ExponentialDistribution

java.lang.Object
- smile.stat.distribution.AbstractDistribution
- - smile.stat.distribution.ExponentialDistribution

All Implemented Interfaces:

Distribution, ExponentialFamily
```
public class ExponentialDistribution
extends AbstractDistribution
implements ExponentialFamily
```
An exponential distribution describes the times between events in a Poisson process, in which events occur continuously and independently at a constant average rate. Exponential variables can also be used to model situations where certain events occur with a constant probability per unit length, such as the distance between mutations on a DNA strand. In real world scenarios, the assumption of a constant rate is rarely satisfied. But if we focus on a time interval during which the rate is roughly constant, the exponential distribution can be used as a good approximate model.
The exponential distribution may be viewed as a continuous counterpart of the geometric distribution, which describes the number of Bernoulli trials necessary for a discrete process to change state. In contrast, the exponential distribution describes the time for a continuous process to change state.
The probability density function of an exponential distribution is f(x; λ) = λe^-λx for x ≥ 0. The cumulative distribution function is given by F(x; λ) = 1 - e^{-λ x} for x ≥ 0. An important property of the exponential distribution is that it is memoryless. This means that if a random variable T is exponentially distributed, its conditional probability obeys Pr(T > s + t | T > s) = Pr(T > t) for all s, t ≥ 0.
In queuing theory, the service times of agents in a system are often modeled as exponentially distributed variables. Reliability theory and reliability engineering also make extensive use of the exponential distribution. Because of the memoryless property of this distribution, it is well-suited to model the constant hazard rate portion of the bathtub curve used in reliability theory. The exponential distribution is however not appropriate to model the overall lifetime of organisms or technical devices, because the "failure rates" here are not constant: more failures occur for very young and for very old systems.

Constructor Summary

Constructors
Constructor and Description

ExponentialDistribution(double lambda)
Constructor.

ExponentialDistribution(double[] data)
Constructor.

Constructors
Constructor and Description
`ExponentialDistribution(double lambda)` Constructor.
`ExponentialDistribution(double[] data)` Constructor.

Method Summary

All Methods Instance Methods Concrete Methods
Modifier and Type	Method and Description
`double`	`cdf(double x)` Cumulative distribution function.
`double`	`entropy()` Shannon entropy of the distribution.
`double`	`getLambda()` Returns the rate parameter.
`double`	`logp(double x)` The density at x in log scale, which may prevents the underflow problem.
`Mixture.Component`	`M(double[] x, double[] posteriori)` The M step in the EM algorithm, which depends the specific distribution.
`double`	`mean()` The mean of distribution.
`int`	`npara()` The number of parameters of the distribution.
`double`	`p(double x)` The probability density function for continuous distribution or probability mass function for discrete distribution at x.
`double`	`quantile(double p)` The quantile, the probability to the left of quantile is p.
`double`	`rand()` Generates a random number following this distribution.
`double`	`sd()` The standard deviation of distribution.
`java.lang.String`	`toString()`
`double`	`var()` The variance of distribution.

Methods inherited from class smile.stat.distribution.AbstractDistribution
inverseTransformSampling, likelihood, logLikelihood, quantile, quantile, rejection

Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

- Constructor Detail
  - ExponentialDistribution
```
public ExponentialDistribution(double lambda)
```
    Constructor.
    
    Parameters:
    
    lambda - rate parameter.
  - ExponentialDistribution
```
public ExponentialDistribution(double[] data)
```
    Constructor. Parameter will be estimated from the data by MLE.
- Method Detail
  - getLambda
```
public double getLambda()
```
    Returns the rate parameter.
  - npara
```
public int npara()
```
    Description copied from interface: Distribution
    
    The number of parameters of the distribution.
    
    Specified by:
    
    npara in interface Distribution
  - mean
```
public double mean()
```
    Description copied from interface: Distribution
    
    The mean of distribution.
    
    Specified by:
    
    mean in interface Distribution
  - var
```
public double var()
```
    Description copied from interface: Distribution
    
    The variance of distribution.
    
    Specified by:
    
    var in interface Distribution
  - sd
```
public double sd()
```
    Description copied from interface: Distribution
    
    The standard deviation of distribution.
    
    Specified by:
    
    sd in interface Distribution
  - entropy
```
public double entropy()
```
    Description copied from interface: Distribution
    
    Shannon entropy of the distribution.
    
    Specified by:
    
    entropy in interface Distribution
  - toString
```
public java.lang.String toString()
```
    Overrides:
    
    toString in class java.lang.Object
  - rand
```
public double rand()
```
    Description copied from interface: Distribution
    
    Generates a random number following this distribution.
    
    Specified by:
    
    rand in interface Distribution
  - p
```
public double p(double x)
```
    Description copied from interface: Distribution
    
    The probability density function for continuous distribution or probability mass function for discrete distribution at x.
    
    Specified by:
    
    p in interface Distribution
  - logp
```
public double logp(double x)
```
    Description copied from interface: Distribution
    
    The density at x in log scale, which may prevents the underflow problem.
    
    Specified by:
    
    logp in interface Distribution
  - cdf
```
public double cdf(double x)
```
    Description copied from interface: Distribution
    
    Cumulative distribution function. That is the probability to the left of x.
    
    Specified by:
    
    cdf in interface Distribution
  - quantile
```
public double quantile(double p)
```
    Description copied from interface: Distribution
    
    The quantile, the probability to the left of quantile is p. It is actually the inverse of cdf.
    
    Specified by:
    
    quantile in interface Distribution
  - M
```
public Mixture.Component M(double[] x,
                           double[] posteriori)
```
    Description copied from interface: ExponentialFamily
    
    The M step in the EM algorithm, which depends the specific distribution.
    
    Specified by:
    
    M in interface ExponentialFamily
    
    Parameters:
    
    x - the input data for estimation
    
    posteriori - the posteriori probability.
    
    Returns:
    
    the (unnormalized) weight of this distribution in the mixture.

Class ExponentialDistribution

Constructor Summary

Method Summary

Methods inherited from class smile.stat.distribution.AbstractDistribution

Methods inherited from class java.lang.Object

Constructor Detail

ExponentialDistribution

ExponentialDistribution

Method Detail

getLambda

npara

mean

var

sd

entropy

toString

rand

p

logp

cdf

quantile

M