在基于数组的列表的程序中,我有一个调用以下 3 次的函数:
list->remove(1)
我的问题是,当它打印出来时,它说它删除了原来的第一项 3 次,而不是转到下一个新的第一项。
具体来说,在test.cpp
中调用这段代码时:
for (int i = 1; i <= 3 && ! list->isEmpty(); i++) {
cout << "Deleting item " << i << "." << endl;
try {
cout << "Deleted item " << list->remove(1) << endl;
}
catch (int e) {
cout << "*** Error deleting from position " << i << " ***" << endl;
}
}
它返回以下内容:
Deleting item 1.
Deleted item 1
Deleting item 2.
Deleted item 1
Deleting item 3.
Deleted item 1
这是我创建的 ABList
类。我认为错误出在我的 remove
方法中,但我不知道在哪里:
#ifndef _ABLIST_H_
#define _ABLIST_H_
#define LIST_MAX 10
#include <iostream>
#include "ABCList.hpp"
using namespace std;
template <class T>
class ABList : public ABCList<T> {
private:
T a[LIST_MAX];
int size;
public:
ABList ();
~ABList ();
virtual bool isEmpty ();
virtual int getLength ();
virtual void insert (int pos, T item);
virtual T remove (int pos);
virtual T retrieve (int pos);
};
template <class T>
ABList<T>::ABList () {
size = 0;
}
template <class T>
bool ABList<T>::isEmpty () {
if (size == 0)
return true;
return false;
}
// returns size, which is updated whenever the list is lengthened or shortened
template <class T>
int ABList<T>::getLength() {
return size;
}
template <class T>
void ABList<T>::insert(int pos, T item) {
try {
// if list is at maximum size
if (size >= LIST_MAX)
throw "Size is greater than or equal to LIST_MAX\n";
// if "pos" is a negative number
if (pos < 0)
throw "Pos must be greater than 0\n";
// if "pos" is outside the bounds of the existing list
if (pos > size + 1)
throw "Pos must be less than or equal to list size\n";
//shift all items at indices > pos one index up
for (int i = size; i >= pos; --i) {
a[i] = a[i-1];
}
//insert new item
a[pos-1] = item;
//increment size
size++;
} catch (char* message) {
cout << "Error: " << message;
throw 1; // any int will do, to catch exception flag in test.cpp
}
}
template <class T>
T ABList<T>::remove(int pos) {
try {
if (pos < 1)
throw "Pos cannot be less than 1";
if (pos > size)
throw "Pos cannot be greater than size";
//find T to be removed, to return later
T temp = retrieve(pos);
//shift all items greater than pos down one index
for (int i = pos + 1; i <= size; i++)
a[i] = a[i+1];
// decrement size
size--;
return temp;
} catch (char* message) {
cout << "Error: " << message;
throw 1;
}
}
template <class T>
T ABList<T>::retrieve(int pos) {
try {
//check that pos is valid
if (pos < 1 || pos > size)
throw "Position is outside bounds of ABList\n";
return a[pos-1];
} catch (char* message) {
cout << "Error: " << message;
}
}
#endif
这是主程序。 (为了这个问题,我不得不假设它是不可变的):
int main () {
// Testing the List implmenetations; first, get a list:
ABCList<string> * list = getList();
// Test "insert"
cout << "Let's populate the list with a few items." << endl;
for (int i = 1; i <= TEST_SIZE; i++) {
int pos = getPos(i); // Randomise "i" to test
string input;
cout << "Enter a word to place into the list at position " << pos << ": ";
cin >> input;
try {
list->insert(pos, input);
cout << "Successfully inserted at position " << pos << "." << endl;
}
catch (int e) {
cout << "*** Error inserting at position " << pos << " ***" << endl;
}
}
// Test "retrieve" & "getLength"
cout << "List is as follows: \n\t";
for (int i = 1; i <= list->getLength(); i++) {
cout << list->retrieve(i) << " ";
}
cout << endl;
// Test "delete" and "isEmpty"
for (int i = 1; i <= 3 && ! list->isEmpty(); i++) {
cout << "Deleting item " << i << "." << endl;
try {
cout << "Deleted item " << list->remove(1) << endl;
}
catch (int e) {
cout << "*** Error deleting from position " << i << " ***" << endl;
}
}
// Done. Let the destructor handle the rest.
delete list;
return 0;
}
最佳答案
在移除过程中,您在获取位于 (1)
的项目(实际上是位于 a[0]
的项目)之后执行此操作:
for (int i = pos + 1; i <= size; i++)
a[i] = a[i+1];
现在输入这个时 pos
是什么?它是 (1)
,因此您正在将列表从 (2)
移动到 size
,将所有内容从 3..( size+1)
到 2...(size)
。 “位置”1 处的项目(同样,实际上位于数组中的插槽 0
中)永远不会被覆盖,因此它始终是报告的返回项目。
这不仅是不正确的,而且当数组完全填充时,它也是 UB(未定义行为),您将访问最后一个可行位置后一个插槽的数据。
您想在 pos
处扔掉元素,这是从 1 开始的(您的设计;不是我的)。它的底层元素位于数组中的 a[pos-1]
位置。所以试试这个:
for (int i = pos; i < size; ++i)
a[i-1] = a[i];
如果您检查以确保 pos
始终为 1
或更大,这应该是安全的(看起来您确实如此,事实上,确实如此)这)。就我个人而言,我会坚持使用从零开始的索引系统,但我敢肯定,您有自己的设计原因。
关于c++ - 删除基于数组的列表中的项目,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/14025045/