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std::acos

Defined in header <cmath> 
float       acos( float arg );
 (1) 
double      acos( double arg );
 (2) 
long double acos( long double arg );
 (3) 
double      acos( Integral arg );
 (4) (since C++11)

Computes the principal value of the arc cosine of arg.

4) A set of overloads or a function template accepting an argument of any integral type. Equivalent to 2) (the argument is cast to double).

Parameters

arg - value of a floating-point or Integral type

Return value

If no errors occur, the arc cosine of arg (arccos(arg)) in the range [0 ; π], is returned.

If a domain error occurs, an implementation-defined value is returned (NaN where supported).

If a range error occurs due to underflow, the correct result (after rounding) is returned.

Error handling

Errors are reported as specified in math_errhandling.

Domain error occurs if arg is outside the range [-1.0; 1.0]

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

  • If the argument is +1, the value +0 is returned.
  • If |arg| > 1, a domain error occurs and NaN is returned.
  • if the argument is NaN, NaN is returned

Example

#include <cmath>
#include <iostream>
#include <cerrno>
#include <cfenv>
#include <cstring>
 
#pragma STDC FENV_ACCESS ON
int main()
{
    std::cout << "acos(-1) = " << acos(-1) << '\n'
              << "acos(0.0) = " << acos(0.0) << " 2*acos(0.0) = " << 2*acos(0) << '\n'
              << "acos(0.5) = " << acos(0.5) << " 3*acos(0.5) = " << 3*acos(0.5) << '\n'
              << "acos(1) = " << acos(1) << '\n';
    // error handling 
    errno = 0; std::feclearexcept(FE_ALL_EXCEPT);
    std::cout << "acos(1.1) = " << acos(1.1) << '\n';
    if(errno == EDOM)
        std::cout << "    errno == EDOM: " << std::strerror(errno) << '\n';
    if(std::fetestexcept(FE_INVALID))
        std::cout << "    FE_INVALID raised" << '\n';
}

Output:

acos(-1) = 3.14159
acos(0.0) = 1.5708 2*acos(0.0) = 3.14159
acos(0.5) = 1.0472 3*acos(0.5) = 3.14159
acos(1) = 0
acos(1.1) = nan
    errno == EDOM: Numerical argument out of domain
    FE_INVALID raised

See also

computes arc sine (arcsin(x))
(function)
computes arc tangent (arctan(x))
(function)
arc tangent, using signs to determine quadrants
(function)
computes cosine (cos(x))
(function)
(C++11)
 computes arc cosine of a complex number (arccos(z))
(function template)
applies the function std::acos to each element of valarray
(function template)
C documentation for acos

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