I'd like to get the sign of a float
value as an int
value of -1 or 1.
Avoiding conditionals is always a good idea in reducing computational cost. For instance, one way I can think of would be to use a fast bit-shift
to get the sign:
float a = ...;
int sign = a >> 31; //0 for pos, 1 for neg
sign = ~sign; //1 for pos, 0 for neg
sign = sign << 1; //2 for pos, 0 for neg
sign -= 1; //-1 for pos, 1 for neg -- perfect.
Or more concisely:
int sign = (~(a >> 31) << 1) - 1;
Any reasons why you don't simply use:
int sign = (int) Math.signum(a); //1 cast for floating-points, 2 for Integer types
Additionally most Number implementations have a signum method taking a primitive of that type and returning an int, so you can avoid casting for extra performance.
int sign1 = Integer.signum(12); //no casting
int sign2 = Long.signum(-24l); //no casting
It will return +1 / 0 / -1 and it has been optimized to deliver a good performance.
For reference, you can have a look at the implementation in openJDK. The relevant bits are:
public static float signum(float f) {
return (f == 0.0f || isNaN(f)) ? f : copySign(1.0f, f);
}
public static boolean isNaN(float f) {
return (f != f);
}
public static float copySign(float magnitude, float sign) {
return rawCopySign(magnitude, (isNaN(sign) ? 1.0f : sign));
}
public static float rawCopySign(float magnitude, float sign) {
return Float.intBitsToFloat((Float.floatToRawIntBits(sign)
& (FloatConsts.SIGN_BIT_MASK))
| (Float.floatToRawIntBits(magnitude)
& (FloatConsts.EXP_BIT_MASK
| FloatConsts.SIGNIF_BIT_MASK)));
}
static class FloatConsts {
public static final int SIGN_BIT_MASK = -2147483648;
public static final int EXP_BIT_MASK = 2139095040;
public static final int SIGNIF_BIT_MASK = 8388607;
}