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Defined in header |
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float log2f( float arg ); |
(1) |
(since C99) |
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double log2( double arg ); |
(2) |
(since C99) |
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long double log2l( long double arg ); |
(3) |
(since C99) |
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Defined in header |
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#define log2( arg ) |
(4) |
(since C99) |
1-3) Computes the base2logarithm ofarg.
4) Type-generic macro: Ifarghas typelong double,log2lis called. Otherwise, ifarghas integer type or the typedouble,log2is called. Otherwise,log2fis called.
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arg |
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floating point value |
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If no errors occur, the base-2logarithm ofarg(log
2(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_VALLis returned.
Errors are reported as specified in math_errhandling.
Domain error occurs ifargis less than zero.
Pole error may occur ifargis zero.
If the implementation supports IEEE floating-point arithmetic (IEC 60559),
If the argument is ±0, -∞ is returned andFE_DIVBYZEROis raised.
If the argument is 1, +0 is returned
If the argument is negative, NaN is returned andFE_INVALIDis raised.
If the argument is +∞, +∞ is returned
If the argument is NaN, NaN is returned
For integerarg, the binary logarithm can be interpreted as the zero-based index of the most significant 1 bit in the input.
#include#include #include #include #include #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) = 16.000000log2(0.125) = -3.000000log2(0x020f) = 9.041659 (highest set bit is in position 9)base-5 logarithm of 125 = 3.000000log2(1) = 0.000000log2(+Inf) = inflog2(0) = -inf errno == ERANGE: Numerical result out of range FE_DIVBYZERO raised
C11 standard (ISO/IEC 9899:2011):
7.12.6.10 The log2 functions (p: 246)
7.25 Type-generic math
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
F.9.3.10 The log2 functions (p: 459)
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loglogflogl (C99)(C99) |
computes natural (base-e) logarithm (ln(x)) (function) |
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log10log10flog10l (C99)(C99) |
computes common (base-10) logarithm (log10(x)) (function) |
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log1plog1pflog1pl (C99)(C99)(C99) |
computes natural (base-e) logarithm of 1 plus the given number (ln(1+x)) (function) |
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exp2exp2fexp2l (C99)(C99)(C99) |
computes 2 raised to the given power (2x) (function) |
| C++ documentation for log2 |
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