| Home | Libraries | People | FAQ | More

## Sign Manipulation Functions

##### Synopsis

    #include <boost/math/special_functions/sign.hpp>

    namespace boost{ namespace math{

    template<class T>
    int signbit(T x);

    template <class T>
    int sign (const T& z);

    template <class T, class U>
    T copysign (const T& x, const U& y);

    template <class T>
    calculated-result-type changesign (const T& z);

    }} // namespaces

##### Description

    template<class T>
    int signbit(T x);

Returns a non-zero value if the sign bit is set in variable x, otherwise `0`.

| Important
The return value from this function is zero or not-zero and not zero or one.

    template <class T>
    int sign (const T& z);

Returns `1` if x `> 0`, `-1` if x `< 0`, and `0` if x is zero.

    template <class T, class U>
    calculated-result-type copysign (const T& x, const U& y);

Sets the sign of x to be the same as the sign of y.

See C99 7.12.11.1 The copysign functions for more detail.

    template <class T>
    T changesign (const T& z);

Returns a floating point number with a binary representation where the signbit is the opposite of the sign bit in x, and where the other bits are the same as in x.

This function is widely available, but not specified in any standards.

Rationale: Not specified by TR1, but `changesign(x)` is both easier to read and more efficient than

    copysign(x, signbit(x) ? 1.0 : -1.0);

For finite values, this function has the same effect as simple negation, the assignment z = -z, but for nonfinite values, infinities and NaNs, the `changesign(x)` function may be the only portable way to ensure that the sign bit is changed.

###### Sign bits

One of the bits in the binary representation of a floating-point number gives the sign, and the remaining bits give the absolute value. That bit is known as the sign bit. The sign bit is set = 1 for negative numbers, and is not set = 0 for positive numbers. (This is true for all binary representations of floating point numbers that are used by modern microprocessors.)

C++ TR1 specifies `copysign` functions and function templates for accessing the sign bit.

For user-defined types (UDT), the sign may be stored in some other way. They may also not provide infinity or NaNs. To use these functions with a UDT, it may be necessary to explicitly specialize then for UDT type T.

###### Examples

    signbit(3.5) is zero (or false)
    signbit(-7.1) is 1 (or true)
    copysign(4.2, 7.9) is 4.2
    copysign(3.5 -1.4) is -3.5
    copysign(-4.2, 1.0) is 4.2
    copysign(-8.6, -3.3) is -8.6
    changesign(6.9) is -6.9
    changesign(-1.8) is 1.8

###### Portability

The library supports the following binary floating-point formats:

- IEEE 754 single precision
- IEEE 754 double precision
- IEEE 754 extended double precision with 15 exponent bits
- Intel extended double precision
- PowerPC extended double precision
- Motorola 68K extended double precision

The library does not support the VAX floating-point formats. (These are available on VMS, but the default on VMS is the IEEE 754 floating-point format.)

The main portability issues are:

- Unsupported floating point formats
- The library depends on the header boost/detail/endian.hpp
- Code such as `#if defined(__ia64) || defined(__ia64__) || defined(_M_IA64)` is used to determine the processor type.

The library has passed all tests on the following platforms:

- Win32 / MSVC 7.1 / 10.0 / x86
- Win32 / Intel C++ 7.1, 8.1, 9.1 / x86
- Mac OS X / GCC 3.3, 4.0 / ppc
- Linux / Intel C++ 9.1 / x86, ia64
- Linux / GCC 3.3 / x86, x64, ia64, ppc, hppa, mips, m68k
- Linux / GCC 3.4 / x64
- HP-UX / aCC, GCC 4.1 / ia64
- HP-UX / aCC / hppa
- Tru64 / Compaq C++ 7.1 / alpha
- VMS / HP C++ 7.1 / alpha (in IEEE floating point mode)
- VMS / HP C++ 7.2 / ia64 (in IEEE floating point mode)