log2, log2f, log2l - cppreference.com (original) (raw)

1-3) Computes the base 2 logarithm of arg.

1. Type-generic macro: If arg has type long double, log2l is called. Otherwise, if arg has integer type or the type double, log2 is called. Otherwise, log2f is called.

Contents

• 1 Parameters
• 2 Return value
• 3 Error handling
• 4 Notes
• 5 Example
• 6 References
• 7 See also

[edit] Parameters

[edit] Return value

If no errors occur, the base-2 logarithm of arg (log2(arg) or lb(arg)) 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 arg is less than zero.

Pole error may occur if arg 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 arg, the binary logarithm can be interpreted as the zero-based index of the most significant 1 bit in the input.

[edit] Example

#include <stdio.h> #include <math.h> #include <float.h> #include <errno.h> #include <fenv.h> // #pragma STDC FENV_ACCESS ON int main(void) { printf("log2(65536) = %f\n", log2(65536)); printf("log2(0.125) = %f\n", log2(0.125)); printf("log2(0x020f) = %f (highest set bit is in position 9)\n", log2(0x020f)); printf("base-5 logarithm of 125 = %f\n", log2(125)/log2(5)); // special values printf("log2(1) = %f\n", log2(1)); printf("log2(+Inf) = %f\n", log2(INFINITY)); //error handling errno = 0; feclearexcept(FE_ALL_EXCEPT); printf("log2(0) = %f\n", log2(0)); if(errno == ERANGE) perror(" errno == ERANGE"); if(fetestexcept(FE_DIVBYZERO)) puts(" FE_DIVBYZERO raised"); }

Possible output:

log2(65536) = 16.000000 log2(0.125) = -3.000000 log2(0x020f) = 9.041659 (highest set bit is in position 9) base-5 logarithm of 125 = 3.000000 log2(1) = 0.000000 log2(+Inf) = inf log2(0) = -inf errno == ERANGE: Numerical result out of range FE_DIVBYZERO raised

[edit] References

• C17 standard (ISO/IEC 9899:2018):

• 7.12.6.10 The log2 functions (p: 179)

• 7.25 Type-generic math <tgmath.h> (p: 272-273)

• F.10.3.10 The log2 functions (p: 381)

• C11 standard (ISO/IEC 9899:2011):

• 7.12.6.10 The log2 functions (p: 246)

• 7.25 Type-generic math <tgmath.h> (p: 373-375)

• F.10.3.10 The log2 functions (p: 522)

• C99 standard (ISO/IEC 9899:1999):

• 7.12.6.10 The log2 functions (p: 226)

• 7.22 Type-generic math <tgmath.h> (p: 335-337)

• F.9.3.10 The log2 functions (p: 459)

[edit] See also

| | computes natural (base-e) logarithm (\({\small \ln{x} }\)ln(x)) (function) [edit] | | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | | computes common (base-10) logarithm (\({\small \log_{10}{x} }\)log10(x)) (function) [edit] | | | computes natural (base-e) logarithm of 1 plus the given number (\({\small \ln{(1+x)} }\)ln(1+x)) (function) [edit] | | | computes 2 raised to the given power (\({\small 2^x}\)2x) (function) [edit] | | |