Copy part of a matrix and paste on another one C++

Question

The user input a matrix, and the output must be a new matrix with an additional column of zeros. If we apply the script to a 2 square matrix like : {1,2,3,4} the new matrix output will be a 2 rows & 3 columns : {1,2,32,3,4,0}. I don't understand the number 32 output.

#include <iostream>

int main(){

    int m,n;

    std::cout << "Input the size of the square matrix :  ";
    std::cin >> m;
    n=m;

    int A[m][n]={};
    int M[m][n+1]={0};

    for (int i(0);i<m;i++){
        for(int j(0);j<n;j++){
            std::cout << "Input element A["<<i<<"]["<<j<<"] : ";
            std::cin >> A[i][j];
            M[i][j]=A[i][j];                    
        }
    }
    for (int i(0);i<m;i++){
        for(int j(0);j<=n;j++){
            std::cout << M[i][j] << " ";
        }
        std::cout << "\n";
    }

    return 0;
}

You know, I'm not sure how Variable Length Arrays handle initialization. It's kind of Wild West territory because Variable Length Arrays are not part of C++. Some compilers have added support to make C++ a bit more like C, but you'd have to check your compiler's documentation to see how it handles `int M[m][n+1]={0};` — user4581301, Jun 05 '20 at 17:00
Since the goal of VLAs in C++ its to be the same as VLAs in C, this C question may be relevant: [Initializing a dynamically-sized Variable-Length Array (VLA) to 0](https://stackoverflow.com/questions/4393779/initializing-a-dynamically-sized-variable-length-array-vla-to-0) — user4581301, Jun 05 '20 at 17:07
Poking around at g++, it looks like the request for initialization is mostly ignored. Only the first element is being set. — user4581301, Jun 05 '20 at 17:15
Side note: [A simple, fast, and safe dynamically-sized matrix class](https://stackoverflow.com/a/2076668/4581301). As an added bonus it also automatically initializes to all zeroes. — user4581301, Jun 05 '20 at 17:40

Paul Sanders · Accepted Answer · 2020-06-05T17:13:10.193

2

Variable length arrays (VLAs) are a non-portable gcc extension and evidently don't initialise as you would expect.

One solution is to use std::vector instead which is portable and will do what you want, something like this:

#include <iostream>
#include <vector>

int main(){
    int m,n;
    std::cout << "Input the size of the square matrix :  ";
    std::cin >> m;
    n=m;

    std::vector <std::vector <int>> A;
    std::vector <std::vector <int>> M;
    A.resize (m);
    M.resize (m);

    for (int i = 0; i < m; ++i)
    {
        A [i].resize (n);
        M [i].resize (n + 1);
    }

    for (int i(0);i<m;i++){
        for(int j(0);j<n;j++){
            std::cout << "Input element A["<<i<<"]["<<j<<"] : ";
            std::cin >> A[i][j];
            M[i][j]=A[i][j];    
            std::cout << "\n";
        }
    }

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

Live demo

edited Jun 05 '20 at 17:13

answered Jun 05 '20 at 17:02

Paul Sanders

24,133
4
26
48

So there's no way to solve that "manually", without using the vector class ? – Hibou Jun 05 '20 at 17:13
1

Yes there is, you can `memset` your VLAs as described in the link posted by user4581301 above. But if you're writing C++, you shouldn't be using VLAs - like I say, there non-standard. – Paul Sanders Jun 05 '20 at 17:14
1

I have a couple reasons beyond non-standard: VLA's totally the compile-time behaviour of `sizeof`and they are a really great way to allow the user to trigger a stack overflow. Input an `m` of 250 on many systems and sit back to watch the fun. – user4581301 Jun 05 '20 at 17:22

score 1 · Answer 2 · answered Jun 05 '20 at 18:22

1

In the bad old days of C, you could realloc() the array bigger and memset() the new column (only good for the last dimension, where items are adjacent).

answered Jun 05 '20 at 18:22

David G. Pickett

387
1
8

Copy part of a matrix and paste on another one C++

2 Answers2