Is there a standard sign function (signum, sgn) in

Question

I want a function that returns -1 for negative numbers and +1 for positive numbers. http://en.wikipedia.org/wiki/Sign_function It's easy enough to write my own, but it seems like something that ought to be in a standard library somewhere.

Edit: Specifically, I was looking for a function working on floats.

Answer 1

If all you want is to test the sign, use signbit (returns true if its argument has a negative sign). Not sure why you would particularly want -1 or +1 returned; copysign is more convenient for that, but it sounds like it will return +1 for negative zero on some platforms with only partial support for negative zero, where signbit presumably would return true.

Answer 2

use:

`#define sgn(x) (x<0)` 

for example:

`if(sng(n)) { etc ....}`

Or you may want to use some elaborated code, but casting first:

inline bool sgn_long(long x) { return ((x<0)? true: false); }

Answer 3

No, it doesn't exist in c++, like in matlab. I use a macro in my programs for this.

#define sign(a) ( ( (a) < 0 )  ?  -1   : ( (a) > 0 ) )

Answer 4

Bit off-topic, but I use this:

template<typename T>
constexpr int sgn(const T &a, const T &b) noexcept{
    return (a > b) - (a < b);
}

template<typename T>
constexpr int sgn(const T &a) noexcept{
    return sgn(a, T(0));
}

and I found first function - the one with two arguments, to be much more useful from "standard" sgn(), because it is most often used in code like this:

int comp(unsigned a, unsigned b){
   return sgn( int(a) - int(b) );
}

vs.

int comp(unsigned a, unsigned b){
   return sgn(a, b);
}

there is no cast for unsigned types and no additional minus.

in fact i have this piece of code using sgn()

template <class T>
int comp(const T &a, const T &b){
    log__("all");
    if (a < b)
        return -1;

    if (a > b)
        return +1;

    return 0;
}

inline int comp(int const a, int const b){
    log__("int");
    return a - b;
}

inline int comp(long int const a, long int const b){
    log__("long");
    return sgn(a, b);
}

Answer 5

int sign(float n)
{     
  union { float f; std::uint32_t i; } u { n };
  return 1 - ((u.i >> 31) << 1);
}

This function assumes:

• binary32 representation of floating point numbers
• a compiler that make an exception about the strict aliasing rule when using a named union

Answer 6

Surprised no one has posted the branchless, type-safe C++ version yet:

template <typename T> int sgn(T val) {
    return (T(0) < val) - (val < T(0));
}

Benefits:

• Actually implements signum (-1, 0, or 1). Implementations here using copysign only return -1 or 1, which is not signum. Also, some implementations here are returning a float (or T) rather than an int, which seems wasteful.
• Works for ints, floats, doubles, unsigned shorts, or any custom types constructible from integer 0 and orderable.
• Fast! copysign is slow, especially if you need to promote and then narrow again. This is branchless and optimizes excellently
• Standards-compliant! The bitshift hack is neat, but only works for some bit representations, and doesn't work when you have an unsigned type. It could be provided as a manual specialization when appropriate.
• Accurate! Simple comparisons with zero can maintain the machine's internal high-precision representation (e.g. 80 bit on x87), and avoid a premature round to zero.

Caveats:

• It's a template so it'll take forever to compile.
• Apparently some people think use of a new, somewhat esoteric, and very slow standard library function that doesn't even really implement signum is more understandable.

• The < 0 part of the check triggers GCC's -Wtype-limits warning when instantiated for an unsigned type. You can avoid this by using some overloads:

  template <typename T> inline constexpr
  int signum(T x, std::false_type is_signed) {
      return T(0) < x;
  }
  
  template <typename T> inline constexpr
  int signum(T x, std::true_type is_signed) {
      return (T(0) < x) - (x < T(0));
  }
  
  template <typename T> inline constexpr
  int signum(T x) {
      return signum(x, std::is_signed<T>());
  }
  

  (Which is a good example of the first caveat.)