Given a function that generates random number from 1-7, write a function that generates random numbers from 1-10.

Question: You are given a function rand7() – that generates random numbers from 1-7. Write a function rand10() – that uses rand7() to generate random numbers from 1-10.

This appear to be one of those probabilistic analysis questions. You should be familiar with the concept of expected value, as it could be extremely helpful in probabilistic analysis.

Hint:
Assume you could generate a random integer in the range 1 to 49. How would you generate a random integer in the range of 1 to 10? What would you do if the generated number is in the desired range? What if it’s not?

Solution:
This solution is based upon Rejection Sampling. The main idea is when you generate a number in the desired range, output that number immediately. If the number is out of the desired range, reject it and re-sample again. As each number in the desired range has the same probability of being chosen, a uniform distribution is produced.

Obviously, we have to run rand7() function at least twice, as there are not enough numbers in the range of 1 to 10. By running rand7() twice, we can get integers from 1 to 49 uniformly. Why?

A table is used to illustrate the concept of rejection sampling. Calling rand7() twice will get us row and column index that corresponds to a unique position in the table above. Imagine that you are choosing a number randomly from the table above. If you hit a number, you return that number immediately. If you hit a *, you repeat the process again until you hit a number.

Since 49 is not a multiple of tens, we have to use rejection sampling. Our desired range is integers from 1 to 40, which we can return the answer immediately. If not (the integer falls between 41 to 49), we reject it and repeat the whole process again.

int rand10()
{
    int row, col, idx;
    do
    {
        row = rand7();
        col = rand7();
        idx = col + (row-1)*7;
    }
    while (idx > 40);

    return 1 + (idx-1)%10;
}