c++ - How to make Random Numbers unique

Question

I am making a random number generator. It asks how many digits the user wants to be in the number. for example it they enter 2 it will generate random numbers between 10 and 99. I have made the generator but my issue is that the numbers are not unique.

Here is my code. I am not sure why it is not generating unique number. I thought srand(time(null)) would do it.

void TargetGen::randomNumberGen()
{
srand (time(NULL));
if (intLength == 1)
{

    for (int i = 0; i< intQuantity; i++)
    {
        int min = 1;
        int max = 9;
        int number1 = rand();

        if (intQuantity > max)
        {
            intQuantity = max;
        }

        cout << number1 % max + min << "";

    }

}
else if (intLength == 2)
{
    for (int i = 0; i<intQuantity; i++)
    {
        int min = 10;
        int max = 90;
        int number1 = rand();

        if (intQuantity > max)
        {
            intQuantity = max;
        }

        cout << number1 % max + min << "";


    }

}

if (intLength == 3)
{
    for (int i = 0; i<intQuantity; i++)
    {
        int min = 100;
        int max = 900;
        int number1 = rand();

        if (intQuantity > max)
        {
            intQuantity = max;
        }

        cout << number1 % max + min << "";
    }

}
else if (intLength == 4)
{
    for (int i = 0; i<intQuantity; i++)
    {
        int min = 1000;
        int max = 9000;
        int number1 = rand();

        if (intQuantity > max)
        {
            intQuantity = max;
        }

        cout << number1 % max + min << "";
    }

}

if (intLength == 5)
{
    for (int i = 0; i<intQuantity; i++)
    {
        int min = 10000;
        int max = 90000;
        int number1 = rand();

        if (intQuantity > max)
        {
            intQuantity = max;
        }

        cout << number1 % max + min << "";
    }

}
else if (intLength == 6)
{

    for (int i = 0; i<intQuantity; i++)
    {
        int min = 100000;
        int max = 900000;
        int number1 = rand();

        if (intQuantity > max)
        {
            intQuantity = max;
        }

        cout << number1 % max + min << "";


    }

}

if (intLength == 7)
{
    for (int i = 0; i<intQuantity; i++)
    {
        int min = 1000000;
        int max = 9000000;
        int number1 = rand();

        if (intQuantity > max)
        {
            intQuantity = max;
        }

        cout << number1 % max + min << "";
    }

}
else if (intLength == 8)
{
    for (int i = 0; i <intQuantity; i++)
    {
        int min = 10000000;
        int max = 89999999;
        int number1 = rand();

        if (intQuantity > max)
        {
            intQuantity = max;
        }

        cout << number1 % max + min << "";
    }

}

if (intLength == 9)
{
    for (int i = 0; i < intQuantity; i++)
    {
        int min = 100000000;
        int max = 900000000;
        int number1 = rand();

        if (intQuantity > max)
        {
            intQuantity = max;
        }

        cout << number1 % max + min << "";
    }

}
}

Okay so I thought I figured out a way to do this without arrays but It isn't working before I switch to the fisher yates method. Can someone tell me why this isn't working? It is supposed to essentially take the random number put that into variable numGen. Then in variable b = to numgen. Just to hold what numGen used to be so when the loop goes through and generates another random number it will compare it to what the old number is and if it is not equal to it, then it will output it. If it is equal to the old number than rather than outputting it, it will deincrement i so that it will run through the loop without skipping over the number entirely. However, when I do this is infinitely loops. And I am not sure why.

if (intLength == 1) {

    for (int i = 0; i< intQuantity; ++i)
    {

        int min = 1;
        int max = 9;
        int number1 = rand();
        int numGen = number1 % max + min;


        if (intQuantity > max)
        {
            intQuantity = max;
        }

        for (int k = 0; k < 1; k++)
        {
            cout << numGen << "";
            int b = numGen;
        }
        int b = numGen;
        if (b != numGen )
        {
            cout << numGen << "";
        }
        else
        {
            i--;
        }

    }

}

Answer 1

Everyone has interesting expectations for random numbers -- apparently, you expect random numbers to be unique! If you use any good random number generator, your random numbers will never be guaranteed to be unique.

To make this most obvious, if you wanted to generate random numbers in the range [1, 2], and you were to generate two numbers, you would (normally expect to) get one of the following four possibilities with equal probability:

1, 2
2, 1
1, 1
2, 2

It does not make sense to ask a good random number generator to generate the first two, but not the last two.

Now, take a second to think what to expect if you asked to generate three numbers in the same range... 1, 2, then what??

Uniqueness, therefore, is not, and will not be a property of a random number generator.

Your specific problem may require uniqueness, though. In this case, you need to do some additional work to ensure uniqueness.

One way is to keep a tab on which numbers are already picked. You can keep them in a set, and re-pick if you get one you got earlier. However, this is effective only if you pick a small set of numbers compared to your range; if you pick most of the range, the end of the process gets ineffective.

If the number count you are going to pick corresponds to most of the range, then using an array of the range, and the using a good shuffling algorithm to shuffle the numbers around is a better solution. (The Fisher-Yates shuffle should do the trick.)