Question: when using C++ vector, time spent is 718 milliseconds,
while when I use Array, time is almost 0 milliseconds.

Why so much performance difference?

```
int _tmain(int argc, _TCHAR* argv[])
{
const int size = 10000; 
clock_t start, end; 
start = clock();
vector<int> v(size*size); 
for(int i = 0; i < size; i++)
{  
    for(int j = 0; j < size; j++)
    {   
        v[i*size+j] = 1;  
    } 
} 
end = clock();
cout<< (end - start)
    <<" milliseconds."<<endl; // 718 milliseconds

int f = 0;
start = clock(); 
int arr[size*size]; 
for(int i = 0; i < size; i++)
{  
    for(int j = 0; j < size; j++)
    {   
        arr[i*size+j] = 1;  
    } 
} 
end = clock();
cout<< ( end - start)
    <<" milliseconds."<<endl; // 0 milliseconds
return 0;
}

```

Answer: 
Change assignment to eg. `arr[i*size+j] = i*j`, or some other non-constant expression. I think compiler optimizes away whole loop, as assigned values are never used, or replaces array with some precalculated values, so that loop isn't even executed and you get 0 milliseconds.

Having changed 1 to `i*j`, `i` get the same timings for both vector and array, unless pass `-O1` flag to gcc, then in both cases I get 0 milliseconds.

So, first of all, double-check whether your loops are **actually** executed.
When you declare the array, it lives in the stack (or in static memory zone), which it's very fast, but can't increase its size.

When you declare the vector, it assign dynamic memory, which it's not so fast, but is more flexible in the memory allocation, so you can change the size and not dimension it to the maximum size.