edu.northwestern.at.utils.math

Class Probability

java.lang.Object

edu.northwestern.at.utils.math.Probability

All Implemented Interfaces:

XCloneable, java.io.Serializable, java.lang.Cloneable, java.lang.Comparable

public class Probability
extends java.lang.Object
implements XCloneable, java.lang.Comparable, java.io.Serializable

Probability: stores and operates on log probabilties.

When working with large numbers of small probabilities, it is helpful to work in "log space", e.g., the natural log of the probabilities, to avoid problems with overflow and underflow. This class provides for storing a probability in log form, and adding, substracting, multiplying, and dividing the log probabilities to minimize accuracy loss.

Probabilities must be floating point values from zero through one inclusive, e.g., [0 , 1].

A probability of zero is stored as -infinity, which correctly propagates through arithmetic operations. The value of exp(-infinity) is 0.0 as it should be.

See Also:

Serialized Form

Field Summary

Fields

Modifier and Type	Field and Description
protected double	logProbability Natural log of the probability.
static Probability	ONE_PROBABILITY One probability.
static Probability	ZERO_PROBABILITY Zero probability.

Constructor Summary

Constructors

Constructor and Description
Probability() Create a Probability.
Probability(double probability) Create a Probability.

Method Summary

Methods

Modifier and Type	Method and Description
java.lang.Object	clone() Get clone of this object.
int	compareTo(java.lang.Object object) Compare this probability to another.
boolean	equals(java.lang.Object object) Test for this probability equaling another.
double	getLogProbability() Get log probability.
double	getProbability() Get probability.
int	hashCode() Get a hash code for this object.
Probability	multiply(Probability otherProbability) Multiply this probability by another.
Probability	multiply(Probability otherProbability1, Probability otherProbability2) Multiply this probability by two others.
Probability	multiply(Probability otherProbability1, Probability otherProbability2, Probability otherProbability3) Multiply this probability by three others.
void	setLogProbability(double logProbability) Set the log probability.
void	setProbability(double probability) Set the probability.
java.lang.String	toString() Return string version of probability.

Methods inherited from class java.lang.Object

finalize, getClass, notify, notifyAll, wait, wait, wait

Field Detail

logProbability

protected double logProbability

Natural log of the probability.

ZERO_PROBABILITY

public static final Probability ZERO_PROBABILITY

Zero probability.

ONE_PROBABILITY

public static final Probability ONE_PROBABILITY

One probability.

Constructor Detail

Probability

public Probability()
            throws java.lang.IllegalArgumentException

Create a Probability.

Throws:

java.lang.IllegalArgumentException

Probability

public Probability(double probability)
            throws java.lang.IllegalArgumentException

Create a Probability.

Parameters:

probability - Probability value in range 0 through 1 inclusive.

Throws:

java.lang.IllegalArgumentException

Method Detail

setProbability

public void setProbability(double probability)
                    throws java.lang.IllegalArgumentException

Set the probability.

Parameters:

probability - Probability value in range 0 through 1 inclusive.

Throws:

java.lang.IllegalArgumentException - If probability is not in the range 0 through 1 inclusive.

setLogProbability

public void setLogProbability(double logProbability)
                       throws java.lang.IllegalArgumentException

Set the log probability.

Parameters:

logProbability - Log probability value in range -infinity through 0 inclusive.

Throws:

java.lang.IllegalArgumentException - If probability is not in the range -infinity through 0 inclusive.

getProbability

public double getProbability()

Get probability.

Returns:

probability from 0 through 1 inclusive.

Note: This can overflow!

getLogProbability

public double getLogProbability()

Get log probability.

Returns:

log probability from -infinity through 0 inclusive.

multiply

public Probability multiply(Probability otherProbability)

Multiply this probability by another.

Parameters:

otherProbability - The other probability.

Returns:

Resulting probability.

multiply

public Probability multiply(Probability otherProbability1,
                   Probability otherProbability2)

Multiply this probability by two others.

Parameters:

otherProbability1 - First other probability.

otherProbability2 - Second other probability.

Returns:

Resulting probability.

multiply

public Probability multiply(Probability otherProbability1,
                   Probability otherProbability2,
                   Probability otherProbability3)

Multiply this probability by three others.

Parameters:

otherProbability1 - First other probability.

otherProbability2 - Second other probability.

otherProbability3 - Third other probability.

Returns:

Resulting probability.

toString

public java.lang.String toString()

Return string version of probability.

Overrides:

toString in class java.lang.Object

Returns:

Probability converted to string.

clone

public java.lang.Object clone()

Get clone of this object.

Specified by:

clone in interface XCloneable

Overrides:

clone in class java.lang.Object

Returns:

Clone of this object.

compareTo

public int compareTo(java.lang.Object object)

Compare this probability to another.

Specified by:

compareTo in interface java.lang.Comparable

Parameters:

object - The other probability.

equals

public boolean equals(java.lang.Object object)

Test for this probability equaling another.

Overrides:

equals in class java.lang.Object

Parameters:

object - Other object, presumably a Probability.

Returns:

true if other object is equal to this one.

hashCode

public int hashCode()

Get a hash code for this object.

Overrides:

hashCode in class java.lang.Object

Returns:

The hash code.

Hash code same as for a Double.