Anything wrong with this function for comparing floats?
- by Michael Borgwardt
When my Floating-Point Guide was yesterday published on slashdot, I got a lot of flak for my suggested comparison function, which was indeed inadequate. So I finally did the sensible thing and wrote a test suite to see whether I could get them all to pass. Here is my result so far. And I wonder if this is really as good as one can get with a generic (i.e. not application specific) float comparison function, or whether I still missed some edge cases.
import static org.junit.Assert.assertFalse;
import static org.junit.Assert.assertTrue;
import org.junit.Test;
public class NearlyEqualsTest
{
public static boolean nearlyEqual(float a, float b)
{
final float epsilon = 0.000001f;
final float absA = Math.abs(a);
final float absB = Math.abs(b);
final float diff = Math.abs(a-b);
if (a*b==0) { // a or b or both are zero
// relative error is not meaningful here
return diff < Float.MIN_VALUE / epsilon;
} else { // use relative error
return diff / (absA+absB) < epsilon;
}
}
/** Regular large numbers - generally not problematic */
@Test public void big()
{
assertTrue(nearlyEqual(1000000f, 1000001f));
assertTrue(nearlyEqual(1000001f, 1000000f));
assertFalse(nearlyEqual(10000f, 10001f));
assertFalse(nearlyEqual(10001f, 10000f));
}
/** Negative large numbers */
@Test public void bigNeg()
{
assertTrue(nearlyEqual(-1000000f, -1000001f));
assertTrue(nearlyEqual(-1000001f, -1000000f));
assertFalse(nearlyEqual(-10000f, -10001f));
assertFalse(nearlyEqual(-10001f, -10000f));
}
/** Numbers around 1 */
@Test public void mid()
{
assertTrue(nearlyEqual(1.0000001f, 1.0000002f));
assertTrue(nearlyEqual(1.0000002f, 1.0000001f));
assertFalse(nearlyEqual(1.0002f, 1.0001f));
assertFalse(nearlyEqual(1.0001f, 1.0002f));
}
/** Numbers around -1 */
@Test public void midNeg()
{
assertTrue(nearlyEqual(-1.000001f, -1.000002f));
assertTrue(nearlyEqual(-1.000002f, -1.000001f));
assertFalse(nearlyEqual(-1.0001f, -1.0002f));
assertFalse(nearlyEqual(-1.0002f, -1.0001f));
}
/** Numbers between 1 and 0 */
@Test public void small()
{
assertTrue(nearlyEqual(0.000000001000001f, 0.000000001000002f));
assertTrue(nearlyEqual(0.000000001000002f, 0.000000001000001f));
assertFalse(nearlyEqual(0.000000000001002f, 0.000000000001001f));
assertFalse(nearlyEqual(0.000000000001001f, 0.000000000001002f));
}
/** Numbers between -1 and 0 */
@Test public void smallNeg()
{
assertTrue(nearlyEqual(-0.000000001000001f, -0.000000001000002f));
assertTrue(nearlyEqual(-0.000000001000002f, -0.000000001000001f));
assertFalse(nearlyEqual(-0.000000000001002f, -0.000000000001001f));
assertFalse(nearlyEqual(-0.000000000001001f, -0.000000000001002f));
}
/** Comparisons involving zero */
@Test public void zero()
{
assertTrue(nearlyEqual(0.0f, 0.0f));
assertFalse(nearlyEqual(0.00000001f, 0.0f));
assertFalse(nearlyEqual(0.0f, 0.00000001f));
}
/** Comparisons of numbers on opposite sides of 0 */
@Test public void opposite()
{
assertFalse(nearlyEqual(1.000000001f, -1.0f));
assertFalse(nearlyEqual(-1.0f, 1.000000001f));
assertFalse(nearlyEqual(-1.000000001f, 1.0f));
assertFalse(nearlyEqual(1.0f, -1.000000001f));
assertTrue(nearlyEqual(10000f*Float.MIN_VALUE, -10000f*Float.MIN_VALUE));
}
/**
* The really tricky part - comparisons of numbers
* very close to zero.
*/
@Test public void ulp()
{
assertTrue(nearlyEqual(Float.MIN_VALUE, -Float.MIN_VALUE));
assertTrue(nearlyEqual(-Float.MIN_VALUE, Float.MIN_VALUE));
assertTrue(nearlyEqual(Float.MIN_VALUE, 0));
assertTrue(nearlyEqual(0, Float.MIN_VALUE));
assertTrue(nearlyEqual(-Float.MIN_VALUE, 0));
assertTrue(nearlyEqual(0, -Float.MIN_VALUE));
assertFalse(nearlyEqual(0.000000001f, -Float.MIN_VALUE));
assertFalse(nearlyEqual(0.000000001f, Float.MIN_VALUE));
assertFalse(nearlyEqual(Float.MIN_VALUE, 0.000000001f));
assertFalse(nearlyEqual(-Float.MIN_VALUE, 0.000000001f));
assertFalse(nearlyEqual(1e20f*Float.MIN_VALUE, 0.0f));
assertFalse(nearlyEqual(0.0f, 1e20f*Float.MIN_VALUE));
assertFalse(nearlyEqual(1e20f*Float.MIN_VALUE, -1e20f*Float.MIN_VALUE));
}
}
