GeographicLib  1.45
GeographicLib::Accumulator< T > Class Template Reference

An accumulator for sums. More...

#include <GeographicLib/Accumulator.hpp>

## Public Member Functions

Accumulator (T y=T(0))

Accumulatoroperator= (T y)

operator() () const

operator() (T y) const

Accumulatoroperator+= (T y)

Accumulatoroperator-= (T y)

Accumulatoroperator*= (int n)

Accumulatoroperator*= (T y)

bool operator== (T y) const

bool operator!= (T y) const

bool operator< (T y) const

bool operator<= (T y) const

bool operator> (T y) const

bool operator>= (T y) const

## Detailed Description

### template<typename T = Math::real> class GeographicLib::Accumulator< T >

An accumulator for sums.

This allows many numbers of floating point type T to be added together with twice the normal precision. Thus if T is double, the effective precision of the sum is 106 bits or about 32 decimal places.

The implementation follows J. R. Shewchuk, Adaptive Precision Floating-Point Arithmetic and Fast Robust Geometric Predicates, Discrete & Computational Geometry 18(3) 305–363 (1997).

Approximate timings (summing a vector<double>)

• double: 2ns
• Accumulator<double>: 23ns

In the documentation of the member functions, sum stands for the value currently held in the accumulator.

Example of use:

// Example of using the GeographicLib::Accumulator class
#include <iostream>
#include <exception>
using namespace std;
using namespace GeographicLib;
int main() {
try {
// Compare using Accumulator and ordinary summation for a sum of large and
// small terms.
double sum = 0;
Accumulator<> acc = 0;
sum += 1e20; sum += 1; sum += 2; sum += 100; sum += 5000; sum += -1e20;
acc += 1e20; acc += 1; acc += 2; acc += 100; acc += 5000; acc += -1e20;
cout << sum << " " << acc() << "\n";
}
catch (const exception& e) {
cerr << "Caught exception: " << e.what() << "\n";
return 1;
}
return 0;
}

Definition at line 40 of file Accumulator.hpp.

## Constructor & Destructor Documentation

template<typename T = Math::real>
 GeographicLib::Accumulator< T >::Accumulator ( T y = T(0) )
inline

Construct from a T. This is not declared explicit, so that you can write Accumulator<double> a = 5;.

Parameters
 [in] y set sum = y.

Definition at line 101 of file Accumulator.hpp.

## Member Function Documentation

template<typename T = Math::real>
 Accumulator& GeographicLib::Accumulator< T >::operator= ( T y )
inline

Set the accumulator to a number.

Parameters
 [in] y set sum = y.

Definition at line 110 of file Accumulator.hpp.

template<typename T = Math::real>
 T GeographicLib::Accumulator< T >::operator() ( ) const
inline

Return the value held in the accumulator.

Returns
sum.

Definition at line 116 of file Accumulator.hpp.

template<typename T = Math::real>
 T GeographicLib::Accumulator< T >::operator() ( T y ) const
inline

Return the result of adding a number to sum (but don't change sum).

Parameters
 [in] y the number to be added to the sum.
Returns
sum + y.

Definition at line 123 of file Accumulator.hpp.

template<typename T = Math::real>
 Accumulator& GeographicLib::Accumulator< T >::operator+= ( T y )
inline

Add a number to the accumulator.

Parameters
 [in] y set sum += y.

Definition at line 129 of file Accumulator.hpp.

template<typename T = Math::real>
 Accumulator& GeographicLib::Accumulator< T >::operator-= ( T y )
inline

Subtract a number from the accumulator.

Parameters
 [in] y set sum -= y.

Definition at line 135 of file Accumulator.hpp.

template<typename T = Math::real>
 Accumulator& GeographicLib::Accumulator< T >::operator*= ( int n )
inline

Multiply accumulator by an integer. To avoid loss of accuracy, use only integers such that n × T is exactly representable as a T (i.e., ± powers of two). Use n = −1 to negate sum.

Parameters
 [in] n set sum *= n.

Definition at line 143 of file Accumulator.hpp.

template<typename T = Math::real>
 Accumulator& GeographicLib::Accumulator< T >::operator*= ( T y )
inline

Multiply accumulator by a number. The fma (fused multiply and add) instruction is used (if available) in order to maintain accuracy.

Parameters
 [in] y set sum *= y.

Definition at line 150 of file Accumulator.hpp.

References GeographicLib::Math::fma().

template<typename T = Math::real>
 bool GeographicLib::Accumulator< T >::operator== ( T y ) const
inline

Test equality of an Accumulator with a number.

Definition at line 159 of file Accumulator.hpp.

template<typename T = Math::real>
 bool GeographicLib::Accumulator< T >::operator!= ( T y ) const
inline

Test inequality of an Accumulator with a number.

Definition at line 163 of file Accumulator.hpp.

template<typename T = Math::real>
 bool GeographicLib::Accumulator< T >::operator< ( T y ) const
inline

Less operator on an Accumulator and a number.

Definition at line 167 of file Accumulator.hpp.

template<typename T = Math::real>
 bool GeographicLib::Accumulator< T >::operator<= ( T y ) const
inline

Less or equal operator on an Accumulator and a number.

Definition at line 171 of file Accumulator.hpp.

template<typename T = Math::real>
 bool GeographicLib::Accumulator< T >::operator> ( T y ) const
inline

Greater operator on an Accumulator and a number.

Definition at line 175 of file Accumulator.hpp.

template<typename T = Math::real>
 bool GeographicLib::Accumulator< T >::operator>= ( T y ) const
inline

Greater or equal operator on an Accumulator and a number.

Definition at line 179 of file Accumulator.hpp.

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