3 ms·
I was curious about this. On the one hand, comparing doubles with == is rarely a good idea but, on the other hand, your explanation seems valid. After some te
by WantonQuantum 1y ago
I was curious about this. On the one hand, comparing doubles with == is rarely a good idea but, on the other hand, your explanation seems valid.
After some testing I discovered a problem but not with the comparison. The problem is with calculating the halfway value. There are some doubles where their difference cannot be represented as a double:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <float.h>
double random_real(double low, double high, int (*random_bit)(void)) {
if (high < low)
return random_real(high, low, random_bit);
double halfway, previous = low;
while (1) {
halfway = low + (high - low) / 2;
if (halfway == previous)
break;
if (random_bit() & 1)
low = halfway;
else
high = halfway;
previous = halfway;
}
return halfway;
}
int main(int argc, char *argv[]) {
srand(time(NULL));
for (int i = 0; i < 1000000; i++) {
double r = random_real(-DBL_MAX, DBL_MAX, rand);
printf("%f\n", r);
}
}
- spyrja 1y agoActually, the problem is that the algorithm is having to calculate DBL_MAX+DBL_MAX, which of course is going to exceed the maximum value for a double-precision number (by definition). That isn't a very realistic use-case either, but in any case you could just clamp the inputs like so: double random_real(double low, double high, int (*random_bit)(void)) { if (high < low) return random_real(high, low, random_bit); const double max = DBL_MAX / 2; if (high > max) high = max; const double min = -max; if (low < min) low = min; double halfway, previous = low; while (1) { halfway = low + (high - low) / 2; if (halfway == previous) break; if (random_bit() & 1) low = halfway; else high = halfway; previous = halfway; } return halfway; }
- WantonQuantum 1y agoFixed it! #include <stdio.h> #include <stdlib.h> #include <time.h> #include <float.h> double random_real(double low, double high, int (*random_bit)(void)) { if (high < low) return random_real(high, low, random_bit); double halfway, previous = low; while (1) { halfway = low + (high/2 - low/2); if (halfway == previous) break; if (random_bit() & 1) low = halfway; else high = halfway; previous = halfway; } return halfway; } int main(int argc, char *argv[]) { srand(time(NULL)); for (int i = 0; i < 1000000; i++) { double r = random_real(-DBL_MAX, DBL_MAX, rand); printf("%f\n", r); } }
- spyrja 1y agoYep, that works too. You likely won't ever need a random number that large, of course, but if you did that would be the way to do it.