How can I separate negative zero from positive zero?

Question

I want to be able to test if zero is positive or negative in swift.

let neg: CGFloat = -0
let pos: CGFloat = +0

if pos == neg {
    // this gets executed, but I don't want this
}

The code above does not work like I need it to. Can someone help me?

When would you need to know positive vs negative 0? – Michael Ozeryansky Aug 23 '20 at 04:16 — Michael Ozeryansky, Aug 23 '20 at 04:16

Martin R · Accepted Answer · 2018-10-25T07:19:28.080

7

There are “negative zero” and “positive zero” floating point numbers. You can disambiguate them by checking the .sign property, but they (intentionally) compare as equal:

let neg: CGFloat = -0.0
let pos: CGFloat = +0.0

print(neg.sign) // minus
print(pos.sign) // plus
print(neg == pos) // true

if (neg == pos && neg.sign == pos.sign) {
    // This is not executed.
}

Note that you have to use floating point literals ("-0.0" or "+0.0") in the initialization. The integer literals ("+0", "-0") are equal and converted to positive zero.

edited Oct 25 '18 at 07:19

answered Oct 25 '18 at 07:11

Martin R

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Thank you ;) Worked out for me – swift-lynx Oct 25 '18 at 09:07

Ahmad F · Answer 2 · 2018-10-25T07:26:14.250

Basically, from the ordinary arithmetic point view both neg and pos are identical zeros, therefore if pos == neg is true.

However, when it comes to representing a floating-point number, CGFloat has sign property as FloatingPointSign enum:

minus if the sign bit of self is set, and plus otherwise. Implements the IEEE 754 sign bit operation.

Therefore, your if-statement would be:

if neg.sign == pos.sign { } // false

How can I separate negative zero from positive zero?

2 Answers2