公共成员函数没有返回值...我错过了什么？

时间：2014-07-31 15:17:49

标签： c++ class

我是C ++的新手（以及一般的OOP）。我正在编写一个对2个多项式执行某些数学运算的类。这是我的代码：

#include "stdafx.h"
#include <iostream>
#include <cstdlib>

using namespace std;

class Poly
{

private:

    int order;                                  // the order of the polynomial
    int *coeff;                                 // pointer to an array of coefficients
                                                // size of the coefficient array is predicated on [order + 1]

public:

//  Poly();                                     // the default constructor
    int setOrderAndCoeff();                     // sets the order and coefficients
    int display();                              // displays the resutling polynomial
    void addition(Poly P1, Poly P2);            // adds 2 polynomials
    void subtraction (Poly P1, Poly P2);        // subtracts 2 polynomials
    void multiplication (Poly P1, Poly P2);     // multiplies 2 polynomials

//  ~Poly();                                    // the destructor
};

//Poly::Poly()
//{
//  order = 0;
//  *coeff = 0;
//}

int Poly::display()
{
    int i;
    int j;
    for (i = order; i >= 0; i--)
    {
        cout << coeff[i] << "x^" << i;
        if ((i - 1) != -1)
        {
            cout << "+";
        }
    }
    cout << "\n";
    return 0;
}

int Poly::setOrderAndCoeff()
{
    int i;
    cout << "Please enter the order of the polynomial: ";
    cin >> order;
    coeff = new int[order + 1];
    for (i = order; i >= 0; i--)
    {
        cout << "Please enter the coefficient of x^" << i << " :";
        cin >> coeff[i];
    }
    return 0;
}

void Poly::addition(Poly P1, Poly P2)
{
    int max;
    int i;
    max = (P1.order > P2.order) ? P1.order : P2.order;
    int *add = new int[max + 1];

    if (P1.order == P2.order)
    {
        for (i = P1.order; i >= 0; i--)
        {
            add[i] = P1.coeff[i] + P2.coeff[i];
        }
    }

    if (P1.order > P2.order)
    {
        for (i = P1.order; i > P2.order; i--)
        {
            add[i] = P1.coeff[i];
        }
        for (i = P2.order; i >= 0; i--)
        {
            add[i] = P1.coeff[i] + P2.coeff[i];
        }
    }

    if (P1.order < P2.order)
    {
        for (i = P2.order; i > P1.order; i--)
        {
            add[i] = P2.coeff[i];
        }
        for (i = P1.order; i >= 0; i--)
        {
            add[i] = P1.coeff[i] + P2.coeff[i];
        }
    }
    cout << "\nAddition:";
    display();
    cout << "\n";
}

void Poly::subtraction(Poly P1, Poly P2)
{
    int max;
    int i;
    max = (P1.order > P2.order) ? P1.order : P2.order;
    int *sub = new int[max + 1];

    if (P1.order == P2.order)
    {
        for (i = P1.order; i >= 0; i--)
        {
            sub[i] = P1.coeff[i] - P2.coeff[i];
        }
    }

    if (P1.order > P2.order)
    {
        for (i = P1.order; i > P2.order; i--)
        {
            sub[i] = P1.coeff[i];
        }
        for (i = P2.order; i >= 0; i--)
        {
            sub[i] = P1.coeff[i] - P2.coeff[i];
        }
    }

    if (P1.order < P2.order)
    {
        for (i = P2.order; i > P1.order; i--)
        {
            sub[i] = -P2.coeff[i];
        }
        for (i = P1.order; i >= 0; i--)
        {
            sub[i] = P1.coeff[i] - P2.coeff[i];
        }
    }
    cout << "\nSubtraction:";
    display();
    cout << "\n";
}

void Poly::multiplication(Poly P1, Poly P2)
{
    int i;
    int j;
    int max;

    max = P1.order + P2.order;
    int *mult = new int[max + 1];

    for (i = P1.order; i >= 0; i--)
    for (j = P2.order; j >= 0; j--)
    {
        mult[i + j] += P1.coeff[i] * P2.coeff[i];
    }
        cout << "\nMultiplication:";
        display();
}

int main()
{
    int choice;
    Poly P1, P2, P3;
    cout << "-------- Instructions --------" << endl;

    cout << "For polynomial 1... " << endl;
    P1.setOrderAndCoeff();

    cout << endl;

    cout << "For polynomial 2... " << endl;
    P2.setOrderAndCoeff();

    while (1)
    {
        cout << "\n******** Menu Selection ********" << endl;
        cout << "1: Addition\n2: Subtraction\n3: Mutiplication\n0: Exit" << endl;
        cout << "Please enter your choice (1, 2, 3 or 0):";
        cin >> choice;

        switch (choice)
        {
        case 1:
            cout << "\n-------- Addition --------\n";
            cout << "Polynomial 1: ";
            P1.display();
            cout << "Polynomial 2: ";
            P2.display();
            P3.addition(P1, P2);
            cout << "--------------------------\n";
            break;

        case 2:
            cout << "\n-------- Subtraction --------\n";
            cout << "Polynomial 1: ";
            P1.display();
            cout << "Polynomial 2: ";
            P2.display();
            P3.subtraction(P1, P2);
            cout << "--------------------------\n";
            break;

        case 3:
            cout << "\n-------- Multiplication --------\n";
            cout << "Polynomial 1: ";
            P1.display();
            cout << "Polynomial 2: ";
            P2.display();
            P3.multiplication(P1, P2);
            cout << "--------------------------\n";
            break;

        case 0:
            cout << "The program will now terminate.  Thank you." << endl;
            exit(0);

        default:
            cout << endl;
            cout << "You have entered an invalid selection." << endl;
            cout << "Please enter a positive integer between 0 and 3.";
            cout << endl;
        }
    }
    return 0;
}

所有内容都会编译，并提示用户输入订单和一些系数。不幸的是，addition（），subttraction（）和multiplication（）函数不会返回结果多项式。

我知道我错过了一些非常基本的东西，但我无法弄清楚它是什么！

提前感谢您提供任何指导！

PS：我甚至需要默认的构造函数吗？

编辑（根据建议的修订更新的代码）

以下是基于建议的修订代码：

#include "stdafx.h"
#include <iostream>
#include <cstdlib>

using namespace std;

class Poly
{

private:

    int order;                                  // the order of the polynomial
    int *coeff;                                 // pointer to an array of coefficients
                                                // size of the coefficient array is predicated on [order + 1]
    int *add;
    int *sub;
    int *mult;

public:

//  Poly();                                     // the default constructor
    int setOrderAndCoeff();                     // sets the order and coefficients
    int display(int *data, int count);          // displays the resutling polynomial
    void addition(Poly P1, Poly P2);            // adds 2 polynomials
    void subtraction (Poly P1, Poly P2);        // subtracts 2 polynomials
    void multiplication (Poly P1, Poly P2);     // multiplies 2 polynomials

//  ~Poly();                                    // the destructor
};


int Poly::display(int *data, int count)
{
    for (i = cout; i >= 0; i--)
    {
        cout << data[i] << "x^" << i;
        if ((i - 1) != -1)
        {
            cout << "+";
        }
    }
    cout << "\n";
    return 0;
}

int Poly::setOrderAndCoeff()
{
    int i;
    cout << "Please enter the order of the polynomial: ";
    cin >> order;
    coeff = new int[order + 1];
    for (i = order; i >= 0; i--)
    {
        cout << "Please enter the coefficient of x^" << i << " :";
        cin >> coeff[i];
    }
    return 0;
}

void Poly::addition(Poly P1, Poly P2)
{
    int max;
    int i;
    max = (P1.order > P2.order) ? P1.order : P2.order;
    add = new int [max + 1];

    if (P1.order == P2.order)
    {
        for (i = P1.order; i >= 0; i--)
        {
            add[i] = P1.coeff[i] + P2.coeff[i];
        }
    }

    if (P1.order > P2.order)
    {
        for (i = P1.order; i > P2.order; i--)
        {
            add[i] = P1.coeff[i];
        }
        for (i = P2.order; i >= 0; i--)
        {
            add[i] = P1.coeff[i] + P2.coeff[i];
        }
    }

    if (P1.order < P2.order)
    {
        for (i = P2.order; i > P1.order; i--)
        {
            add[i] = P2.coeff[i];
        }
        for (i = P1.order; i >= 0; i--)
        {
            add[i] = P1.coeff[i] + P2.coeff[i];
        }
    }
    cout << "\nAddition:";
    display(add, count);
    cout << "\n";
}

void Poly::subtraction(Poly P1, Poly P2)
{
    int max;
    int i;
    max = (P1.order > P2.order) ? P1.order : P2.order;
    int *sub = new int[max + 1];

    if (P1.order == P2.order)
    {
        for (i = P1.order; i >= 0; i--)
        {
            sub[i] = P1.coeff[i] - P2.coeff[i];
        }
    }

    if (P1.order > P2.order)
    {
        for (i = P1.order; i > P2.order; i--)
        {
            sub[i] = P1.coeff[i];
        }
        for (i = P2.order; i >= 0; i--)
        {
            sub[i] = P1.coeff[i] - P2.coeff[i];
        }
    }

    if (P1.order < P2.order)
    {
        for (i = P2.order; i > P1.order; i--)
        {
            sub[i] = -P2.coeff[i];
        }
        for (i = P1.order; i >= 0; i--)
        {
            sub[i] = P1.coeff[i] - P2.coeff[i];
        }
    }
    cout << "\nSubtraction:";
    display(sub, count);
    cout << "\n";
}

void Poly::multiplication(Poly P1, Poly P2)
{
    int i;
    int j;
    int max;

    max = P1.order + P2.order;
    int *mult = new int[max + 1];

    for (i = P1.order; i >= 0; i--)
    for (j = P2.order; j >= 0; j--)
    {
        mult[i + j] += P1.coeff[i] * P2.coeff[i];
    }
        cout << "\nMultiplication:";
        display(mult, count);
}

int main()
{
    int choice;
    Poly P1, P2, P3;
    cout << "-------- Instructions --------" << endl;

    cout << "For polynomial 1... " << endl;
    P1.setOrderAndCoeff();

    cout << endl;

    cout << "For polynomial 2... " << endl;
    P2.setOrderAndCoeff();

    while (1)
    {
        cout << "\n******** Menu Selection ********" << endl;
        cout << "1: Addition\n2: Subtraction\n3: Mutiplication\n0: Exit" << endl;
        cout << "Please enter your choice (1, 2, 3 or 0):";
        cin >> choice;

        switch (choice)
        {
        case 1:
            cout << "\n-------- Addition --------\n";
            cout << "Polynomial 1: ";
            P1.display(add, count);
            cout << "Polynomial 2: ";
            P2.display(add, count);
            P3.addition(P1, P2);
            cout << "--------------------------\n";
            break;

        case 2:
            cout << "\n-------- Subtraction --------\n";
            cout << "Polynomial 1: ";
            P1.display(sub, count);
            cout << "Polynomial 2: ";
            P2.display(sub, count);
            P3.subtraction(P1, P2);
            cout << "--------------------------\n";
            break;

        case 3:
            cout << "\n-------- Multiplication --------\n";
            cout << "Polynomial 1: ";
            P1.display(mult, count);
            cout << "Polynomial 2: ";
            P2.display(sub, count);
            P3.multiplication(P1, P2);
            cout << "--------------------------\n";
            break;

        case 0:
            cout << "The program will now terminate.  Thank you." << endl;
            exit(0);

        default:
            cout << endl;
            cout << "You have entered an invalid selection." << endl;
            cout << "Please enter a positive integer between 0 and 3.";
            cout << endl;
        }
    }
    return 0;
}

我仍然遇到一些构建错误，即：

1）我没有在

中定义 
int Poly::display(int *data, int count)

2）在开关的情况1中添加是未声明的标识符（与sub和mult的情况一样，但它的错误相同）

3）in 

void Poly::addition(Poly P1, Poly P2`)

无法将参数2转换为int

提前感谢大家一起来看看。

-Ryan

2 个答案:

答案 0 :(得分：1)

您如何期望函数不返回值来返回值？当你有

void addition(Poly P1, Poly P2);

此函数无法返回值。作为第一步，您应该将其更改为

Poly addition(Poly P1, Poly P2);

并在函数中添加适当的return语句。但它仍然不是通常在C ++中完成的方式。您不希望按值传递Poly对象，因为它很昂贵，而且根据您当前的定义，它会泄漏内存。你应该使用

Poly addition(const Poly& P1, const Poly& P2);

并且作为最后 - 但最重要的 - 评论。不要使用指针。替换你的

int *coeff;

与

std::vector<int> coeff;

只有当您知道自己在做什么以及在低级实现中时，才应使用手动内存分配。为了您的目的，std::vector要好得多。

答案 1 :(得分：0)

我认为returning代表displaying。问题是sub，add和mult在Poly::display函数中不可见。很简单，尝试将此变量添加为Poly类的成员。

class Poly
{

private:

    int order;                                  // the order of the polynomial
    int *coeff;                                 // pointer to an array of coefficients
    int *add, *sub, *mult;                       

// ...
};

然后你不必在每个函数中声明它们，例如Poly::addition(Poly P1, Poly P2)你可以改变：

int *add = new int[max + 1];

通过

add = new int[max + 1]; // Or 
                        // this->add = new int[max + 1];
                        // if you prefer.

编辑：您正在功能coeff中显示display内容，并且coeff在您拨打任何操作时都不会更改。

您必须对函数display进行一些修改，最简单的方法是重新定义显示以接受2个参数：

class Poly
{

puplic:
    // ...
    void display(int *data, int count);
    // ...
};

int Poly::display(int *data, int count)
{
    for (int i = count; i >= 0; i--)
    {
        cout << data[i] << "x^" << i;
        if ((i - 1) != -1)
        {
            cout << "+";
        }
    }
    cout << "\n";
    return 0;
}

然后在（例如）Poly::addition：

void Poly::addition(Poly P1, Poly P2)
{
    //...
    display(add, count)  // Here count is the ammount if elements in data.
}
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