I need to generate random numbers in C [duplicate]

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I need high quality random numbers in C, but I have no idea what to really do. I need to be able to get numbers from 1-100. Any help or maybe point me to where I can find help.

Answer 1

By "fair distribution", I assume you mean that you're not generally satisfied by rand(). In this case, you should probably use OS-specific methods that produce cryptographically secure random numbers - /dev/random or /dev/urandom (depending on your needs) on Unix, and CryptGenRandom or RtlGetRandom on Win32. On Win32 specifically, if you're using VS2005 or later, you may just use rand_s.

Answer 2

As an high-quality random number generator, please do not use rand(), or not-Quality-Assured code. It is extremely easy to generate random number incorrectly (check Knuth's funny story in "The art of Computer Programming: Seminumerical Algorithms"). The easiest way to proceed is maybe to use the generators in GNU's scientific library. It installs with one click on *nux, and there are several ports of the GSL to Windows. I've used it. It is easy to call and works well.

Answer 3

You can try something like this:

main()
{
  srand(time(0));

  for(int i=0;i<1000;++i)
    printf("%f ", ((float)rand())/RAND_MAX*99+1);

  return 0;
}

It's as uniform a distribution as the standard rand() can give.

Answer 4

The standard C library has rand which will probably be sufficient, unless you have a need for a prng with a particular statistical distribution..

Answer 5

This is the simplest method of producing uniformly distributed random numbers in C:

Step 1. Be sure to include the standard library header to get the necessary function prototypes

#include <stdlib.h>

Step 2. Seed the random number generator using srand(). The seed determines where the random numbers start. The sequence of random numbers will always be exactly the same for a given seed. This allows you to have random, yet reproducible results. If you don't need it to be reproducible, a good thing to seed with is the current time, so that the random sequence will be different on each run.

srand(time(NULL));

(be sure to include time.h if you do this). Also, only seed the generator once per program run unless you are generating a huge number (millions or billions) of random numbers. Seeding frequently makes the sequence less random.

Step 3. Get your random number.

rand()

This function returns a random number between 0 and RAND_MAX, which is a macro that is defined as a rather large integer.

Step 4. Get your random number into the range you want. The general formula for doing so is this:

int random_number = rand() % range + min;

Where range is how many (consecutive) numbers you want to choose from, and min is the smallest of these. So to generate a number between 1 and 100, range is 100 and min is 1:

int random_number = rand() % 100 + 1;

Some people object to this formula because it uses the low-order bits of the number given by rand(), and in older implementations of software pseudo-random number generators these were often less random than the high order bits, but on any modern system this method should be perfectly fine.

Answer 6

The other posts have good advice. If you really want to dive into the guts of random number generation, take a look at Numerical Recipes in C. Start with Chapter 7.