init(_:)

Discussion 解説

The string passed as text can represent a real number in decimal or hexadecimal format or can be in a special format representing infinity or NaN (“not a number”). If text is not in a recognized format, the optional initializer will fail and return nil.

The text string consists of an optional plus or minus sign character (+ or -) followed by one of the following:

• A decimal string contains a significand consisting of one or more decimal digits that may include a decimal point:

  
  let c = Double("-1.0")
  // c == -1.0
  
  
  let d = Double("28.375")
  // d == 28.375

  A decimal string may also include an exponent following the significand, indicating the power of 10 by which the significand should be multiplied. If included, the exponent is separated by a single character, e or E, and consists of an optional plus or minus sign character and a sequence of decimal digits. もし含むならば、指数は１つの文字eもしくはEによって区切られます、そして随意のプラスまたはマイナス符号文字と一連の10進の桁から成り立ちます。

  
  let e = Double("2837.5e-2")
  // e == 28.375

• A hexadecimal string contains a significand consisting of 0X or 0x followed by one or more hexadecimal digits that may include a decimal point.

  
  let f = Double("0x1c.6")
  // f == 28.375

  A hexadecimal string may also include an exponent indicating the power of 2 by which the significand should be multiplied. If included, the exponent is separated by a single character, p or P, and consists of an optional plus or minus sign character and a sequence of decimal digits. もし含むならば、指数は１つの文字pもしくはPによって区切られます、そして随意のプラスまたはマイナス符号文字と一連の10進の桁から成り立ちます。

  
  let g = Double("0x1.c6p4")
  // g == 28.375

• The input strings "inf" or "infinity" (case insensitive) are converted to an infinite result:

  
  let i = Double("inf")
  // i == Double.infinity
  
  
  let j = Double("-Infinity")
  // j == -Double.infinity

• An input string of "nan" (case insensitive) is converted into a NaN value:

  
  let n = Double("-nan")
  // n?.isNaN == true
  // n?.sign == .minus

  A NaN string may also include a payload in parentheses following the "nan" keyword. The payload consists of a sequence of decimal digits, or the characters 0X or 0x followed by a sequence of hexadecimal digits. If the payload contains any other characters, it is ignored. If the value of the payload is larger than can be stored as the payload of a Double.nan, the least significant bits are used. ペイロードは、一連の10進の桁、または文字0Xか0xに続く一連の16進の桁から成ります。ペイロードが何か他の文字を含むならば、それは無視されます。ペイロードの値がDouble.nanのペイロードとして格納可能であるより大きいならば、最下位ビットが使われます。

  
  let p = Double("nan(0x10)")
  // p?.isNaN == true
  // String(p!) == "nan(0x10)"

A string in any other format than those described above or containing additional characters results in a nil value. For example, the following conversions result in nil:

  Double(" 5.0")      // Includes whitespace
  Double("±2.0")      // Invalid character
  Double("0x1.25e4")  // Incorrect exponent format

A decimal or hexadecimal string is converted to a Double instance using the IEEE 754 roundTiesToEven (default) rounding attribute. Values with absolute value smaller than Double.leastNonzeroMagnitude are rounded to plus or minus zero. Values with absolute value larger than Double.greatestFiniteMagnitude are rounded to plus or minus infinity.

  let y = Double("1.23e-9999")
  // y == 0.0
  // y?.sign == .plus


  let z = Double("-7.89e-7206")
  // z == -0.0
  // z?.sign == .minus


  let r = Double("1.23e17802")
  // r == Double.infinity


  let s = Double("-7.89e7206")
  // s == Double.-infinity