C Interview Questions and Answers

 

How can I generate floating-point random numbers?

drand48 is a Unix System V routine which returns floating point random numbers (presumably
with 48 bits of precision) in the half-open interval [0, 1). (Its companion seed
routine is srand48; neither is in the C Standard.) It's easy to write a low-precision
replacement:



#include <stdlib.h>



double drand48()



{



return rand() / (RAND_MAX + 1.); }





To more accurately simulate drand48's semantics, you can try to give it closer
to 48 bits worth of precision:



#define PRECISION 2.82e14 /* 2**48, rounded up */



double drand48()



{



double x = 0;



double denom = RAND_MAX + 1.;



double need;





for(need = PRECISION; need > 1;



need /= (RAND_MAX + 1.)) {



x += rand() / denom;



denom *= RAND_MAX + 1.;



}





return x;



}





Before using code like this, though, beware that it is numerically suspect, particularly
if (as is usually the case) the period of rand is on the order of RAND_MAX. (If
you have a longer-period random number generator available, such as BSD random,
definitely use it when simulating drand48.)

Posted by:Richards