What is use of underScore in Double value in swift? [duplicate]

Question

I just recently look into Apple documentation I found code like below

extension Double {
    var km: Double { return self * 1_000.0 }
    var m: Double { return self }
    var cm: Double { return self / 100.0 }
    var mm: Double { return self / 1_000.0 }
    var ft: Double { return self / 3.28084 }
}

What is the use of "_" in double value? it's working without error why?

Answer 1

Just a way to improve readability. According to Swift documentation:

Numeric literals can contain extra formatting to make them easier to read. Both integers and floats can be padded with extra zeros and can contain underscores to help with readability. Neither type of formatting affects the underlying value of the literal.