Use of qsrand, random method that is not random

Question

I'm having a strange problem here, and I can't manage to find a good explanation to it, so I thought of asking you guys :

Consider the following method :

int MathUtility::randomize(int Min, int Max)
{
    qsrand(QTime::currentTime().msec());

    if (Min > Max)
    {
        int Temp = Min;
        Min = Max;
        Max = Temp;
    }
    return ((rand()%(Max-Min+1))+Min);
}

I won't explain you gurus what this method actually does, I'll instead explain my problem :

I realised that when I call this method in a loop, sometimes, I get the same random number over and over again... For example, this snippet...

for(int i=0; i<10; ++i)
{
    int Index = MathUtility::randomize(0, 1000);
    qDebug() << Index;
}

...will produce something like :

567 567 567 567...etc...

I realised too, that if I don't call qsrand everytime, but only once during my application's lifetime, it's working perfectly...

My question : Why ?

Answer 1

Because if you call randomize more than once in a millisecond (which is rather likely at current CPU clock speeds), you are seeding the RNG with the same value. This is guaranteed to produce the same output from the RNG.

Random-number generators are only meant to be seeded once. Seeding them multiple times does not make the output extra random, and in fact (as you found) may make it much less random.

Answer 2

If you make the call fast enough the value of QTime::currentTime().msec() will not change, and you're basically re-seeding qsrand with the same seed, causing the next random number generated to be the same as the prior one.