Printing out elements of dynamic array or matrix

Question

Why does this simple code for printing out an array not work?

void main()
{

    cout<<"Simple for\n";
    int n;
    cout<<"Enter the lenght of array:";
    cin>>n;
    int* a= new int[n];
    for(int j=0; j<n; j++){
        a[j]=0;
        cout<<setw(8)<<a[j];
        //getchar();

    }
    getchar();  
    delete[]a;


}

The output disappears quickly.

When n is constant its works but when n comes from input it doesn't work. When getchar put in the For, it only prints out two elements of the array with any length.

What is wrong?

Answer 1

You probably want this:

int* a= new int[n];

C++ is not C; you don't allocate "bytes" with new; you allocate objects. Arrays, types, etc. So if you want an array of n values of type int, then you allocate that. No need for sizeof, multiplication, etc.

Note that anything you allocate with the array version of new (ie: new Type[]) must be deleted with the array version of delete:

delete []a;

The size is unnecessary; you just need to make sure that new[] is paired with delete[]

Answer 2

This allocation

int* a= new int(n*sizeof(int)); 

only allocates a single int and sets its initial value to n*sizeof(int). Not exactly what you wanted.

The correct way to allocate n ints is

int* a = new int[n];

or even better

std::vector<int>   a(n);

Answer 3

Thanks everyone The problem is solved.. But I don,t know why. Can anyone explain the role of another getchar() here?

---

This is new code:

int main()
{
cout<<"Simple for\n";
int n;
cout<<"Enter the lenght of array:";
cin>>n;
int* a= new int[n];
for(int j=0; j<n; j++){
        a[j]=0;
        cout<<setw(8)<<a[j];
                  }
    delete[]a;
    getchar();  
    getchar();
}