C++ 程序使用指针和排序崩溃

Question

我正在尝试编写一个程序，从用户那里获取学生的姓名和他们的分数。 然后对这些分数进行排序并取平均值。 它使用指针来实现这一点。 当我运行它时，它会进入要求名称和分数的部分。 但是它在收集数据后崩溃了。

#include <iostream>
#include <iomanip>
#include <string>

using namespace std;

struct Data
{
    string Name;
    double Grade;
};

// Function prototypes
void getData(Data *, int);
void selectionSort(Data *, int);
double getAverage(Data *, int);
void displayData(Data *, int, double);

int main()
{
    Data *Test;         // To dynamically allocate an array
    double Average;     // To hold the average of the scores
    int Scores;         // To hold number of scores

    // Get number of scores
    cout << "How many scores do you have to average? ";
    cin  >> Scores;

    // Dynamically allocate an array larger enough
    // to hold the user-defined number of scores
    Test = new Data[Scores];    // Allocate memory

    getData(Test, Scores);

    selectionSort(Test, Scores);

    Average = getAverage(Test, Scores);

    displayData(Test, Scores, Average);

    delete [] Test;
    Test = 0;

    return 0;
}

void getData(Data *Test, int Scores)
{
    cout << "Enter the names and scores for each student.\n";
    for (int i = 0; i < Scores; i++)
    {
        cout << "Student #" << (i + 1) << endl;
        cout << "   Name: ";
        cin.ignore();
        getline(cin, (Test + i)->Name);
        do
        {
            cout << "   Score :";
            cin  >> (Test + i)->Grade;
            if ((Test + i)->Grade < 0)
                cout << "Scores must be greater than 0.\n" << "Re-enter ";

            cout << endl;
        } while ((Test + i)->Grade < 0);
    }
}

void selectionSort(Data *Test, int Scores)
{
    int startscan, minIndex;
    Data *minValue;

    for (startscan = 0; startscan < (Scores - 1); startscan++)
    {
        minIndex = startscan;
        *minValue = Test[startscan];
        for (int i = startscan + 1; i < Scores; i++)
        {
            if ((Test + i)->Grade < minValue->Grade)
            {
                *minValue = Test[i];
                minIndex = i;
            }
        }
        Test[minIndex] = Test[startscan];
        Test[startscan] = * minValue;
    }
}

double getAverage(Data *Test, int Scores)
{
    double Total;
    for (int i = 0; i < Scores; i++)
        Total += (Test + i)->Grade;

    return Total / Scores;
}

void displayData(Data *Test, int Scores, double Avg)
{
    cout << "    Test scores\n";
    cout << "Number of scores: " << Scores << endl;
    cout << "Scores in ascending-order:\n";
    for (int i = 0; i < Scores; i++)
        cout << (Test + i)->Name << ": " << (Test + i)->Grade << endl;

    cout << fixed << showpoint << setprecision(2);
    cout << "Average of scores: " << Avg << endl;
}

Answer 1

我可以看到两个错误：

错误 1：

Data *minValue;

for (startscan = 0; startscan < (Scores - 1); startscan++)
{
    minIndex = startscan;
    *minValue = Test[startscan];  // <-- minValue is an uninitialized pointer

     //...
        if ((Test + i)->Grade < minValue->Grade)
        {
            *minValue = Test[i]; // <-- Same issue
            minIndex = i;
        }

您正在取消引用未初始化的指针，因此程序将表现出未定义的行为。 修复，（如果你仍然想使用指针）：

Data *minValue;

for (startscan = 0; startscan < (Scores - 1); startscan++)
{
    minIndex = startscan;
    minValue = &Test[startscan];  

    //...

        if ((Test + i)->Grade < minValue->Grade)
        {
            minValue = &Test[i]; 
            minIndex = i;
        }

---

错误 2：

double getAverage(Data *Test, int Scores)
{
    double Total;
    for (int i = 0; i < Scores; i++)
        Total += (Test + i)->Grade;  // <-- Total is uninitialized

Total未初始化，因此分配给它是未定义的行为。 修复很简单，只需在循环之前将Total设置为 0。

double getAverage(Data *Test, int Scores)
{
    double Total = 0;