Difference between revisions of "cpp/numeric/math/log2"
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{{cpp/title|log2|log2f|log2l}} | {{cpp/title|log2|log2f|log2l}} | ||
{{cpp/numeric/math/navbar}} | {{cpp/numeric/math/navbar}} | ||
| − | {{ | + | {{cpp/numeric/math/declarations |
| − | + | |family=log2 | |
| − | + | |param1=num | |
| − | + | |constexpr_since=26 | |
| − | + | |desc=Computes the {{enwiki|Binary logarithm|binary (base-''2'') logarithm}} of {{c|num}}. | |
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}} | }} | ||
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===Parameters=== | ===Parameters=== | ||
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If the implementation supports IEEE floating-point arithmetic (IEC 60559), | If the implementation supports IEEE floating-point arithmetic (IEC 60559), | ||
| − | * If the argument is ±0, -∞ is returned and {{lc|FE_DIVBYZERO}} is raised | + | * If the argument is ±0, -∞ is returned and {{lc|FE_DIVBYZERO}} is raised. |
| − | * If the argument is 1, +0 is returned | + | * If the argument is 1, +0 is returned. |
| − | * If the argument is negative, NaN is returned and {{lc|FE_INVALID}} is raised | + | * If the argument is negative, NaN is returned and {{lc|FE_INVALID}} is raised. |
| − | * If the argument is +∞, +∞ is returned | + | * If the argument is +∞, +∞ is returned. |
| − | * If the argument is NaN, NaN is returned | + | * If the argument is NaN, NaN is returned. |
===Notes=== | ===Notes=== | ||
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===Example=== | ===Example=== | ||
| − | {{example|code= | + | {{example |
| + | |code= | ||
#include <cerrno> | #include <cerrno> | ||
#include <cfenv> | #include <cfenv> | ||
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#include <cmath> | #include <cmath> | ||
| + | #include <cstring> | ||
#include <iostream> | #include <iostream> | ||
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// #pragma STDC FENV_ACCESS ON | // #pragma STDC FENV_ACCESS ON | ||
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<< "log2(0x020f) = " << std::log2(0x020f) | << "log2(0x020f) = " << std::log2(0x020f) | ||
<< " (highest set bit is in position 9)\n" | << " (highest set bit is in position 9)\n" | ||
| − | << "base-5 logarithm of 125 = " << std::log2(125)/std::log2(5) << '\n'; | + | << "base-5 logarithm of 125 = " |
| − | + | << std::log2(125) / std::log2(5) << '\n'; | |
| + | |||
// special values | // special values | ||
std::cout << "log2(1) = " << std::log2(1) << '\n' | std::cout << "log2(1) = " << std::log2(1) << '\n' | ||
<< "log2(+Inf) = " << std::log2(INFINITY) << '\n'; | << "log2(+Inf) = " << std::log2(INFINITY) << '\n'; | ||
| − | + | ||
// error handling | // error handling | ||
errno = 0; | errno = 0; | ||
std::feclearexcept(FE_ALL_EXCEPT); | std::feclearexcept(FE_ALL_EXCEPT); | ||
| − | + | ||
std::cout << "log2(0) = " << std::log2(0) << '\n'; | std::cout << "log2(0) = " << std::log2(0) << '\n'; | ||
| − | + | ||
if (errno == ERANGE) | if (errno == ERANGE) | ||
std::cout << " errno == ERANGE: " << std::strerror(errno) << '\n'; | std::cout << " errno == ERANGE: " << std::strerror(errno) << '\n'; | ||
Latest revision as of 09:33, 15 October 2023
| Defined in header <cmath>
|
||
| (1) | ||
float log2 ( float num ); double log2 ( double num ); |
(until C++23) | |
| /* floating-point-type */ log2 ( /* floating-point-type */ num ); |
(since C++23) (constexpr since C++26) |
|
| float log2f( float num ); |
(2) | (since C++11) (constexpr since C++26) |
| long double log2l( long double num ); |
(3) | (since C++11) (constexpr since C++26) |
| Additional overloads (since C++11) |
||
| Defined in header <cmath>
|
||
| template< class Integer > double log2 ( Integer num ); |
(A) | (constexpr since C++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 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, the base-2 logarithm of num (log2(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.
[edit] 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.
[edit] 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)).
[edit] 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[edit] See also
| (C++11)(C++11) |
computes natural (base e) logarithm (ln(x)) (function) |
| (C++11)(C++11) |
computes common (base 10) logarithm (log10(x)) (function) |
| (C++11)(C++11)(C++11) |
natural logarithm (to base e) of 1 plus the given number (ln(1+x)) (function) |
| (C++11)(C++11)(C++11) |
returns 2 raised to the given power (2x) (function) |
| C documentation for log2
| |
