I've used HashSet and Dictionary a lot in C#, and found them very fast...
I've tried using std::map and std::hash_map and am finding them very slow in comparision. Does this sound like expected behaviour? Is there something I might be doing wrong in my use of std::hash_map?
Or, is there a better C++ Hash container out there?
I'm hashing int32s, usually around 100,000 of them.
Update: I created a repro in C# and C++. It runs two trials, they take 19ms and 13ms in C#, and about 11,000ms in C++. There must be something really wrong with my C++ code :)
(Both were run as Release builds, both are Console apps)
C# Output:
Found 511 values in the intersection, in 19 ms
Found 508 values in the intersection, in 13 ms
C++ Output:
Found 308 values in the intersection, in 11764.7ms
Found 316 values in the intersection, in 11742.8ms
C++ Output (using stdext::hash_map instead of std::map)
Found 300 values in the intersection, in 383.552ms
Found 306 values in the intersection, in 2277.02ms
C++ Output (using stdext::hash_map, a release x64 build)
Found 292 values in the intersection, in 1037.67ms
Found 302 values in the intersection, in 3663.71ms
Notes:
- Set2 is not getting populated quite as I wanted in C++, I was expecting it to have a 50% intersection with Set1 (as it does in C#), but I had to multiply my random number by 10 for some reason to even get them to partially not intersect
C#:
static void Main(string[] args)
{
int start = DateTime.Now.Millisecond;
int intersectionSize = runIntersectionTest();
int duration = DateTime.Now.Millisecond - start;
Console.WriteLine(String.Format("Found {0} values in the intersection, in {1} ms", intersectionSize, duration));
start = DateTime.Now.Millisecond;
intersectionSize = runIntersectionTest();
duration = DateTime.Now.Millisecond - start;
Console.WriteLine(String.Format("Found {0} values in the intersection, in {1} ms", intersectionSize, duration));
Console.ReadKey();
}
static int runIntersectionTest()
{
Random random = new Random(DateTime.Now.Millisecond);
Dictionary<int,int> theMap = new Dictionary<int,int>();
List<int> set1 = new List<int>();
List<int> set2 = new List<int>();
// Create 100,000 values for set1
for ( int i = 0; i < 100000; i++ )
{
int value = 1000000000 + i;
set1.Add(value);
}
// Create 1,000 values for set2
for ( int i = 0; i < 1000; i++ )
{
int value = 1000000000 + (random.Next() % 200000 + 1);
set2.Add(value);
}
// Now intersect the two sets by populating the map
foreach( int value in set1 )
{
theMap[value] = 1;
}
int intersectionSize = 0;
foreach ( int value in set2 )
{
int count;
if ( theMap.TryGetValue(value, out count ) )
{
intersectionSize++;
theMap[value] = 2;
}
}
return intersectionSize;
}
C++:
int runIntersectionTest()
{
std::map<int,int> theMap;
vector<int> set1;
vector<int> set2;
// Create 100,000 values for set1
for ( int i = 0; i < 100000; i++ )
{
int value = 1000000000 + i;
set1.push_back(value);
}
// Create 1,000 values for set2
for ( int i = 0; i < 1000; i++ )
{
int random = rand() % 200000 + 1;
random *= 10;
int value = 1000000000 + random;
set2.push_back(value);
}
// Now intersect the two sets by populating the map
for ( vector<int>::iterator iterator = set1.begin(); iterator != set1.end(); iterator++ )
{
int value = *iterator;
theMap[value] = 1;
}
int intersectionSize = 0;
for ( vector<int>::iterator iterator = set2.begin(); iterator != set2.end(); iterator++ )
{
int value = *iterator;
map<int,int>::iterator foundValue = theMap.find(value);
if ( foundValue != theMap.end() )
{
theMap[value] = 2;
intersectionSize++;
}
}
return intersectionSize;
}
int _tmain(int argc, _TCHAR* argv[])
{
srand ( time(NULL) );
Timer timer;
int intersectionSize = runIntersectionTest();
timer.Stop();
cout << "Found " << intersectionSize << " values in the intersection, in " << timer.GetMilliseconds() << "ms" << endl;
timer.Reset();
intersectionSize = runIntersectionTest();
timer.Stop();
cout << "Found " << intersectionSize << " values in the intersection, in " << timer.GetMilliseconds() << "ms" << endl;
getchar();
return 0;
}