init(_:)

The string passed as text can represent a real number in decimal or hexadecimal format or special floating-point values for infinity and NaN (“not a number”).

The given string may begin with a plus or minus sign character (+ or -). The allowed formats for each of these representations is then as follows:

• A decimal value contains the significand, a sequence of 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 value 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.

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

• A hexadecimal value contains the significand, either 0X or 0x, followed by a sequence of hexadecimal digits. The significand may include a decimal point.

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

  A hexadecimal value may also include an exponent following the significand, 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.

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

• A value of infinity contains one of the strings "inf" or "infinity", case insensitive.

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

• A value of NaN contains the string "nan", case insensitive.

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

  A NaN value 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.

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

Passing any other format or any additional characters as text results in nil. For example, the following conversions result in nil:

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