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: 
If you are compiling this with a Microsoft compiler, to make it a fair comparison you need to switch off iterator security checks and iterator debugging, by defining \_SECURE\_SCL=0 and \_HAS\_ITERATOR\_DEBUGGING=0.

Secondly, the constructor you are using initialises each vector value with zero, and you are not memsetting the array to zero before filling it. So you are traversing the vector twice.

Try:

```
vector<int> v; 
v.reserve(size*size);

```
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.