Generating random numbers in C

Question

While searching for Tutorials on generating random numbers in C I found This Topic

When I try to use the rand() function with parameters, I always get the random number generated 0. When I try to use the rand() function with parameters, I always get the value 41.

And whenever I try to use arc4random() and random() functions, I get a LNK2019 error.

Here's what I'm doing:

#include <stdlib.h>
int main()
{
int x;
x = rand(6);
printf("%d", x);
}

This code always generate 41.

Where am I going wrong??

P.S. : I'm running Windows XP SP3 and using VS2010 Command Prompt as compiler. P.P.S. : Took me 15 minutes to learn how to format properly.

Answer 1

You should call srand() before calling rand to initialize the random number generator.

Either call it with a specific seed, and you will always get the same pseudo-random sequence

#include <stdlib.h>

int main ()
{
  srand ( 123 );
  int random_number = rand();
  return 0;
}

or call it with a changing sources, ie the time function

#include <stdlib.h>
#include <time.h>

int main ()
{
  srand ( time(NULL) );
  int random_number = rand();
  return 0;
}

In response to Moon's Comment rand() generates a random number with an equal probability between 0 and RAND_MAX (a macro pre-defined in stdlib.h)

You can then map this value to a smaller range, e.g.

int random_value = rand(); //between 0 and RAND_MAX

//you can mod the result
int N = 33;
int rand_capped = random_value % N;  //between 0 and 32
int S = 50;
int rand_range = rand_capped + S; //between 50 and 82

//you can convert it to a float
float unit_random = random_value / (float) RAND_MAX; //between 0 and 1 (floating point)

This might be sufficient for most uses, but its worth pointing out that in the first case using the mod operator introduces a slight bias if N does not divide evenly into RAND_MAX+1.

Random number generators are interesting and complex, it is widely said that the rand() generator in the C standard library is not a great quality random number generator, read (http://en.wikipedia.org/wiki/Random_number_generation for a definition of quality).

http://en.wikipedia.org/wiki/Mersenne_twister (source http://www.math.sci.hiroshima-u.ac.jp/~m-mat/MT/emt.html ) is a popular high quality random number generator.

Also, I am not aware of arc4rand() or random() so I cannot comment.

Answer 2

You need to seed your PRNG so it starts with a different value each time.

A simple but low quality seed is to use the current time:

srand(time(0));

This will get you started but is considered low quality (i.e. for example, don't use that if you are trying to generate RSA keys).

Background. Pseudo-random number generators do not create true random number sequences but just simulate them. Given a starting point number, a PRNG will always return the same sequence of numbers. By default, they start with the same internal state so will return the same sequence.

To not get the same sequence, you change the internal state. The act of changing the internal state is called "seeding".

Answer 3

You first need to seed the generator because it doesn't generate real random numbers!

Try this:

#include <stdlib.h>
#include <time.h>
int main()
{
    // random seed, time!
    srand( time(NULL) ); // hackish but gets the job done.
    int x;
    x = rand(); // everytime it is different because the seed is different.
    printf("%d", x);
}

Answer 4

#include <stdlib.h>

int main()
{
    int x;
    x = rand(6);
    printf("%d", x);
}

Especially as a beginner, you should ask your compiler to print every warning about bad code that it can generate. Modern compilers know lots of different warnings which help you to program better. For example, when you compile this program with the GNU C Compiler:

$ gcc -W -Wall rand.c
rand.c: In function `main':
rand.c:5: error: too many arguments to function `rand'
rand.c:6: warning: implicit declaration of function `printf'

You get two warnings here. The first one says that the rand function only takes zero arguments, not one as you tried. To get a random number between 0 and n, you can use the expression rand() % n, which is not perfect but ok for small n. The resulting random numbers are normally not evenly distributed; smaller values are returned more often.

The second warning tells you that you are calling a function that the compiler doesn't know at that point. You have to tell the compiler by saying #include <stdio.h>. Which include files are needed for which functions is not always simple, but asking the Open Group specification for portable operating systems works in many cases: http://www.google.com/search?q=opengroup+rand.

These two warnings tell you much about the history of the C programming language. 40 years back, the definition of a function didn't include the number of parameters or the types of the parameters. It was also ok to call an unknown function, which in most cases worked. If you want to write code today, you should not rely on these old features but instead enable your compiler's warnings, understand the warnings and then fix them properly.

Answer 5

Also, linear congruential PRNGs tend to produce more randomness on the higher bits that on the lower bits, so to cap the result don't use modulo, but instead use something like:

j = 1 + (int) (10.0 * (rand() / (RAND_MAX + 1.0)));

(This one is from "Numerical Recipes in C", ch.7)