Using a random number to generate a series of repeatable numbers in C++

Question

I have an unsigned int storing a random number. I need to use this number to generate a series of "random like" numbers and have it generate those exact same numbers if this original random number turns up again.

i.e.

Random number: 123456789

First "random like number": 3

Second "random like number": 7

Third "random like number": 1

Fourth "random like number": 9

and so on.

Next time that random number comes around, the exact same numbers need to be generated from it.

The non random numbers can be duplicated and/or wrap around (to start again if whatever algorithm is used runs out of numbers, it can start again from the beginning). Just as long as each time, those same numbers are generated.

I realise this is exactly what rand() does (seeding with the random number), however I cannot use rand. So perhaps a more concise question would be how do I replicate a "mini" rand function (it doesn't have to be anything complicated, just so long as the numbers coming out look somewhat randomised).

Also I cannot use boost (unfortunately).

Any pointers/tips?

look somewhat randomised is too vague. What is your criterion? Also, how long do you want the sequence to be before it repeats? — Björn Pollex

paxdiablo paxdiablo · Accepted Answer · 2011-10-31T12:27:17

I don't know exactly why you cannot use rand itself but, if not, then you could just roll your own. Use your integer to seed a number then use the standard X_n+1 = a * X_n + c mod m.

The Wikipedia page for linear congruential method has some sample values for a, c and m.

Now LCM isn't the greatest random number generator in the world but, if you just want numbers that "look somewhat randomised", it should be sufficient.

As an example of an LCM algorithm, the following function, based on the Microsoft Visual/Quick C/C++ entry from that linked page above:

// LCM pseudo-random number generator.
// Outputs numbers from 0..32767 inclusive.
// With protection against identical sequences.
//   due to full 32-bit cycle but only returning 15 bits.

uint32_t myRand (void) {
    const static uint32_t a = 214013U;
    const static uint32_t c = 2531011U;
    // m is, of course, 2^32 since the values will wrap.

    static uint32_t seed = 1;
    seed = seed * a + c;
    return (seed >> 16) & 0x7FFF;
}

has a cycle time of the full range of 32-bit numbers but only returns certain bits of the seed each time, which reduces the appearance of identical sequences.

If you want a C++ class to do this so that all random number generators are independent of each other (as suggested by TomZ), you can use something like:

class myRand {
    public:
        myRand ();
        myRand (unsigned int newSeed);
        ~myRand ();
        void setSeed (unsigned int newSeed);
        unsigned int getSeed (void);
        unsigned int next (void);
    private:
        unsigned int seed;
        const static unsigned int a = 214013U;
        const static unsigned int c = 2531011U;
};

myRand::myRand () { seed = 1; }
myRand::myRand (unsigned int newSeed) { setSeed (newSeed); }
myRand::~myRand () { }
void myRand::setSeed (unsigned int newSeed) { seed = newSeed; }
unsigned int myRand::getSeed (void) { return seed; }
unsigned int myRand::next (void) {
    seed = (seed * a + c) & 0xffffffff;
    return (seed >> 16) & 0x7fff;
}

#include <iostream>

int main (void) {
    myRand r (5);

    std::cout << r.next() << "\n";          //    54
    std::cout << r.next() << "\n";          // 28693

    unsigned int saveSeed = r.getSeed();
    std::cout << r.next() << "\n";          // 12255
    std::cout << r.next() << "\n";          // 24449

    r.setSeed (saveSeed);
    std::cout << r.next() << "\n";          // 12255
    std::cout << r.next() << "\n";          // 24449

    return 0;
}

Using a random number to generate a series of repeatable numbers in C++

4 Answers