我的函数没有更新全局变量/ main 中使用的变量

Question

我的项目是一些简单的矩阵运算，它应该可以正常工作，但我的输入函数似乎没有更新全局变量，它们可能是我正在尝试做的错误的数据类型，但我不确定。这是应该发生的事情的链接http://cpp.sh/22kie。 这里还有一个链接到所有带有 Makefile https://drive.google.com/open?id=1tmIbdEWXJJ54moQKy8rkZODFkXKDU2Nz

这是我的头文件：

#ifndef project1
#define project1

static int choice, rows1, cols1, rows2, cols2, k;
static char choice2;
static int *mat1 = new int[10 * 10];
static int *mat2 = new int[10 * 10];
static int *matf = new int[10 * 10];

void output(int *matf, int rows, int cols);

void addition(int *mat1, int *mat2, int *matf, int rows, int cols);

void subtraction(int *mat1, int *mat2, int *matf, int rows, int cols);

void multiplication(int *mat1, int *mat2, int *matf, int rows1, int cols1, int rows2, int cols2);

int input();
#endif

这是：我的主要内容：

#include <string>
#include "project1.h"
using namespace std;

int main()
{

    input();

    switch (choice)
    { //this switch block will call the correct fucntion with respect to the user's choice

    case 1:
        addition(mat1, mat2, matf, rows1, cols1);

        break;

    case 2:
        subtraction(mat1, mat2, matf, rows1, cols1);

        break;

    case 3:
        multiplication(mat1, mat2, matf, rows1, cols1, rows2, cols2);

        break;

    case 0:
        return 0;
    default:
        cout << "invalid 2input";
    }
    //the end of the main function will repeat if the user would like

    cout << "\n Would you like to do another operation y/n?" << endl;
    cin >> choice2;

    if (choice2 == 'y')
    {
        main();
    }
    else
    {
        return 0;
    }
}

这是输入函数：

int input()

{

    //input function

    cout << "Menu"
         << "\n 1. Addition"
         << "\n 2. Subtraction"
         << "\n 3. Multiplication"
         << "\n 0. Exit"
         << "\n Enter the number of your choice" << endl;
    cin >> choice;

    if (choice == 0)
    { // this will kill the main function if "Exit" is chosen, or repeat if an invalid number is chosen
        return 0;
    }
    else if (choice > 3)
    {
        cout << "invalid input" << endl;
        input();
    }
    else
    {
    }
    //this part of the code will prompt the user for the rows and columns for each matrix
    cout << "Number of rows for the first Matrix? (max 10)" << endl;
    cin >> rows1;

    cout << "Number of columns for the first Matrix? (max 10)" << endl;
    cin >> cols1;

    cout << "Number of rows for the second Matrix? (max 10)" << endl;
    cin >> rows2;

    cout << "Number of columns for the second Matrix? (max 10)" << endl;
    cin >> cols2;

    //this if/else statement checks that the matrices are the right size to be operated
    if ((choice == 1 || choice == 2) && ((cols1 != cols2) || (rows1 != rows2)))
    {
        cout << " error: for addition and subraction the two matrices must have the same amount of elements";
        choice = 0;
        return 0;
    }
    else if ((choice == 3) && (rows2 != cols1))
    {
        cout << " error: for multiplication the columns of the first must equal the rows of the second";
        choice = 0;
        return 0;
    }
    else
    {
    }

    // this for loop allows the user to input each element for the first matrix
    cout << "input the elements of the first matrix: " << endl;

    for (int i = 0; i < rows1; i++)
    {
        for (int j = 0; j < cols1; j++)
        {
            cout << "Enter element "
                 << "(" << i << "," << j << "): ";
            cin >> k;
            *(mat1 + i * cols1 + j) = k;
        }
    }

    output(mat1, rows1, cols1); //this call to output will allow the user to see what they have imputed

    // this for loop allows the user to input each element for the second matrix
    cout << "input the elements of the second matrix: " << endl;
    for (int i = 0; i < rows2; i++)
    {
        for (int j = 0; j < cols2; j++)
        {
            cout << "Enter element "
                 << "(" << i << "," << j << "): ";
            cin >> k;
            *(mat2 + i * cols2 + j) = k;
        }
    }

    output(mat2, rows2, cols2); //this call to output will allow the user to see what they have imputed
}```

编辑：将所有内容切换为局部变量后的新 main：

主要：

#include <iostream>
#include <string>
#include "project1.h"
using namespace std;

int choice, rows1, cols1, rows2, cols2, k;
char choice2;
int *mat1 = new int[10 * 10];
int *mat2 = new int[10 * 10];
int *matf = new int[10 * 10];

int main()
{


    input( choice, rows1, cols1, rows2, cols2, k, mat1, mat2, matf);
cout<<rows1;
    switch (choice)
    { //this switch block will call the correct fucntion with respect to the user's choice

    case 1:
        addition(mat1, mat2, matf, rows1, cols1);

        break;

    case 2:
        subtraction(mat1, mat2, matf, rows1, cols1);

        break;

    case 3:
        multiplication(mat1, mat2, matf, rows1, cols1, rows2, cols2);

        break;

    case 0:
        return 0;
    default:
        cout << "invalid input";
    }
    //the end of the main fucntion will repeat if the user would like

    cout << "\n Would you like to do another operation y/n?" << endl;
    cin >> choice2;

    if (choice2 == 'y')
    {
        main();
    }
    else
    {
        return 0;
    }
}

标题：

#ifndef project1
#define project1


int output(int *matf, int rows1, int cols1);

int addition(int *mat1, int *mat2, int *matf, int rows1, int cols1);

int subtraction(int *mat1, int *mat2, int *matf, int rows1, int cols1);

int multiplication(int *mat1, int *mat2, int *matf, int rows1, int cols1, int rows2, int cols2);

int input(int choice,int rows1,int cols1,int rows2,int cols2,int k, int *mat1, int*mat2, int*matf);
#endif

Answer 1

您的变量仍然是全局变量（在函数外部声明）。您应该在 main 中声明它们（尽管这不是导致问题的原因）。

现在的问题是您将值按值传递给input。这意味着，input 被调用，堆栈增加，您发送的值被复制到input 的堆栈空间中。当您在input 中修改它们时，它们会在input 的堆栈上进行修改。 input 然后返回，这些更新的值被删除。您需要将 references 发送到这些变量。这意味着变量的地址将被复制到input的堆栈中，input将使用这些地址回到main函数中的变量并直接更新这些值。

将输入的函数签名更改为

void input(int& choice, int& rows1, int& cols1, int& rows2, int& cols2, int& k, int *mat1, int* mat2, int* matf);

注意input的返回值从int变为void。这是因为当你在main 中调用它时，你没有将input 的返回值分配给任何东西，所以我认为你不需要它。 input 中的任何 return 语句都需要更改为 return;（而不是 return 0; 之类的东西）。

另请注意，在发送到输入的前 6 个参数之后现在有与号 (&)。这样，input 就会有他们的地址并且可以直接更改它们。 mat1、mat2 和 matf 已作为指针传递，因此无需更改。

What's the difference between passing by reference vs. passing by value?