How do I scale down numbers from rand()?

Question

The following code outputs a random number each second:

int main ()
{
    srand(time(NULL)); // Seeds number generator with execution time.

    while (true)         


        

Answer 1

You can do

cout << rawRand % 100 << endl; // Outputs between 0 and 99

cout << rawRand % 101 << endl; // outputs between 0 and 100

For the people downvoting; note one minute after this was originally posted I left the comment:

From http://www.cplusplus.com/reference/clibrary/cstdlib/rand "Notice though that this modulo operation does not generate a truly uniformly distributed random number in the span (since in most cases lower numbers are slightly more likely), but it is generally a good approximation for short spans."

With 64-bit ints and using 100 numbers as output, the numbers 0-16 are represented with 1.00000000000000000455 % of the numbers (an relative accuracy to identically distributed of 1% by about 10^-18), while the numbers 17-99 are represented with 0.99999999999999999913 % of the numbers. Yes, not perfectly distributed, but a very good approximation for small spans.

Also note, where does the OP ask for identically distributed numbers? For all we know these are being used for purposes where a small deviations doesn't matter (e.g., anything other than cryptography -- and if they are using the numbers for cryptography this question is much too naive for them to be writing their own cryptography).

EDIT - For people who are truly concerned with having a uniform distribution of random numbers the following code works. Note this isn't necessarily optimal as with 64-bit random ints, it will require two calls of rand() once every 10^18 calls.

unsigned N = 100; // want numbers 0-99
unsigned long randTruncation = (RAND_MAX / N) * N; 
// include every number the N times by ensuring rawRand is between 0 and randTruncation - 1 or regenerate.
unsigned long rawRand = rand();

while (rawRand >= randTruncation) {
    rawRand = rand();  
// with 64-bit int and range of 0-99 will need to generate two random numbers
// about 1 in every (2^63)/16 ~ 10^18 times (1 million million times)

// with 32-bit int and range of 0-99 will need to generate two random numbers 
// once every 46 million times.

}
cout << rawRand % N << stdl::endl;