如何从文件中读取值，然后使用它们初始化二维数组 C++

Question

我希望读取文本文件，例如

第一行是二维数组的第一维，第二行是二维数组的第二维，第三行是一个值。在读取前三个值并将其存储到变量 n、m、k 后，我想初始化 2D int 数组 int x[n][m].

#include <iostream>
#include <fstream>
#include <string>
#include <math.h>

using namespace std;

int n,m,k;

int main()
{
    ifstream File;
    File.open("text.txt");

    if (File.fail()) {
        cout << "error opening file";
    }

    while (!File.eof())
    {
            File >> n;
            File >> m;
            File >> k;

            int x[n][m];

            for (int i = 0; i < 3; i++) {
                for (int j = 0; j < 5; j++) {
                    File >> x[i][j];
                }
            }

    }
    return 0;
}

但是，我无法初始化数组，因为表达式 n 和 m 似乎没有常量值。如果我将变量设置为 const int n,m,k，我将无法使用 >> 从文件中读取它们。我如何读取数组大小调整值，使用它们创建数组，然后将值存储在其中？

编辑：

#include <iostream>
#include <fstream>
#include <string>
#include <vector>
#include <math.h>

using namespace std;
std::vector < std::vector <int>> x;

int n,m,k;

int main()
{
    ifstream File;
    File.open("test.txt");

    if (File.fail()) {
        cout << "error opening file";
    }

    while (!File.eof())
    {
            File >> n;
            File >> m;
            File >> k;

            for (int i = 0; i < n; i++) {
                vector <int> row;
                for (int j = 0; j < m; j++) {
                    int readFromFile = 0;
                    File >> readFromFile;
                    row.push_back(readFromFile);
                }
                x.push_back(row);
            }

    }

    cout << n;
    cout << "\n";
    cout << m;
    cout << "\n";
    cout << k;
    cout << "\n";

    for (int i = 0; i < n; i++) {
        for (int j = 0; j < m; j++) {
            cout << x[i][j];
            cout << " ";
        }
        cout << "\n";
    }
    return 0;

}

Answer 1

#include <cstdlib>  // EXIT_FAILURE
#include <vector>
#include <fstream>
#include <iostream>

int main()
{
    char const * file_name{ "test.txt" };
    std::ifstream is{ file_name };

    if (!is.is_open()) {  // exit if the file couldn't be opened.
        std::cerr << "Couldn't open \"" << file_name << "\" for reading!\n\n";
        return EXIT_FAILURE;
    }

    int m, n, k;
    if (!(is >> m >> n >> k)) {  // whatever k might be.
        std::cerr << "Data error!\n\n";
        return EXIT_FAILURE;
    }

    std::vector<std::vector<int>> data(m, std::vector<int>(n)); // m rows, n cols

    for (int row = 0; row < m; ++row) {
        for (int col = 0; col < n; ++col) {
            if (!(is >> data[row][col])) {  // try to extract an integer
                std::cerr << "Data error!\n\n";  // give an error message and
                return EXIT_FAILURE;             // exit if it fails
            }
        }
    }

    for (auto const &row : data) {
        for (auto const &col : row) {
            std::cout << col << ' ';
        }
        std::cout.put('\n');
    }
}

输出：

如何从文件中读取值，然后使用它们初始化二维数组 C++

How to read values from a file and then use them to initialize a 2D array C++

c++

arrays

fstream

declaration