C++ random number generator

Question

I am writing an lottory application. I have a function called generateLotteryNumbers this takes in an array and fills that array with 5 random numbers. What I want to do is have this function produce a different set of random numbers every time this function is called.

void generateLotteryNumbers(int lotteryNumbers[])
{

    srand (time(NULL));
    const int arraySize = 5;
    int index = 0;

    while (index < arraySize)
    {
        lotteryNumbers[index] = rand() % 50 + 1;
        cout << lotteryNumbers[index] << endl;
        index++;
    }
}

The output at the moment is eg:

Repeated twice.

http://channel9.msdn.com/Events/GoingNative/2013/rand-Considered-Harmful <- why you shouldn't use rand() and srand(). And why you should avoid "rand() % N". And srand(time(NULL))... — Exceptyon, Nov 07 '13 at 13:44
You may take a look at the [c++11 ](http://en.cppreference.com/w/cpp/numeric/random) features that provide better pseudo-random number generator (e.g. Mersenne Twister ) as well as a `random_device` interface that may be used for seeding the generator with an hardware entropy source. — zakinster, Nov 07 '13 at 13:52

score 6 · Accepted Answer · answered Nov 07 '13 at 13:45

6

Call srand exactly once, usually early in the code in main.

answered Nov 07 '13 at 13:45

Pete Becker

74,985
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score 3 · Answer 2 · answered Nov 07 '13 at 13:51

3

Width C++11 standard, you can use new number generators. To obtain always different results, usually you must set a different seed at every program execution, for example with time.

answered Nov 07 '13 at 13:51

Jepessen

11,744
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score 1 · Answer 3 · answered Nov 07 '13 at 14:02

You may take a look at the c++11 features that provide better pseudo-random number generator (e.g. Mersenne Twister ) as well as a random_device interface that may be used for seeding the generator with an hardware entropy source.

Example with std::vector and c++11 <random> features:

vector<int> generateLotteryNumbers(int size)
{
    static std::random_device rseed;
    static mt19937 rgen(rseed());
    uniform_int_distribution<int> idist(1,50); 

    vector<int> result;
    for(int i = 0; i < size; ++i) {
        result.push_back(idist(rgen));
        cout << result[i] << endl;
    }
    return result;
}

Also note that if you're generating lottery numbers, you may not want the same value twice in the same array, in which case, you'll have to add a bit more logic to your code.

score 0 · Answer 4 · answered Nov 07 '13 at 14:23

You should not call srand multiple times. Furthermore you should not use rand this way to generate lottery numbers (ok, depends on the lottery but I think duplicate numbers are not allowed). You can do it very easy using std::random_shuffle

int main()
{
    int numbers[49];  // all numbers
    std::iota(begin(numbers), end(numbers), 1);  // fill with 1-49

    // shuffle numbers
    std::random_shuffle(begin(numbers), end(numbers));  

    // use first 5 numbers:
    std::copy(begin(numbers), begin(numbers) + 5, 
        std::ostream_iterator<int>(std::cout, " "));
}

C++ random number generator

4 Answers4