Generating random numbers in C -

while searching tutorials on generating random numbers in c found this topic

when try use rand() function without parameters, 0. when try use rand() function parameters, value 41. , whenever try use arc4random() , random() functions, lnk2019 error.

here's i've done:

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

this code generates 41. going wrong? i'm running windows xp sp3 , using vs2010 command prompt compiler.

you should call srand() before calling rand initialize random number generator.

either call specific seed, , same pseudo-random sequence

#include <stdlib.h>  int main () {   srand ( 123 );   int random_number = rand();   return 0; } 

or call changing sources, ie time function

#include <stdlib.h> #include <time.h>  int main () {   srand ( time(null) );   int random_number = rand();   return 0; } 

in response moon's comment rand() generates random number equal probability between 0 , rand_max (a macro pre-defined in stdlib.h)

you can map value smaller range, e.g.

int random_value = rand(); //between 0 , rand_max  //you can mod result int n = 33; int rand_capped = random_value % n;  //between 0 , 32 int s = 50; int rand_range = rand_capped + s; //between 50 , 82  //you can convert float float unit_random = random_value / (float) rand_max; //between 0 , 1 (floating point) 

this might sufficient uses, worth pointing out in first case using mod operator introduces slight bias if n not divide evenly rand_max+1.

random number generators interesting , complex, said rand() generator in c standard library not great quality random number generator, read (http://en.wikipedia.org/wiki/random_number_generation definition of quality).

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

also, not aware of arc4rand() or random() cannot comment.