Random generator test. Where is error? - c++

Question

Strange tests behaviour.
I have class that generate random values.
std::random_device RandomProvider::rd;
std::mt19937 RandomProvider::rnb(RandomProvider::rd());
#define mainDataType unsigned int
mainDataType RandomProvider::GetNextValue(mainDataType upperLimit)
{
static std::uniform_int_distribution<int> uniform_dist(1, upperLimit);
return uniform_dist(rnb);
}
And unit-test that test it's behavior.
TEST_METHOD(TestRandomNumber)
{
CreateOper(RandomNumber);
int one = 0, two = 0, three = 0, unlim = 0;
const int cycles = 10000;
for (int i = 0; i < cycles; i++)
{
mainDataType res = RandomProvider::GetNextValue(3);
if (res == 1) one++;
if (res == 2) two++;
if (res == 3) three++;
}
double onePerc = one / (double)cycles;
double twoPerc = two / (double)cycles;
double threePerc = three / (double)cycles;
Assert::IsTrue(onePerc > 0.20 && onePerc < 0.40);
Assert::IsTrue(twoPerc > 0.20 && twoPerc < 0.40);
Assert::IsTrue(threePerc > 0.20 && threePerc < 0.40);
}
Test passed all-times in debug and if i chose it and Run only it. But it fails all times when i
run it with other tests. I added debug output to text file and got unreal values onePerc = 0.0556, twoPerc= 0.0474 and threePerc = 0.0526... What is going on here? (i am using VS2013 RC)

Answer 1

Since you use a static uniform_int_distribution the first time you call GetNextValue the max limit is set, never being changed in any subsequent call. Presumably in the test case you mentioned, your first call to GetNextValue had a different value than 3. Judging from the values returned it looks like probably either 19 or 20 was used in the first such call.