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Difference between revisions of "cpp/numeric/math/erf"

From cppreference.com
< cpp‎ | numeric‎ | math
m (Use since= and until= params of {{dcl}} template.)
m (., fmt, "" -> '')
 
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{{cpp/title|erf}}
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{{cpp/title|erf|erff|erfl}}
 
{{cpp/numeric/math/navbar}}
 
{{cpp/numeric/math/navbar}}
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{{dcl begin}}
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{{cpp/numeric/math/declarations
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{{dcl header | cmath}}
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|family=erf
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{{dcl | since=c++11 |
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|param1=num
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float      erf( float arg );
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|constexpr_since=26
 +
|desc=Computes the {{enwiki|Error function|error function}} of {{c|num}}.
 
}}
 
}}
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{{dcl | since=c++11 |
 
−
double      erf( double arg );
 
−
}}
 
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{{dcl | since=c++11 |
 
−
long double erf( long double arg );
 
−
}}
 
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{{dcl | since=c++11 |
 
−
double      erf( Integral arg );
 
−
}}
 
−
{{dcl end}}
 
−
 
−
Computes the [[enwiki:Error function|error function]] of {{tt|arg}}.
 
  
 
===Parameters===
 
===Parameters===
 
{{par begin}}
 
{{par begin}}
−
{{par | arg | floating point value}}
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{{par|num|floating-point or integer value}}
 
{{par end}}
 
{{par end}}
  
 
===Return value===
 
===Return value===
 +
If no errors occur, value of the error function of {{c|num}}, that is {{mathjax-or|\(\frac{2}{\sqrt{\pi} }\int_{0}^{num}{e^{-{t^2} }\mathsf{d}t}\)|{{mfrac|2|{{mrad|&pi;}}}}{{minteg|0|num|{{mexp|-t{{su|p=2}}}}d''t''}}}}, is returned.<br><!-- a blank line does not work here -->
 +
If a range error occurs due to underflow, the correct result (after rounding), that is {{mathjax-or|\(\frac{2\cdot num}{\sqrt{\pi} }\)|{{mfrac|2*num|{{mrad|&pi;}}}}}} is returned.
  
−
The value of the error function of {{tt|arg}}, that is {{math|{{mfrac|2|{{mrad|&pi;}}}}{{minteg|0|arg|{{mexp|-t{{su|p=2}}}}d''t''}}}}.
+
===Error handling===
 +
Errors are reported as specified in {{lc|math_errhandling}}.
 +
 
 +
If the implementation supports IEEE floating-point arithmetic (IEC 60559),
 +
* If the argument is ±0, ±0 is returned.
 +
* If the argument is ±∞, ±1 is returned.
 +
* If the argument is NaN, NaN is returned.
 +
 
 +
===Notes===
 +
Underflow is guaranteed if {{c|{{!}}num{{!}} < DBL_MIN * (std::sqrt(π) / 2)}}.
 +
 
 +
{{mathjax-or|\(\operatorname{erf}(\frac{x}{\sigma \sqrt{2} })\)|erf({{mfrac|x|&sigma;{{mrad|2}}}})}} is the probability that a measurement whose errors are subject to a normal distribution with standard deviation {{mathjax-or|\(\sigma\)|&sigma;}} is less than {{mathjax-or|\(x\)|x}} away from the mean value.
 +
 
 +
{{cpp/numeric/math/additional integer overload note|erf}}
  
 
===Example===
 
===Example===
 
{{example
 
{{example
−
| The following example calculates the probability that a normal variate is on the interval (x1, x2)
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|The following example calculates the probability that a normal variate is on the interval (x1, x2):
−
| code=
+
|code=
−
#include <iostream>
+
 
#include <cmath>
 
#include <cmath>
 
#include <iomanip>
 
#include <iomanip>
 +
#include <iostream>
 +
 
double phi(double x1, double x2)
 
double phi(double x1, double x2)
 
{
 
{
−
     return (std::erf(x2/std::sqrt(2)) - std::erf(x1/std::sqrt(2)))/2;
+
     return (std::erf(x2 / std::sqrt(2)) - std::erf(x1 / std::sqrt(2))) / 2;
 
}
 
}
 +
 
int main()
 
int main()
 
{
 
{
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     std::cout << "normal variate probabilities:\n";
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     std::cout << "Normal variate probabilities:\n"
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     for(int n=-4; n<4; ++n)
+
              << std::fixed << std::setprecision(2);
−
         std::cout << "[" << std::setw(2) << n << ":" << std::setw(2) << n+1 << "]: "
+
     for (int n = -4; n < 4; ++n)
−
                   << std::setw(5) << std::fixed << std::setprecision(2)
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         std::cout << '[' << std::setw(2) << n
−
                  << 100*phi(n, n+1) << "%\n";
+
                  << ':' << std::setw(2) << n + 1 << "]: "
 +
                   << std::setw(5) << 100 * phi(n, n + 1) << "%\n";
 +
   
 +
    std::cout << "Special values:\n"
 +
              << "erf(-0) = " << std::erf(-0.0) << '\n'
 +
              << "erf(Inf) = " << std::erf(INFINITY) << '\n';
 
}
 
}
−
| output=
+
|output=
−
normal variate probabilities:
+
Normal variate probabilities:
 
[-4:-3]:  0.13%
 
[-4:-3]:  0.13%
 
[-3:-2]:  2.14%
 
[-3:-2]:  2.14%
Line 57: Line 68:
 
[ 2: 3]:  2.14%
 
[ 2: 3]:  2.14%
 
[ 3: 4]:  0.13%
 
[ 3: 4]:  0.13%
 +
Special values:
 +
erf(-0) = -0.00
 +
erf(Inf) = 1.00
 
}}
 
}}
−
 
  
 
===See also===
 
===See also===
−
 
 
{{dsc begin}}
 
{{dsc begin}}
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{{dsc inc | cpp/numeric/math/dsc erfc}}
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{{dsc inc|cpp/numeric/math/dsc erfc}}
 +
{{dsc see c|c/numeric/math/erf}}
 
{{dsc end}}
 
{{dsc end}}
  
 
===External links===
 
===External links===
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[http://mathworld.wolfram.com/Erf.html Weisstein, Eric W. "Erf."] From MathWorld--A Wolfram Web Resource.
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{{eli|[https://mathworld.wolfram.com/Erf.html Weisstein, Eric W. "Erf."] From MathWorld &mdash; A Wolfram Web Resource.}}
  
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[[de:cpp/numeric/math/erf]]
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{{langlinks|de|es|fr|it|ja|pt|ru|zh}}
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[[es:cpp/numeric/math/erf]]
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[[it:cpp/numeric/math/erf]]
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[[pt:cpp/numeric/math/erf]]
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[[ru:cpp/numeric/math/erf]]
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[[zh:cpp/numeric/math/erf]]
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Latest revision as of 08:49, 15 October 2023

 
C++
 
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Defined in header <cmath>
(1)
float       erf ( float num );

double      erf ( double num );

long double erf ( long double num );
 (until C++23)
/* floating-point-type */
            erf ( /* floating-point-type */ num );
 (since C++23)
(constexpr since C++26)
float       erff( float num );
 (2) (since C++11)
(constexpr since C++26)
long double erfl( long double num );
 (3) (since C++11)
(constexpr since C++26)
Additional overloads (since C++11)
Defined in header <cmath>
template< class Integer >
double      erf ( Integer num );
 (A) (constexpr since C++26)
1-3) Computes the error function of num. The library provides overloads of std::erf for all cv-unqualified floating-point types as the type of the parameter.(since C++23)
A) Additional overloads are provided for all integer types, which are treated as double.
 (since C++11)

Contents

[edit] Parameters

num - floating-point or integer value

[edit] Return value

If no errors occur, value of the error function of num, that is
2
√π
∫num0e-t2dt, is returned.
If a range error occurs due to underflow, the correct result (after rounding), that is
2*num
√π
 is returned.

[edit] Error handling

Errors are reported as specified in math_errhandling.

If the implementation supports IEEE floating-point arithmetic (IEC 60559),

  • If the argument is ±0, ±0 is returned.
  • If the argument is ±∞, ±1 is returned.
  • If the argument is NaN, NaN is returned.

[edit] Notes

Underflow is guaranteed if |num| < DBL_MIN * (std::sqrt(π) / 2).

erf(
x
σ√2
) is the probability that a measurement whose errors are subject to a normal distribution with standard deviation σ is less than x away from the mean value.

The additional overloads are not required to be provided exactly as (A). They only need to be sufficient to ensure that for their argument num of integer type, std::erf(num) has the same effect as std::erf(static_cast<double>(num)).

[edit] Example

The following example calculates the probability that a normal variate is on the interval (x1, x2):

Run this code
#include <cmath>
#include <iomanip>
#include <iostream>
 
double phi(double x1, double x2)
{
    return (std::erf(x2 / std::sqrt(2)) - std::erf(x1 / std::sqrt(2))) / 2;
}
 
int main()
{
    std::cout << "Normal variate probabilities:\n"
              << std::fixed << std::setprecision(2);
    for (int n = -4; n < 4; ++n)
        std::cout << '[' << std::setw(2) << n
                  << ':' << std::setw(2) << n + 1 << "]: "
                  << std::setw(5) << 100 * phi(n, n + 1) << "%\n";
 
    std::cout << "Special values:\n"
              << "erf(-0) = " << std::erf(-0.0) << '\n'
              << "erf(Inf) = " << std::erf(INFINITY) << '\n';
}

Output: 

Normal variate probabilities:
[-4:-3]:  0.13%
[-3:-2]:  2.14%
[-2:-1]: 13.59%
[-1: 0]: 34.13%
[ 0: 1]: 34.13%
[ 1: 2]: 13.59%
[ 2: 3]:  2.14%
[ 3: 4]:  0.13%
Special values:
erf(-0) = -0.00
erf(Inf) = 1.00

[edit] See also

(C++11)(C++11)(C++11)
 complementary error function
(function) [edit]
C documentation for erf

[edit] External links

Weisstein, Eric W. "Erf." From MathWorld — A Wolfram Web Resource.
Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/numeric/math/erf&oldid=160724"