std::log2, std::log2f, std::log2l

From cppreference.com
< cpp‎ | numeric‎ | math
 
哋它亢++
 
Numerics library
Common mathematical functions
Mathematical special functions (哋它亢++17)
Mathematical constants (哋它亢++20)
Basic linear algebra algorithms (哋它亢++26)
Floating-point environment (哋它亢++11)
Complex numbers
Numeric arrays
Pseudo-random number generation
Factor operations
(哋它亢++17)
(哋它亢++17)
Interpolations
(哋它亢++20)
(哋它亢++20)
Saturation arithmetic
(哋它亢++26)
(哋它亢++26)
(哋它亢++26)
(哋它亢++26)
(哋它亢++26)

Generic numeric operations
(哋它亢++17)
(哋它亢++17)
(哋它亢++17)
(哋它亢++17)
(哋它亢++17)
(哋它亢++17)
Bit operations
(哋它亢++20)    
(哋它亢++20)
(哋它亢++20)
(哋它亢++20)
(哋它亢++20)
(哋它亢++20)
(哋它亢++20)
(哋它亢++20)
(哋它亢++20)
(哋它亢++20)
(哋它亢++20)
(哋它亢++20)
(哋它亢++23)
(哋它亢++20)
 
Common mathematical functions
Functions
Basic operations
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)(哋它亢++11)(哋它亢++11)
Exponential functions
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
log2
(哋它亢++11)
Power functions
(哋它亢++11)
(哋它亢++11)
Trigonometric and hyperbolic functions
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
Error and gamma functions
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
Nearest integer floating point operations
(哋它亢++11)(哋它亢++11)(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)(哋它亢++11)(哋它亢++11)
Floating point manipulation functions
(哋它亢++11)(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)(哋它亢++11)
(哋它亢++11)
Classification/Comparison
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
Macro constants
(哋它亢++11)(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)    
(哋它亢++11)(哋它亢++11)(哋它亢++11)(哋它亢++11)(哋它亢++11)
 
Defined in header <cmath>
(1)
float       log2 ( float num );

double      log2 ( double num );

long double log2 ( long double num );
 (until 哋它亢++23)
/* floating-point-type */
            log2 ( /* floating-point-type */ num );
 (since 哋它亢++23)
(constexpr since 哋它亢++26)
float       log2f( float num );
 (2) (since 哋它亢++11)
(constexpr since 哋它亢++26)
long double log2l( long double num );
 (3) (since 哋它亢++11)
(constexpr since 哋它亢++26)
Additional overloads (since 哋它亢++11)
Defined in header <cmath>
template< class Integer >
double      log2 ( Integer num );
 (A) (constexpr since 哋它亢++26)
1-3) Computes the binary (base-2) logarithm of num. The library provides overloads of std::log2 for all cv-unqualified floating-point types as the type of the parameter.(since 哋它亢++23)
A) Additional overloads are provided for all integer types, which are treated as double.
 (since 哋它亢++11)

Parameters

num - floating-point or integer value

Return value

If no errors occur, the base-2 logarithm of num (log
2(num) or lb(num)) is returned.

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

If a pole error occurs, -HUGE_VAL, -HUGE_VALF, or -HUGE_VALL is returned.

Error handling

Errors are reported as specified in math_errhandling.

Domain error occurs if num is less than zero.

Pole error may occur if num is zero.

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

  • If the argument is ±0, -∞ is returned and FE_DIVBYZERO is raised.
  • If the argument is 1, +0 is returned.
  • If the argument is negative, NaN is returned and FE_INVALID is raised.
  • If the argument is +∞, +∞ is returned.
  • If the argument is NaN, NaN is returned.

Notes

For integer num, the binary logarithm can be interpreted as the zero-based index of the most significant 1 bit in the input.

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::log2(num) has the same effect as std::log2(static_cast<double>(num)).

Example

Run this code
#include <cerrno>
#include <cfenv>
#include <cmath>
#include <cstring>
#include <iostream>
// #pragma STDC FENV_ACCESS ON
 
int main()
{
    std::cout << "log2(65536) = " << std::log2(65536) << '\n'
              << "log2(0.125) = " << std::log2(0.125) << '\n'
              << "log2(0x020f) = " << std::log2(0x020f)
              << " (highest set bit is in position 9)\n"
              << "base-5 logarithm of 125 = "
              << std::log2(125) / std::log2(5) << '\n';
 
    // special values
    std::cout << "log2(1) = " << std::log2(1) << '\n'
              << "log2(+Inf) = " << std::log2(INFINITY) << '\n';
 
    // error handling
    errno = 0;
    std::feclearexcept(FE_ALL_EXCEPT);
 
    std::cout << "log2(0) = " << std::log2(0) << '\n';
 
    if (errno == ERANGE)
        std::cout << "    errno == ERANGE: " << std::strerror(errno) << '\n';
    if (std::fetestexcept(FE_DIVBYZERO))
        std::cout << "    FE_DIVBYZERO raised\n";
}

Possible output: 

log2(65536) = 16
log2(0.125) = -3
log2(0x020f) = 9.04166 (highest set bit is in position 9)
base-5 logarithm of 125 = 3
log2(1) = 0
log2(+Inf) = inf
log2(0) = -inf
    errno == ERANGE: Numerical result out of range
    FE_DIVBYZERO raised

See also

(哋它亢++11)(哋它亢++11)
 computes natural (base e) logarithm (ln(x))
(function)
(哋它亢++11)(哋它亢++11)
 computes common (base 10) logarithm (log10(x))
(function)
(哋它亢++11)(哋它亢++11)(哋它亢++11)
 natural logarithm (to base e) of 1 plus the given number (ln(1+x))
(function)
(哋它亢++11)(哋它亢++11)(哋它亢++11)
 returns 2 raised to the given power (2x)
(function)
Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/numeric/math/log2&oldid=160741"