C++ Array vs vector

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 1

Your array arr is allocated on the stack, i.e., the compiler has calculated the necessary space at compile time. At the beginning of the method, the compiler will insert an assembler statement like

sub esp, 10000*10000*sizeof(int)

which means the stack pointer (esp) is decreased by 10000 * 10000 * sizeof(int) bytes to make room for an array of 10000² integers. This operation is almost instant.

The vector is heap allocated and heap allocation is much more expensive. When the vector allocates the required memory, it has to ask the operating system for a contiguous chunk of memory and the operating system will have to perform significant work to find this chunk of memory.

As Andreas says in the comments, all your time is spent in this line:

vector<int> v(size*size); 

Accessing the vector inside the loop is just as fast as for the array.

For an additional overview see e.g.

• [What and where are the stack and heap?

• [http://computer.howstuffworks.com/c28.htm][2]

• [http://www.cprogramming.com/tutorial/virtual_memory_and_heaps.html][3]

Edit:

After all the comments about performance optimizations and compiler settings, I did some measurements this morning. I had to set size=3000 so I did my measurements with roughly a tenth of the original entries. All measurements performed on a 2.66 GHz Xeon:

1. With debug settings in Visual Studio 2008 (no optimization, runtime checks, and debug runtime) the vector test took 920 ms compared to 0 ms for the array test.

   98,48 % of the total time was spent in vector::operator[], i.e., the time was indeed spent on the runtime checks.

2. With full optimization, the vector test needed 56 ms (with a tenth of the original number of entries) compared to 0 ms for the array.

   The vector ctor required 61,72 % of the total application running time.

So I guess everybody is right depending on the compiler settings used. The OP's timing suggests an optimized build or an STL without runtime checks.

As always, the morale is: profile first, optimize second.