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

#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