org.apache.commons.math3.distribution

## Class ZipfDistribution

- java.lang.Object
- - org.apache.commons.math3.distribution.AbstractIntegerDistribution
  - - org.apache.commons.math3.distribution.ZipfDistribution

All Implemented Interfaces:
  java.io.Serializable, IntegerDistribution

    public class ZipfDistribution
    extends AbstractIntegerDistribution

Implementation of the Zipf distribution.

Version:
  $Id: ZipfDistribution.java 1416643 2012-12-03 19:37:14Z tn $
See Also:
  Zipf distribution (MathWorld), Serialized Form

### Field Summary

Fields
`private double` `exponent` Exponent parameter of the distribution.
`private int` `numberOfElements` Number of elements.
`private double` `numericalMean` Cached numerical mean
`private boolean` `numericalMeanIsCalculated` Whether or not the numerical mean has been calculated
`private double` `numericalVariance` Cached numerical variance
`private boolean` `numericalVarianceIsCalculated` Whether or not the numerical variance has been calculated
`private static long` `serialVersionUID` Serializable version identifier.

### Fields inherited from class org.apache.commons.math3.distribution.AbstractIntegerDistribution

`random, randomData`

### Constructor Summary

Constructors
Constructor and Description
`ZipfDistribution(int numberOfElements, double exponent)` Create a new Zipf distribution with the given number of elements and exponent.
`ZipfDistribution(RandomGenerator rng, int numberOfElements, double exponent)` Creates a Zipf distribution.

### Method Summary

Methods
`protected double` `calculateNumericalMean()` Used by `getNumericalMean()`.
`protected double` `calculateNumericalVariance()` Used by `getNumericalVariance()`.
`double` `cumulativeProbability(int x)` For a random variable `X` whose values are distributed according to this distribution, this method returns `P(X <= x)`.
`private double` `generalizedHarmonic(int n, double m)` Calculates the Nth generalized harmonic number.
`double` `getExponent()` Get the exponent characterizing the distribution.
`int` `getNumberOfElements()` Get the number of elements (e.g.
`double` `getNumericalMean()` Use this method to get the numerical value of the mean of this distribution.
`double` `getNumericalVariance()` Use this method to get the numerical value of the variance of this distribution.
`int` `getSupportLowerBound()` Access the lower bound of the support.
`int` `getSupportUpperBound()` Access the upper bound of the support.
`boolean` `isSupportConnected()` Use this method to get information about whether the support is connected, i.e.
`double` `probability(int x)` For a random variable `X` whose values are distributed according to this distribution, this method returns `P(X = x)`.

### Methods inherited from class org.apache.commons.math3.distribution.AbstractIntegerDistribution

`cumulativeProbability, inverseCumulativeProbability, reseedRandomGenerator, sample, sample, solveInverseCumulativeProbability`

### Methods inherited from class java.lang.Object

`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

### Field Detail

#### serialVersionUID

    private static final long serialVersionUID

Serializable version identifier.

See Also:
  Constant Field Values

#### numberOfElements

    private final int numberOfElements

Number of elements.

#### exponent

    private final double exponent

Exponent parameter of the distribution.

#### numericalMean

    private double numericalMean

Cached numerical mean

#### numericalMeanIsCalculated

    private boolean numericalMeanIsCalculated

Whether or not the numerical mean has been calculated

#### numericalVariance

    private double numericalVariance

Cached numerical variance

#### numericalVarianceIsCalculated

    private boolean numericalVarianceIsCalculated

Whether or not the numerical variance has been calculated

### Constructor Detail

#### ZipfDistribution

    public ZipfDistribution(int numberOfElements,
                    double exponent)

Create a new Zipf distribution with the given number of elements and exponent.

Parameters:
  `numberOfElements` - Number of elements.
  `exponent` - Exponent.
Throws:
  `NotStrictlyPositiveException` - if `numberOfElements <= 0` or `exponent <= 0`.

#### ZipfDistribution

    public ZipfDistribution(RandomGenerator rng,
                    int numberOfElements,
                    double exponent)
                     throws NotStrictlyPositiveException

Creates a Zipf distribution.

Parameters:
  `rng` - Random number generator.
  `numberOfElements` - Number of elements.
  `exponent` - Exponent.
Throws:
  `NotStrictlyPositiveException` - if `numberOfElements <= 0` or `exponent <= 0`.
Since:
  3.1

### Method Detail

#### getNumberOfElements

    public int getNumberOfElements()

Get the number of elements (e.g. corpus size) for the distribution.

Returns:
  the number of elements

#### getExponent

    public double getExponent()

Get the exponent characterizing the distribution.

Returns:
  the exponent

#### probability

    public double probability(int x)

For a random variable `X` whose values are distributed according to this distribution, this method returns `P(X = x)`. In other words, this method represents the probability mass function (PMF) for the distribution.

Parameters:
  `x` - the point at which the PMF is evaluated
Returns:
  the value of the probability mass function at `x`

#### cumulativeProbability

    public double cumulativeProbability(int x)

For a random variable `X` whose values are distributed according to this distribution, this method returns `P(X <= x)`. In other words, this method represents the (cumulative) distribution function (CDF) for this distribution.

Parameters:
  `x` - the point at which the CDF is evaluated
Returns:
  the probability that a random variable with this distribution takes a value less than or equal to `x`

#### getNumericalMean

    public double getNumericalMean()

Use this method to get the numerical value of the mean of this distribution. For number of elements `N` and exponent `s`, the mean is `Hs1 / Hs`, where

- `Hs1 = generalizedHarmonic(N, s - 1)`,
- `Hs = generalizedHarmonic(N, s)`.

Returns:
  the mean or `Double.NaN` if it is not defined

#### calculateNumericalMean

    protected double calculateNumericalMean()

Used by `getNumericalMean()`.

Returns:
  the mean of this distribution

#### getNumericalVariance

    public double getNumericalVariance()

Use this method to get the numerical value of the variance of this distribution. For number of elements `N` and exponent `s`, the mean is `(Hs2 / Hs) - (Hs1^2 / Hs^2)`, where

- `Hs2 = generalizedHarmonic(N, s - 2)`,
- `Hs1 = generalizedHarmonic(N, s - 1)`,
- `Hs = generalizedHarmonic(N, s)`.

Returns:
  the variance (possibly `Double.POSITIVE_INFINITY` or `Double.NaN` if it is not defined)

#### calculateNumericalVariance

    protected double calculateNumericalVariance()

Used by `getNumericalVariance()`.

Returns:
  the variance of this distribution

#### generalizedHarmonic

    private double generalizedHarmonic(int n,
                             double m)

Calculates the Nth generalized harmonic number. See Harmonic Series.

Parameters:
  `n` - Term in the series to calculate (must be larger than 1)
  `m` - Exponent (special case `m = 1` is the harmonic series).
Returns:
  the nth generalized harmonic number.

#### getSupportLowerBound

    public int getSupportLowerBound()

Access the lower bound of the support. This method must return the same value as `inverseCumulativeProbability(0)`. In other words, this method must return

`inf {x in Z | P(X <= x) > 0}`.

The lower bound of the support is always 1 no matter the parameters.

Returns:
  lower bound of the support (always 1)

#### getSupportUpperBound

    public int getSupportUpperBound()

Access the upper bound of the support. This method must return the same value as `inverseCumulativeProbability(1)`. In other words, this method must return

`inf {x in R | P(X <= x) = 1}`.

The upper bound of the support is the number of elements.

Returns:
  upper bound of the support

#### isSupportConnected

    public boolean isSupportConnected()

Use this method to get information about whether the support is connected, i.e. whether all integers between the lower and upper bound of the support are included in the support. The support of this distribution is connected.

Returns:
  `true`