;
Cell prev = head;
while (cur.next() != link)
{
prev = cur;
cur = cur.next();
}
if (cur == head)
return removeFirst();
prev.setNext(link);
cur.setNext(null);
counter--;
return cur;
}
public Cell removeAfter(Cell link) // Section Deleting Cells pp 62 - 63
/* pre: Have at least one cell in calling singly linked list object. Have a link in the singly linked list.
* post: Return the cell removed, which is the cell AFTER the given link.
* The counter is modified to reflect the removal of the cell from the singly linked list. */
{
// ADD CODE FOR removeAfter HERE
if (link == null) // nothing after
return null;
if (link.next() == null) // then nothing after link
return null;
Cell cur = link.next();
link.setNext(cur.next());
cur.setNext(null);
counter--;
return cur;
}
}
}
您需要对代码的语法和概念进行一些必要的根本性更改。例如，首先您应该知道 foreach 仅用于实现 System.Collections.IEnumberable 或 System.Collections.Generic.IEnumberable<T> 的对象。这里有一个转折点；如果你想遍历 Developed class 的（可能）成员，你应该使用反射；如果你想促进你的 Developed class 与 foreach 语句一起使用，你必须从前两个 BCL classes 中的一个派生提及。请考虑修改您的代码。
我有一个简单的程序，我们在我的编程课程中忙于单向链表，我正在尝试计算不同类型对象的数量，我有一个 class 单向链表，App , Onceoff : App, Distrubuted : App, Cell
我想要实现的是获取名为 Developed 的单链表中每个项目的对象类型，并尝试使用 "is" 语句确定它是哪种类型并增加所需的计数器，但是 foreach带有下划线的是以下错误 "foreach cannot operate on variables of type 'PRr05.SinglyLinkedList' because 'PRr05.SinglyLinkedList' does not contain a public definition for 'GetEnumarator'"，我不太确定这是什么意思，但如果我是正确的，Enumarator returns 类型，我该如何解决这个问题，SinglyLinkedList class 下面
(ps, 是的，这很实用，但我更进一步 ;) )
FOREACH 方法
public int countApps(char Type)
{
int countO = 0, countD = 0, countR = 0;
foreach (App item in Developed)
{
if (item is Onceoff)
{
countO++;
}
else
if (item is Distributed)
{
countD++;
}
else countR++;
}
if (Type == 'O')
{
return countO;
}
if (Type == 'D')
{
return countD;
}
if (Type == 'R')
{
return countR;
}
}
单链表Class
使用系统；
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace Pr05
{
public class SinglyLinkedList // NO CHANGES PERMITTED TO EXISTING METHODS IN THIS CLASS
{
// Sentinel cell
private int counter;
private Cell head; // pointer to first cell in the list
public SinglyLinkedList()
{
counter = 0;
head = null;
}
System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() { get {return Current;}}
object System.Collections.IEnumerator.Current { get { return Current; } }
public int Count()
{
return counter;
}
public Cell getFirst()
{
return head;
}
public void addFirst(Object newItem) // Section Adding Cells at the Beginning pp 59 - 60
/* pre: Have object to be added to calling singly linked list object, which may be empty.
* post: newItem is the element of the FIRST cell of the singly linked list. All other existing cells of the
* singly linked list retain their ordering AFTER the new first cell.
* The counter is modified to reflect the addition of a new cell to the singly linked list. */
{
// ADD CODE FOR addFirst HERE
Cell newCell = new Cell(newItem);
newCell.setNext(head);
head = newCell;
counter++;
}
public void addLast(Object newItem) // Section Adding Cells at the End pp 60 - 61
/* pre: Have object to be added to calling singly linked list object, which may be empty.
* post: newItem is the element of the LAST cell of the singly linked list. All other existing cells of the
* singly linked list retain their ordering BEFORE the new last cell.
* The counter is modified to reflect the addition of a new cell to the singly linked list.
* CAREFUL: C# has certain restrictions which do not allow direct implemention of the code as specified in the
* prescribed text. Find a way around the restriction. */
{
// ADD CODE FOR addLast HERE
if (this.Count() == 0)
{
this.addFirst(newItem);
return;
}
Cell newCell = new Cell(newItem);
Cell cur = head;
while (cur.next() != null)
cur = cur.next();
cur.setNext(newCell);
counter++;
}
public Cell removeFirst()
/* pre: Have at least one cell in calling singly linked list object.
* post: Return the cell removed, which is the first cell in the list.
* The counter is modified to reflect the removal of the first cell from the singly linked list. */
{
// ADD CODE FOR removeFirst HERE
Cell cur = head;
head = cur.next();
cur.setNext(null);
counter--;
return cur;
}
public Cell removeLast()
/* pre: Have at least one cell in calling singly linked list object.
* post: Return the cell removed, which is the last cell in the list.
* The counter is modified to reflect the removal of the last cell from the singly linked list.
* CAREFUL: C# has certain restrictions - find a way around the restriction. */
{
// ADD CODE FOR removeLast HERE
Cell cur;
if (this.Count() == 1)
{
cur = head;
this.Clear();
return cur;
}
cur = head;
Cell prev = head;
while (cur.next() != null)
{
prev = cur;
cur = cur.next();
}
prev.setNext(null);
counter--;
return cur;
}
public void addBefore(Object newItem, Cell link)
/* pre: Have object to be added to calling singly linked list object, and a link in the singly linked list BEFORE
* which the newItem's cell must be added.
* post: newItem is the element of the added cell of the singly linked list. All other existing cells of the
* singly linked list retain their ordering relevant to the position of the newly added cell.
* The counter is modified to reflect the addition of a new cell to the singly linked list. */
{
// ADD CODE FOR addBefore HERE
if (link == null) // list either empty or must be added at end of list
{
this.addLast(newItem);
return;
}
Cell newCell = new Cell(newItem);
Cell cur = head;
if (cur == link) // must be added as first cell
{
this.addFirst(newItem);
return;
}
while (cur.next() != link)
cur = cur.next();
cur.setNext(newCell);
newCell.setNext(link);
counter++;
}
public void addAfter(Object newItem, Cell link) // Section Inserting Cells After Other Cells pp 61 - 62
/* pre: Have object to be added to calling singly linked list object, and a link in the singly linked list AFTER
* which the newItem's cell must be added.
* post: newItem is the element of the added cell of the singly linked list. All other existing cells of the
* singly linked list retain their ordering relevant to the position of the newly added cell.
* The counter is modified to reflect the addition of a new cell to the singly linked list. */
{
// ADD CODE FOR addAfter HERE
if (link == null)
{
this.addLast(newItem);
return;
}
Cell newCell = new Cell(newItem);
newCell.setNext(link.next());
link.setNext(newCell);
counter++;
}
public void Clear()
/* pre: Have calling singly linked list object, which could be empty.
* post: EFFICIENTLY clear the singly linked list. */
{
// ADD CODE FOR Clear HERE
counter = 0;
head = null;
}
// OPTIONAL TASKS
public Cell removeBefore(Cell link)
/* pre: Have at least one cell in calling singly linked list object. Have a link in the singly linked list.
* post: Return the cell removed, which is the cell BEFORE the given link.
* The counter is modified to reflect the removal of the cell from the singly linked list. */
{
// ADD CODE FOR removeBefore HERE
if (link == head) // nothing to remove in front of link
return null;
if (link == null) // then remove last cell
return removeLast();
Cell cur = head;
Cell prev = head;
while (cur.next() != link)
{
prev = cur;
cur = cur.next();
}
if (cur == head)
return removeFirst();
prev.setNext(link);
cur.setNext(null);
counter--;
return cur;
}
public Cell removeAfter(Cell link) // Section Deleting Cells pp 62 - 63
/* pre: Have at least one cell in calling singly linked list object. Have a link in the singly linked list.
* post: Return the cell removed, which is the cell AFTER the given link.
* The counter is modified to reflect the removal of the cell from the singly linked list. */
{
// ADD CODE FOR removeAfter HERE
if (link == null) // nothing after
return null;
if (link.next() == null) // then nothing after link
return null;
Cell cur = link.next();
link.setNext(cur.next());
cur.setNext(null);
counter--;
return cur;
}
}
}
您需要对代码的语法和概念进行一些必要的根本性更改。例如，首先您应该知道 foreach 仅用于实现 System.Collections.IEnumberable 或 System.Collections.Generic.IEnumberable<T> 的对象。这里有一个转折点；如果你想遍历 Developed class 的（可能）成员，你应该使用反射；如果你想促进你的 Developed class 与 foreach 语句一起使用，你必须从前两个 BCL classes 中的一个派生提及。请考虑修改您的代码。