有没有办法在swift 4中迭代double var的小数部分？

Question

What i'm looking for:

Given a Double (doublenumber) and an Int (n) I wish to iterate trough the 1st decimal, 2nd decimal, 3rd decimal, 4th decimal.... until the 'n'decimal

My first approach was, coverting to String the Double so I could iterate like an array the string, but the problem is that when I convert to string I lose many decimals numbers

    let doubleNumber = 1.00/98                  //0.010204081632653061224489795918367346938775510204081632653...
    var stringFromDouble = String(doubleNumber) //0.010204081632653
    stringFromDouble.removeFirst()              //.010204081632653
    stringFromDouble.removeFirst()              //010204081632653

    for letter in stringFromDouble{
      //cycle to iterate the decimals
    }

Answer 1

If the intention is to get many decimal digits of 1.0/98.0 then you must not store that number in a Double in the first place, because that has a precision of approximately 16 decimal digits only. You could use Decimal which has a precision of 38 decimal digits.

But for more decimal digits you'll have to do “rational arithmetic,” ie work with numerator and denominator of the fraction as integers.

Here is how you can print arbitrarily many decimal digits of a rational number. For simplicity I have assumed that the number is positive and less than one.

func printDecimalDigits(of numerator: Int, dividedBy denominator: Int, count: Int) {
    var numerator = numerator
    for _ in 1...count {
        // Multiply by 10 to get the next digit:
        numerator *= 10
        // Print integer part of `numerator/denominator`:
        print(numerator / denominator, terminator: "")
        // Reduce `numerator/denominator` to its fractional part:
        numerator %= denominator
    }
    print()
}

Example:

printDecimalDigits(of: 1, dividedBy: 98, count: 100)
// 0102040816326530612244897959183673469387755102040816326530612244897959183673469387755102040816326530

Or as a function which returns the digits as a (lazily evaluated) sequence:

func decimalDigits(of numerator: Int, dividedBy denominator: Int) -> AnySequence<Int> {
    return AnySequence(sequence(state: numerator) { num -> Int in
        num *= 10
        let d = num / denominator
        num %= denominator
        return d
    })
}

Example:

let first1000Digits = decimalDigits(of: 1, dividedBy: 98).prefix(1000)
for d in first1000Digits { print(d) }