The Random Number Generator (RNG) that has a specified empirical distribution. More...

#include "random-variable-stream.h"

Inheritance diagram for ns3::EmpiricalRandomVariable:

Collaboration diagram for ns3::EmpiricalRandomVariable:

Public Member Functions
	EmpiricalRandomVariable ()
	Creates an empirical RNG that has a specified, empirical distribution, and configured for interpolating mode.

void	CDF (double v, double c)
	Specifies a point in the empirical distribution.

virtual uint32_t	GetInteger ()
	Get the next random value drawn from the distribution.

double	GetValue () override
	Get the next random value drawn from the distribution.

virtual double	Interpolate ()
	Returns the next value in the empirical distribution using linear interpolation.

bool	SetInterpolate (bool interpolate)
	Switch the mode between sampling the CDF and interpolating.

Public Member Functions inherited from ns3::RandomVariableStream
	RandomVariableStream ()
	Default constructor.

	RandomVariableStream (const RandomVariableStream &)=delete

	~RandomVariableStream () override
	Destructor.

int64_t	GetStream () const
	Returns the stream number for the RngStream.

bool	IsAntithetic () const
	Check if antithetic values will be generated.

RandomVariableStream &	operator= (const RandomVariableStream &)=delete

void	SetAntithetic (bool isAntithetic)
	Specify whether antithetic values should be generated.

void	SetStream (int64_t stream)
	Specifies the stream number for the RngStream.

Public Member Functions inherited from ns3::Object
	Object ()
	Constructor.

	~Object () override
	Destructor.

void	AggregateObject (Ptr< Object > other)
	Aggregate two Objects together.

void	Dispose ()
	Dispose of this Object.

AggregateIterator	GetAggregateIterator () const
	Get an iterator to the Objects aggregated to this one.

TypeId	GetInstanceTypeId () const override
	Get the most derived TypeId for this Object.

template<typename T >
Ptr< T >	GetObject () const
	Get a pointer to the requested aggregated Object.

template<>
Ptr< Object >	GetObject () const
	Specialization of () for objects of type ns3::Object.

template<typename T >
Ptr< T >	GetObject (TypeId tid) const
	Get a pointer to the requested aggregated Object by TypeId.

template<>
Ptr< Object >	GetObject (TypeId tid) const
	Specialization of (TypeId tid) for objects of type ns3::Object.

void	Initialize ()
	Invoke DoInitialize on all Objects aggregated to this one.

bool	IsInitialized () const
	Check if the object has been initialized.

void	UnidirectionalAggregateObject (Ptr< Object > other)
	Aggregate an Object to another Object.

Public Member Functions inherited from ns3::SimpleRefCount< Object, ObjectBase, ObjectDeleter >
	SimpleRefCount ()
	Default constructor.

	SimpleRefCount (const SimpleRefCount &o)
	Copy constructor.

uint32_t	GetReferenceCount () const
	Get the reference count of the object.

SimpleRefCount &	operator= (const SimpleRefCount &o)
	Assignment operator.

void	Ref () const
	Increment the reference count.

void	Unref () const
	Decrement the reference count.

Public Member Functions inherited from ns3::ObjectBase
virtual	~ObjectBase ()
	Virtual destructor.

void	GetAttribute (std::string name, AttributeValue &value, bool permissive=false) const
	Get the value of an attribute, raising fatal errors if unsuccessful.

bool	GetAttributeFailSafe (std::string name, AttributeValue &value) const
	Get the value of an attribute without raising errors.

void	SetAttribute (std::string name, const AttributeValue &value)
	Set a single attribute, raising fatal errors if unsuccessful.

bool	SetAttributeFailSafe (std::string name, const AttributeValue &value)
	Set a single attribute without raising errors.

bool	TraceConnect (std::string name, std::string context, const CallbackBase &cb)
	Connect a TraceSource to a Callback with a context.

bool	TraceConnectWithoutContext (std::string name, const CallbackBase &cb)
	Connect a TraceSource to a Callback without a context.

bool	TraceDisconnect (std::string name, std::string context, const CallbackBase &cb)
	Disconnect from a TraceSource a Callback previously connected with a context.

bool	TraceDisconnectWithoutContext (std::string name, const CallbackBase &cb)
	Disconnect from a TraceSource a Callback previously connected without a context.

Static Public Member Functions
static TypeId	GetTypeId ()
	Register this type.

Static Public Member Functions inherited from ns3::RandomVariableStream
static TypeId	GetTypeId ()
	Register this type.

Static Public Member Functions inherited from ns3::Object
static TypeId	GetTypeId ()
	Register this type.

Static Public Member Functions inherited from ns3::ObjectBase
static TypeId	GetTypeId ()
	Get the type ID.

Private Member Functions
double	DoInterpolate (double r)
	Linear interpolation between two points on the CDF to estimate the value at `r`.

double	DoSampleCDF (double r)
	Sample the CDF as a histogram (without interpolation).

bool	PreSample (double &value)
	Do the initial rng draw and check against the extrema.

void	Validate ()
	Check that the CDF is valid.

Private Attributes
std::map< double, double >	m_empCdf
	The map of CDF points (x, F(x)).

bool	m_interpolate
	If `true` GetValue will interpolate, otherwise treat CDF as normal histogram.

bool	m_validated
	`true` once the CDF has been validated.

Additional Inherited Members
Protected Member Functions inherited from ns3::RandomVariableStream
RngStream *	Peek () const
	Get the pointer to the underlying RngStream.

Protected Member Functions inherited from ns3::Object
	Object (const Object &o)
	Copy an Object.

virtual void	DoDispose ()
	Destructor implementation.

virtual void	DoInitialize ()
	Initialize() implementation.

virtual void	NotifyNewAggregate ()
	Notify all Objects aggregated to this one of a new Object being aggregated.

Protected Member Functions inherited from ns3::ObjectBase
void	ConstructSelf (const AttributeConstructionList &attributes)
	Complete construction of ObjectBase; invoked by derived classes.

virtual void	NotifyConstructionCompleted ()
	Notifier called once the ObjectBase is fully constructed.

Related Symbols inherited from ns3::ObjectBase
static TypeId	GetObjectIid ()
	Ensure the TypeId for ObjectBase gets fully configured to anchor the inheritance tree properly.

Detailed Description

The Random Number Generator (RNG) that has a specified empirical distribution.

Defines a random variable that has a specified, empirical distribution. The cumulative probability distribution function (CDF) is specified by a series of calls to the CDF() member function, specifying a value and the probability that the distribution is less than the specified value. When random values are requested, a uniform random variable $u \in [0, 1]$ is used to select a probability, and the return value is chosen as the largest input value with CDF less than the random value. This method is known as inverse transform sampling.

This generator has two modes: sampling and interpolating. In sampling mode this random variable generator treats the CDF as an exact histogram and returns one of the histogram inputs exactly. This is appropriate when the configured CDF represents the exact probability distribution, for a categorical variable, for example.

In interpolating mode this random variable generator linearly interpolates between the CDF values defining the histogram bins. This is appropriate when the configured CDF is an approximation to a continuous underlying probability distribution.

For historical reasons the default is sampling. To switch modes use the Interpolate Attribute, or call SetInterpolate(). You can change modes at any time.

If you find yourself switching frequently it could be simpler to set the mode to sampling, then use the GetValue() function for sampled values, and Interpolate() function for interpolated values.

The CDF need not start with a probability of zero, nor end with a probability of 1.0. If the selected uniform random value $u \in [0,1]$ is less than the probability of the first CDF point, that point is selected. If is greater than the probability of the last CDF point the last point is selected. In either case the interpolating mode will not interpolate (since there is no value beyond the first/last to work with), but simply return the extremal CDF value, as in sampling.

Example

Here is an example of how to use this class:

// Create the RNG with a non-uniform distribution between 0 and 10.
// in sampling mode.
Ptr<EmpiricalRandomVariable> x = CreateObject<EmpiricalRandomVariable> ();
x->SetInterpolate (false);
x->CDF ( 0.0,  0.0);
x->CDF ( 5.0,  0.25);
x->CDF (10.0,  1.0);
 
double value = x->GetValue ();

The expected values and probabilities returned by GetValue() are

Value	Probability
0.0	0
5.0	25%
10.0	75%

The only two values ever returned are 5 and 10, in the ratio 1:3.

If instead you want linear interpolation between the points of the CDF use the Interpolate() function:

double interp = x->Interpolate ();

This will return continuous values on the range [0,1), 25% of the time less than 5, and 75% of the time between 5 and 10.

See empirical-random-variable-example.cc for an example.

Antithetic Values.

If an instance of this RNG is configured to return antithetic values, the actual value returned, , is generated by using instead. of on [0, 1].

Definition at line 1973 of file random-variable-stream.h.

Constructor & Destructor Documentation

◆ EmpiricalRandomVariable()

ns3::EmpiricalRandomVariable::EmpiricalRandomVariable ( )

Creates an empirical RNG that has a specified, empirical distribution, and configured for interpolating mode.

Definition at line 1554 of file random-variable-stream.cc.

References NS_LOG_FUNCTION.

Member Function Documentation

◆ CDF()

void ns3::EmpiricalRandomVariable::CDF	(	double	v,
		double	c )

Specifies a point in the empirical distribution.

Parameters

[in]	v	The function value for this point
[in]	c	Probability that the function is less than or equal to `v` In other words this is cumulative distribution function at `v`.

Definition at line 1678 of file random-variable-stream.cc.

References m_empCdf, NS_LOG_FUNCTION, and NS_LOG_WARN.

◆ DoInterpolate()

double ns3::EmpiricalRandomVariable::DoInterpolate ( double r )

private

Linear interpolation between two points on the CDF to estimate the value at r.

Parameters

[in] r The argument value to interpolate to.

Returns: The interpolated CDF at r

Definition at line 1651 of file random-variable-stream.cc.

References m_empCdf, and NS_LOG_FUNCTION.

Referenced by GetValue(), and Interpolate().

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◆ DoSampleCDF()

double ns3::EmpiricalRandomVariable::DoSampleCDF ( double r )

private

Sample the CDF as a histogram (without interpolation).

Parameters

[in] r The CDF value at which to sample the CDF.

Returns: The bin value corresponding to r.

Definition at line 1624 of file random-variable-stream.cc.

References m_empCdf, and NS_LOG_FUNCTION.

Referenced by GetValue().

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◆ GetInteger()

uint32_t ns3::RandomVariableStream::GetInteger ( )

virtual

Get the next random value drawn from the distribution.

Returns: A random value.

Note: This does not interpolate the CDF, but treats it as a stepwise continuous function.

Reimplemented from ns3::RandomVariableStream.

Definition at line 144 of file random-variable-stream.cc.

◆ GetTypeId()

TypeId ns3::EmpiricalRandomVariable::GetTypeId ( )

static

Register this type.

Returns: The object TypeId.

Definition at line 1538 of file random-variable-stream.cc.

References m_interpolate, ns3::MakeBooleanAccessor(), ns3::MakeBooleanChecker(), and ns3::TypeId::SetParent().

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◆ GetValue()

double ns3::EmpiricalRandomVariable::GetValue ( )

overridevirtual

Get the next random value drawn from the distribution.

Returns: A random value.

Note: This does not interpolate the CDF, but treats it as a stepwise continuous function.

Implements ns3::RandomVariableStream.

Definition at line 1602 of file random-variable-stream.cc.

References DoInterpolate(), DoSampleCDF(), ns3::RandomVariableStream::GetStream(), m_interpolate, NS_LOG_DEBUG, and PreSample().

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◆ Interpolate()

double ns3::EmpiricalRandomVariable::Interpolate ( )

virtual

Returns the next value in the empirical distribution using linear interpolation.

Returns: The floating point next value in the empirical distribution using linear interpolation.

Definition at line 1635 of file random-variable-stream.cc.

References DoInterpolate(), NS_LOG_FUNCTION, and PreSample().

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◆ PreSample()

bool ns3::EmpiricalRandomVariable::PreSample ( double & value )

private

Do the initial rng draw and check against the extrema.

If the extrema apply, value will have the extremal value and the return will be true.

If the extrema do not apply value will have the URNG value and the return will be false.

Parameters

[out] value The extremal value, or the URNG.

Returns: true if value is the extremal result, or false if value is the URNG value.

Definition at line 1570 of file random-variable-stream.cc.

References ns3::RandomVariableStream::IsAntithetic(), m_empCdf, m_validated, NS_LOG_FUNCTION, ns3::RandomVariableStream::Peek(), ns3::RngStream::RandU01(), and Validate().

Referenced by GetValue(), and Interpolate().

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◆ SetInterpolate()

bool ns3::EmpiricalRandomVariable::SetInterpolate ( bool interpolate )

Switch the mode between sampling the CDF and interpolating.

The default mode is sampling.

Parameters

[in] interpolate If true set to interpolation, otherwise sampling.

Returns: The previous interpolate flag value.

Definition at line 1561 of file random-variable-stream.cc.

References m_interpolate, NS_LOG_FUNCTION, and prev.

◆ Validate()

void ns3::EmpiricalRandomVariable::Validate ( )

private

Check that the CDF is valid.

A valid CDF has

Strictly increasing arguments, and
Strictly increasing CDF.

It is a fatal error to fail validation.

Definition at line 1695 of file random-variable-stream.cc.

References m_empCdf, m_validated, NS_FATAL_ERROR, and NS_LOG_FUNCTION.

Referenced by PreSample().

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Member Data Documentation

◆ m_empCdf

std::map<double, double> ns3::EmpiricalRandomVariable::m_empCdf

private

The map of CDF points (x, F(x)).

The CDF points are stored in the std::map in reverse order, as follows: Key: CDF F(x) [0, 1] | Value: domain value (x) [-inf, inf].

Definition at line 2071 of file random-variable-stream.h.

Referenced by CDF(), DoInterpolate(), DoSampleCDF(), PreSample(), and Validate().

◆ m_interpolate

bool ns3::EmpiricalRandomVariable::m_interpolate

private

If true GetValue will interpolate, otherwise treat CDF as normal histogram.

Definition at line 2076 of file random-variable-stream.h.

Referenced by GetTypeId(), GetValue(), and SetInterpolate().

◆ m_validated

bool ns3::EmpiricalRandomVariable::m_validated

private

true once the CDF has been validated.

Definition at line 2065 of file random-variable-stream.h.

Referenced by PreSample(), and Validate().

The documentation for this class was generated from the following files:

src/core/model/random-variable-stream.h
src/core/model/random-variable-stream.cc

Public Member Functions

Static Public Member Functions

Private Member Functions

Private Attributes

Additional Inherited Members

Detailed Description

Constructor & Destructor Documentation

◆ EmpiricalRandomVariable()

Member Function Documentation

◆ CDF()

◆ DoInterpolate()

◆ DoSampleCDF()

◆ GetInteger()

◆ GetTypeId()

◆ GetValue()

◆ Interpolate()

◆ PreSample()

◆ SetInterpolate()

◆ Validate()

Member Data Documentation

◆ m_empCdf

◆ m_interpolate

◆ m_validated