c++ - 试图弄清楚我需要做哪些更改才能使此代码正常工作。使用 Visual Studio

Question

#include <iostream>
using namespace std;
void getMaximumPositive(int* arr, int size, int* max)
{
    max = nullptr;      //Set default value of max pointer
    for (int i = 0; i < size; i++)
    {
        if (arr[i] > 0)     //Only positive numbers are considered while locating the maximum
        {

            if (max == nullptr)
                max = &arr[i];
            else if (arr[i] > * max)
                max = &arr[i];
        }
    }
}

void getMaximumNegative(int* arr, int size, int* max)
{
    max = nullptr;      //Set default value of max pointer
    for (int i = 0; i < size; i++)
    {
        if (arr[i] < 0)     //Only negative numbers are considered while locating the maximum
        {

            if (max == nullptr)
                max = &arr[i];
            else if (arr[i] > * max)
                max = &arr[i];
        }
    }
}

void printArrayForwards(int* arr, int size)
{
    cout << "Array contents (Forwards): " << endl;
    for (int i = 0; i < size; i++)
        cout << arr[i] << " : ";
    cout << endl;
}

void printArrayBackwards(int* arr, int size)
{
    cout << "Array contents (Backwards): " << endl;
    for (int i = 0; i < size; i++)
        cout << *(arr - i - 1) << " : ";
    cout << endl;
}

void swapMaxtoFront(int* arr, int* max_address)
{
    if (max_address == nullptr)
        return;                 //Do nothing if max_address is null

    //Print the addresses and their corresponding values of the first element and the maximum element of the array
    cout << endl << "Swapping elements:" << endl;
    cout << "Address 1: " << arr << " Value: " << *arr << endl;
    cout << "Address 2: " << max_address << " Value: " << *max_address << endl << endl;

    //Code for the swap
    int temp = *arr;
    *arr = *max_address;
    *max_address = temp;

    max_address = arr;
}

void PrintArray(int* arr, int size)
{
    printArrayForwards(arr, size);
    printArrayBackwards(arr, size);
}

int main()
{
    //Initialize pointers to null
    int* array = nullptr;       //Do Not Change This Variable Definition 
    int* max = nullptr;         //Do Not Change This Variable Definition

    int arr_size;

    cout << "Enter the size of your array: ";
    cin >> arr_size;
    array = new int[arr_size];          //Reserve memory for array of size given by user
    cout << "Enter array elements: " << endl;
    for (int i = 0; i < arr_size; i++)
    {
        cout << "Element " << i + 1 << " of " << arr_size << " : ";
        cin >> *(array++);
    }

    PrintArray(array, arr_size);

    cout << endl << endl;
    cout << "-----------------------------------------------" << endl << "Finding maximum positive number in array..." << endl;

    getMaximumPositive(array, arr_size, max);   //Max should point to the Maximum positive value in the array

    swapMaxtoFront(array, max);         //Swap the maximum positive number with the first element of the array

    PrintArray(array, arr_size);        //Print array with swapped values.

    //Print maximum positive number
    cout << endl;
    if (max == nullptr)
        cout << "*******No positive numbers found in array" << endl;
    else
        cout << "*******Maximum positive number in array: " << *max << endl;        //Max should point to the Maximum positive value in the array, which should now be the first element
    cout << "-----------------------------------------------" << endl;


    cout << endl;
    cout << "-----------------------------------------------" << endl << "Finding maximum negative number in array..." << endl;

    getMaximumNegative(array, arr_size, max);   //Max should point to the Maximum negative value in the array

    swapMaxtoFront(array, max);         //Swap the maximum negative number with the first element of the array

    PrintArray(array, arr_size);        //Print array with swapped values.

    //Print maximum negative number
    cout << endl;
    if (max == nullptr)
        cout << "*******No negative numbers found in array" << endl;
    else
        cout << "*******Maximum negative number in array: " << *max << endl;        //Max should point to the Maximum negative value in the array, which should now be the first element
    cout << "-----------------------------------------------" << endl;

    delete[] array;
    delete max;
    array = nullptr;
    max = nullptr;
    return 0;
}

输出(我正在寻找的):

输入数组的大小:4

输入数组元素:

元素 1，共 4 个:2

元素 2，共 4 个:8

元素 3，共 4 个:4

元素 4，共 4:6

数组内容(转发):

2:8:4:6:

数组内容(向后):

6:4:8:2

---

查找数组中的最大正数...

交换元素:

地址 1:015E10A0 值:2

地址 2:015E10A4 值:8

数组内容(向前)

8:2:4:6:

数组内容(向后)

6:4:2:8:

*********数组中的最大正数:8

---

---

查找数组中的最大负数...

数组内容(转发):

8:2:4:6:

数组内容(向后):

6:4:2:8:

*******数组中没有找到负数

Answer 1

我注意到的最大问题是你对 array 做了什么:

    array = new int[arr_size];

    //...

    for(size_t i = 0; i < arr_size; i++) {
        cin >> *(array++);                 // here you step array
    }

    // ... and you use the final pointer "array" everywhere, undefined behaviour. It
    //     points one element outside of the area you allocated

    delete[] array; // the final nail in the coffin, 

修复:

    for(int i = 0; i < arr_size; i++) {
        cout << "Element " << i + 1 << " of " << arr_size << " : ";
        cin >> array[i];
    }

解决了这个问题，这就成了一个问题:

void printArrayBackwards(int* arr, int size) {
    cout << "Array contents (Backwards): " << endl;

    // arr points at the start, so arr - 0 - 1 points before start here:

    for(int i = 0; i < size; i++) cout << *(arr - i - 1) << " : ";
    cout << endl;
}

修复:

void printArrayBackwards(int* arr, int size) {
    cout << "Array contents (Backwards): " << endl;
    for(int i = size - 1; i >= 0; --i) cout << arr[i] << " : ";
    cout << endl;
}

另一件事是 getMaximumPositive 和 getMaximumNegative 函数。您不会返回指向 max 中最大值的指针。我将其更改为对 int* 的引用:

void getMaximumPositive(int* arr, int size, int*& max) {
    max = nullptr; // Set default value of max pointer
    for(int i = 0; i < size; i++) {
        if(arr[i] > 0)
        { // Only positive numbers are considered while locating the maximum
            if(max == nullptr)
                max = &arr[i];
            else if(arr[i] > *max)
                max = &arr[i];
        }
    }
}

void getMaximumNegative(int* arr, int size, int*& max) {
    max = nullptr; // Set default value of max pointer
    for(int i = 0; i < size; i++) {
        if(arr[i] < 0)
        { // Only negative numbers are considered while locating the maximum
            if(max == nullptr)
                max = &arr[i];
            else if(arr[i] > *max)
                max = &arr[i];
        }
    }
}

我还删除了你的 delete max。它是指向 array 中的 int 的指针，您可以使用 delete[] array 删除它。你不应该删除它。

另一个不通过参数返回值的函数在 swapMaxtoFront 中。我将其更改为:

void swapMaxtoFront(int* arr, int*& max_address) {
//...

Full code用
模拟你的输入std::istringstream cin{"4 2 8 4 6"};