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进行双值相等比较时,epsilon值应该是什么?

java

Cheok Yan Cheng · 技术社区 · 14 年前

下面是下面程序的输出。

value is : 2.7755575615628914E-17
Double.compare with zero : 1
isEqual with zero : true

我的问题是,什么应该是epsilon值?是否有任何可靠的方法来获得价值,而不是从天空中挑选一个数字。

package sandbox;

/**
 *
 * @author yccheok
 */
public class Main {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        double zero = 1.0/5.0 + 1.0/5.0 - 1.0/10.0 - 1.0/10.0 - 1.0/10.0 - 1.0/10.0;
        System.out.println("value is : " + zero);
        System.out.println("Double.compare with zero : " + Double.compare(zero, 0.0));
        System.out.println("isEqual with zero : " + isEqual(zero, 0.0));
    }

    public static boolean isEqual(double d0, double d1) {
        final double epsilon = 0.0000001;
        return d0 == d1 ? true : Math.abs(d0 - d1) < epsilon;
    }
}

6 回复 | 直到 7 年前

mob 14 年前

public static void main(String[] args) {
    double z = 0.0;
    double x = 0.23;
    double y = 1.0 / x;
    int N = 50000;
    for (int i = 0; i < N; i++) {
        z += x * y - 1.0;
    }
    System.out.println("z should be zero, is " + z);
}

~5.55E-12 N

Alexandre C. 12 年前

bool fuzzyEquals(double a, double b)
{
    return abs(a - b) < eps * max(abs(a), abs(b));
}

Jerry Coffin 14 年前

isEqual isNearlyEQual isEqual(a, c) isEqual(a, b) isEqual(b, c)

Hejazi 10 年前

isEqual

epsilon = Math.max(Math.ulp(d0), Math.ulp(d1))

http://docs.oracle.com/javase/6/docs/api/java/lang/Math.html#ulp%28double%29

Richard Gomes 7 年前

Double.ulp()
double d Double.ulp(d)

disastrous examples here

doubles floats

fixed point arithmetic

Geoff Reedy 7 年前

https://randomascii.wordpress.com/2012/02/25/comparing-floating-point-numbers-2012-edition/

java.lang.Double#doubleToLongBits java.lang.Float#floatToIntBits ulp(double) ulp(float) nextUp(double) nextUp(float) nextAfter(double, double) nextAfter(float, float)