Is there a standard sign function (signum, sgn) in C/C++?


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.

1
377
12/24/2009 2:40:14 AM

Accepted Answer

Surprised no one has posted the 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 might take longer to compile in some circumstances.
  • 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.)

474
8/10/2019 2:41:41 PM

I don't know of a standard function for it. Here's an interesting way to write it though:

(x > 0) - (x < 0)

Here's a more readable way to do it:

if (x > 0) return 1;
if (x < 0) return -1;
return 0;

If you like the ternary operator you can do this:

(x > 0) ? 1 : ((x < 0) ? -1 : 0)

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