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std::acos, std::acosf, std::acosl

Defined in header `<cmath>`
	(1)
float acos ( float num ); double acos ( double num ); long double acos ( long double num );		(until C++23)
/* floating-point-type / acos ( / floating-point-type */ num );		(since C++23) (constexpr since C++26)
float acosf( float num );	(2)	(since C++11) (constexpr since C++26)
long double acosl( long double num );	(3)	(since C++11) (constexpr since C++26)
Additional overloads (since C++11)
Defined in header `<cmath>`
template< class Integer > double acos ( Integer num );	(A)	(constexpr since C++26)

1-3) Computes the principal value of the arc cosine of num. The library provides overloads of std::acos for all cv-unqualified floating-point types as the type of the parameter. (since C++23)

double

(since C++11)

Parameters

num

floating-point or integer value

Return value

If no errors occur, the arc cosine of num (arccos(num)) 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 num 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 |num| > 1, a domain error occurs and NaN is returned.
if the argument is NaN, NaN is returned.

Notes

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::acos(num) has the same effect as std::acos(static_cast<double>(num)).

Example

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

asinasinfasinl (C++11)(C++11)	computes arc sine (\({\small\arcsin{x} }\)arcsin(x)) (function)
atanatanfatanl (C++11)(C++11)	computes arc tangent (\({\small\arctan{x} }\)arctan(x)) (function)
atan2atan2fatan2l (C++11)(C++11)	arc tangent, using signs to determine quadrants (function)
coscosfcosl (C++11)(C++11)	computes cosine (\({\small\cos{x} }\)cos(x)) (function)
acos(std::complex) (C++11)	computes arc cosine of a complex number (\({\small\arccos{z} }\)arccos(z)) (function template)
acos(std::valarray)	applies the function `std::acos` to each element of valarray (function template)
C documentation for `acos`