从 c++ 中的 txt 文件中读取二维数组

Question

This is my code

#include<bits/stdc++.h>
    using namespace std;
    int main()
    {
        char arr1[10][10];
        cout <<"Reading Start" <<endl;
        ifstream rfile("test.txt");
        rfile.getline(arr1[10],10);
        int i,j;
        for(i=0;i<6;i++)
        {
        for(j=0;i<6;j++)
        {
          cout << arr1[i][j];
        }
        }
        cout <<"\nRead Done" <<endl<<endl;
        rfile.close();
    }

This is my test.txt file

0 4 7 0 0 0
4 0 0 5 3 0
7 0 0 0 6 0
0 5 3 0 0 2
0 3 4 0 0 2
0 0 0 2 2 0

I want to read this matrix but when using the above code then it shows core dumped output, can anyone give me a better solution to do this thing?

Answer 1

can anyone give me a better solution to do this thing?

A better alternative would be to use a 2D vector as shown below. The advantage of using a vector over an array is that you don't need to specify(know) the rows and columns beforehand. That is, the text input file can have as many rows and columns and there is no need to ask the user (or preallocate) how many rows and columns does the file have. std::vector will take care of it as shown below.

The below program uses a 2D std::vector for storing information(like integers values in this case) in 2D manner. After reading all the values from the file you can process the vector according to your needs. The program shown reads data( int values) from input.txt and store those in a 2D vector . Also, this program works even if there are uneven number of columns. You can use the below program as a reference(starting point).

#include <iostream>
#include <vector>
#include <string>
#include <sstream>
#include<fstream>
int main() {
    std::string line;
    int word;
    
    std::ifstream inFile("input.txt");
    
    //create/use a std::vector instead of builit in array 
    std::vector<std::vector<int>> vec;
    
    if(inFile)
    {
        while(getline(inFile, line, '\n'))        
        {
            //create a temporary vector that will contain all the columns
            std::vector<int> tempVec;
            
            std::istringstream ss(line);
            
            //read word by word(or int by int) 
            while(ss >> word)
            {
                //std::cout<<"word:"<<word<<std::endl;
                //add the word to the temporary vector 
                tempVec.push_back(word);
            }      
            //now all the words from the current line has been added to the temporary vector 
            vec.emplace_back(tempVec);
        }    
    }
    else 
    {
        std::cout<<"file cannot be opened"<<std::endl;
    }
    
    inFile.close();
    //now you can do the whatever processing you want on the vector

    //lets check out the elements of the 2D vector so the we can confirm if it contains all the right elements(rows and columns)
    for(std::vector<int> &newvec: vec)
    {
        for(const int &elem: newvec)
        {
            std::cout<<elem<<" ";
        }
        std::cout<<std::endl;
    }
    return 0;
}

The output of the above program can be seen here . The input file through which int values are read is also given at the above mentioned link.

Advantages of using vector

1. You don't need to ask the user for the number of rows and columns in the input file. That is you don't have to fix(hardcode) the size of your array.

2. The above program works even if there are uneven entries in any particular row.

3. std::vector takes care of memory management for you. So you don't have to use new and delete by yourself which needs more attention/care.(in case you're thinking of creating array on heap)

Answer 2

Becuase there are so many possible solutions, let us just show some.

The basic difference is, if we know the dimensions of the array at compile time,so,if there arecompiletime constants, then we can still use a C-Style array, or better, a std::array .

If we do not know the dimesions of the source data array, then we need a dynamic container that can grow, for example a std::vector .

In allcases, we can use the index operator [] with a standard for loop or a range based for loop with references. There is no difference.

---

Examples:

C-Style array with standard for loops and index based access

#include <iostream>
#include <fstream>

constexpr int NumberOfRows = 6;
constexpr int NumberOfColumns = 6;

int main() {

    // Open the sourcefile
    std::ifstream sourceFileStream{ "test.txt" };

    // And check, if it could be opened
    if (sourceFileStream) {

        // Define 2D array to hold all data and initialize it with all 0
        char array2D[NumberOfRows][NumberOfColumns]{};

        // Read the rows and columns from the source file
        for (int row = 0; row < NumberOfRows; ++row)
            for (int col = 0; col < NumberOfColumns; ++col)
                sourceFileStream >> array2D[row][col];

        // Debug output
        for (int row = 0; row < NumberOfRows; ++row) {
            for (int col = 0; col < NumberOfColumns; ++col)  std::cout << array2D[row][col] << ' ';
            std::cout << '\n';
        }
    }
    else std::cerr << "\nError: Could not open source file\n\n";
}

---

C-Style array with range based for loop and reference access

#include <iostream>
#include <fstream>

constexpr int NumberOfRows = 6;
constexpr int NumberOfColumns = 6;

int main() {

    // Open the sourcefile
    std::ifstream sourceFileStream{ "test.txt" };

    // And check, if it could be opened
    if (sourceFileStream) {

        // Define 2D array toholdall data and initialize it with all 0
        char array2D[NumberOfRows][NumberOfColumns]{};

        // Read the rows and columns from the source file
        for (auto& row : array2D)
            for (auto& col : row)
                sourceFileStream >> col;

        // Debug output
        for (const auto& row : array2D) {
            for (const auto& col : row) std::cout << col << ' ';
            std::cout << '\n';
        }
    }
    else std::cerr << "\nError: Could not open source file\n\n";
}

---

C++ std::array with range based for loop

#include <iostream>
#include <fstream>
#include <array>

constexpr int NumberOfRows = 6;
constexpr int NumberOfColumns = 6;

int main() {

    // Open the sourcefile
    std::ifstream sourceFileStream{ "test.txt" };

    // And check, if it could be opened
    if (sourceFileStream) {

        // Define 2D array toholdall data and initialize it with all 0
        std::array<std::array<char, NumberOfColumns>, NumberOfRows> array2D{};

        // Read the rows and columns from the source file
        for (auto& row : array2D)
            for (auto& col : row)
                sourceFileStream >> col;

        // Debug output
        for (const auto& row : array2D) {
            for (const auto& col : row) std::cout << col << ' ';
            std::cout << '\n';
        }
    }
    else std::cerr << "\nError: Could not open source file\n\n";
}

---

Dynamic solution, with a std::vector

#include <iostream>
#include <fstream>
#include <sstream>
#include <vector>
#include <string>

int main() {

    // Open the sourcefile
    std::ifstream sourceFileStream{ "test.txt" };

    // And check, if it could be opened
    if (sourceFileStream) {

        // Define 2D array to hol dall data and initialize it with all 0
        std::vector<std::vector<char>> array2D{};

        // Read the rows and columns from the source file
        std::string line{};
        while (std::getline(sourceFileStream, line)) {

            // Add a new row to our matrix
            array2D.push_back(std::vector<char>{});

            // Read all column data
            char c{};
            for (std::istringstream iss(line); iss >> c; array2D.back().push_back(c))
                ;
        }
        // Debug output
        for (const auto& row : array2D) {
            for (const auto& col : row) std::cout << col << ' ';
            std::cout << '\n';
        }
    }
    else std::cerr << "\nError: Could not open source file\n\n";
}

---

Basically, everything is the same somehow. . .

Answer 3

There are a lot of other ways to perform the specific task, but i guess your method is not wrong, and you have just made a simple typing mistake in your second for loop condition. so ill just fix your code for you. and also you could just input single values at a time as u go.

#include<bits/stdc++.h>
using namespace std;
int main()
{
    char arr1[10][10];
    cout <<"Reading Start" <<endl;
    ifstream rfile("test.txt");
    int i,j;
    for(i=0;i<6;i++){
        for(j=0;j<6;j++){
            rfile >> arr1[i][j];
            cout << arr1[i][j] << " ";
        }
        cout << endl;
    }

    cout <<"\nRead Done" <<endl<<endl;
    rfile.close();
}

Output: