Standard rand()
function gives numbers not big enough for me: I need unsigned long long
ones. How do we get really big random numbers? I tried modifying a simple hash function but it's too big, takes too long to run and never produces numbers which are less than 1e5!!
Here's a portable C99 solution that returns a random 64-bit number:
unsigned long long llrand() {
unsigned long long r = 0;
for (int i = 0; i < 5; ++i) {
r = (r << 15) | (rand() & 0x7FFF);
}
return r & 0xFFFFFFFFFFFFFFFFULL;
}
Explanation: rand()
returns integers in the range 0 to RAND_MAX
and RAND_MAX
is only guaranteed to be at least 32,767 (15 random bits). long long
is guaranteed to have 64 bits but may be larger.