How to get input and display 2 dimensional array in c++?

Question

Code:-

    #include <iostream> 
    using namespace std;
    
    int main() {
        int r,c,*p;
        cout<<"Rows : ";
        cin>>r;
        cout<<"Columns : ";
        cin>>c;
        
        p=new int[r*c];
        
        cout<<"\nEnter array elements :"<<endl;
        int i,j,k;
        for( i=0;i<r;i++){
          for( j=0;j<c;j++){
            cin>>k;
            *(p+i*c+j)=k;
          }
        }
        
        cout<<"\nThe array elements are:"<<endl;
        for( i=0;i<r;i++){
          for( j=0;j<c;j++){
            cout<<*p<<" ";
            p=p+1;
          }
          cout<<endl;
        }
        cout<<endl;
        
        delete[]p;
        return 0; 
}

Output:-

Output

Error:-

munmap_chunk(): invalid pointer Process finished with exit code -6.

Can anyone explain why the above error occurs?

Please do not upload images of code/errors when asking a question. — vsr
– vsr, Commented Oct 27, 2021 at 14:57
When you delete[]p at the end, p is no longer pointing to the start of the array, so what you're deleting isn't valid. — ChrisMM
– ChrisMM, Commented Oct 27, 2021 at 14:58
Off-topic, but recommended reading: Why is "using namespace std;" considered bad practice? — ChrisMM
– ChrisMM, Commented Oct 27, 2021 at 15:09
Also off-topic, but std::unique_ptr<int[]> p = std::make_unique<int[]>(r * c) is preferable if your compiler supports it. See here and here. — Daniel Dearlove
– Daniel Dearlove, Commented Oct 28, 2021 at 15:14

ChrisMM · Accepted Answer · 2021-10-27 15:06:57Z

1

The problem occurs at the end of your program, when you delete[] p. In the nested for-loop immediately preceding it, you are modifying p, thus, when attempting to delete[] p at the end, you get undefined behaviour.

Potential fixes include, when printing the array elements, access the pointer the same way you did in the first for loop (or as Jarod mentioned, using [i * c + j].

Alternatively, you can use an additional variable.

int *tmp = p;
for( i = 0 ; i < r; i++ ){
   for( j = 0; j < c; j++ ){
       cout << *tmp << " ";
       tmp = tmp + 1; // ++tmp; also works
   }
   cout << endl;
}
cout << endl;
        
delete[] p;

This way, p still points to the original address.

answered Oct 27, 2021 at 15:06

ChrisMM

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