ilogb, ilogbf, ilogbl
Defined in header <math.h>
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int ilogbf( float arg ); |
(1) | (since C99) |
int ilogb( double arg ); |
(2) | (since C99) |
int ilogbl( long double arg ); |
(3) | (since C99) |
Defined in header <tgmath.h>
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#define ilogb( arg ) |
(4) | (since C99) |
Defined in header <math.h>
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#define FP_ILOGB0 /* implementation-defined */ |
(5) | (since C99) |
#define FP_ILOGBNAN /* implementation-defined */ |
(6) | (since C99) |
ilogbl
is called. Otherwise, if arg has integer type or the type double, ilogb
is called. Otherwise, ilogbf
is called.Formally, the unbiased exponent is the integral part of logr|arg| as a signed integral value, for non-zero arg, where r
is FLT_RADIX.
Contents |
[edit] Parameters
arg | - | floating-point value |
[edit] Return value
If no errors occur, the unbiased exponent of arg is returned as a signed int value.
If arg is zero, FP_ILOGB0 is returned.
If arg is infinite, INT_MAX is returned.
If arg is a NaN, FP_ILOGBNAN is returned.
If the correct result is greater than INT_MAX or smaller than INT_MIN, the return value is unspecified and a domain error or range error may occur.
[edit] Error handling
Errors are reported as specified in math_errhandling
.
A domain error or range error may occur if arg is zero, infinite, or NaN.
If the correct result is greater than INT_MAX or smaller than INT_MIN, a domain error or a range error may occur
If the implementation supports IEEE floating-point arithmetic (IEC 60559),
- If the correct result is greater than INT_MAX or smaller than INT_MIN, FE_INVALID is raised.
- If arg is ±0, ±∞, or NaN, FE_INVALID is raised.
- In all other cases, the result is exact (FE_INEXACT is never raised) and the current rounding mode is ignored.
[edit] Notes
If arg is not zero, infinite, or NaN, the value returned is exactly equivalent to (int)logb(arg).
POSIX requires that a domain error occurs if arg is zero, infinite, NaN, or if the correct result is outside of the range of int.
POSIX also requires that, on XSI-conformant systems, the value returned when the correct result is greater than INT_MAX is INT_MAX and the value returned when the correct result is less than INT_MIN is INT_MIN.
The correct result can be represented as int on all known implementations. For overflow to occur, INT_MAX must be less than LDBL_MAX_EXP * log2(FLT_RADIX) or INT_MIN must be greater than LDBL_MIN_EXP - LDBL_MANT_DIG) * log2(FLT_RADIX).
The value of the exponent returned by ilogb
is always 1 less than the exponent retuned by frexp because of the different normalization requirements: for the exponent e
returned by ilogb
, |arg*r-e
| is between 1 and r
(typically between 1 and 2), but for the exponent e
returned by frexp, |arg*2-e
| is between 0.5 and 1.
[edit] Example
#include <fenv.h> #include <float.h> #include <math.h> #include <stdio.h> // #pragma STDC FENV_ACCESS ON int main(void) { double f = 123.45; printf("Given the number %.2f or %a in hex,\n", f, f); double f3; double f2 = modf(f, &f3); printf("modf() makes %.0f + %.2f\n", f3, f2); int i; f2 = frexp(f, &i); printf("frexp() makes %f * 2^%d\n", f2, i); i = ilogb(f); printf("logb()/ilogb() make %f * %d^%d\n", f/scalbn(1.0, i), FLT_RADIX, i); // error handling feclearexcept(FE_ALL_EXCEPT); printf("ilogb(0) = %d\n", ilogb(0)); if (fetestexcept(FE_INVALID)) puts(" FE_INVALID raised"); }
Possible output:
Given the number 123.45 or 0x1.edccccccccccdp+6 in hex, modf() makes 123 + 0.45 frexp() makes 0.964453 * 2^7 logb()/ilogb() make 1.92891 * 2^6 ilogb(0) = -2147483648 FE_INVALID raised
[edit] References
- C23 standard (ISO/IEC 9899:2024):
- 7.12/8 Mathematics <math.h> (p: TBD)
- 7.12.6.5 The ilogb functions (p: TBD)
- 7.25 Type-generic math <tgmath.h> (p: TBD)
- F.10.3.5 The ilogb functions (p: TBD)
- C17 standard (ISO/IEC 9899:2018):
- 7.12/8 Mathematics <math.h> (p: TBD)
- 7.12.6.5 The ilogb functions (p: TBD)
- 7.25 Type-generic math <tgmath.h> (p: TBD)
- F.10.3.5 The ilogb functions (p: TBD)
- C11 standard (ISO/IEC 9899:2011):
- 7.12/8 Mathematics <math.h> (p: 232)
- 7.12.6.5 The ilogb functions (p: 244)
- 7.25 Type-generic math <tgmath.h> (p: 373-375)
- F.10.3.5 The ilogb functions (p: 521)
- C99 standard (ISO/IEC 9899:1999):
- 7.12/8 Mathematics <math.h> (p: 213)
- 7.12.6.5 The ilogb functions (p: 224-225)
- 7.22 Type-generic math <tgmath.h> (p: 335-337)
- F.9.3.5 The ilogb functions (p: 458)
[edit] See also
(C99)(C99) |
breaks a number into significand and a power of 2 (function) |
(C99)(C99)(C99) |
extracts exponent of the given number (function) |
(C99)(C99)(C99)(C99)(C99)(C99) |
computes efficiently a number times FLT_RADIX raised to a power (function) |
C++ documentation for ilogb
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