點符號在迅速

Question

我使用的XCode

正好與此表達發生什麼通過https://github.com/nettlep/learn-swift第一基本遊樂場工作p表示十六進制數字文字？

0xC.3p0 == 12.1875 

我瞭解十六進制文字和特殊的「P」符號表示的2

0xF == 15 

0xFp0 == 15 // 15 * 2^0 

電源如果我嘗試0xC.3我得到的錯誤：十六進制浮點文字必須結束與指數。

我發現這個不錯overview of numeric literals和another deep explanation，但我沒有看到解釋什麼.3p0做什麼。

我分叉了代碼並將本課升級到了XCode 7/Swift 2 - 這裏是specific line。

Answer 1

這是Hexadecimal exponential notation。

By convention, the letter P (or p, for "power") represents times two raised to the power of ... The number after the P is decimal and represents the binary exponent.

...

Example: 1.3DEp42 represents hex(1.3DE) × dec(2^42).

對於你的榜樣，我們得到：

0xC.3p0 represents 0xC.3 * 2^0 = 0xC.3 * 1 = hex(C.3) = 12.1875 

where hex(C.3) = dec(12.{3/16}) = dec(12.1875) 

舉個例子，你可以嘗試0xC.3p1（等於hex(C.3) * dec(2^1)），這將產生雙重價值，即24.375。

您也可以研究在操場二進制指數增長的十六進制值1：

// ... 
print(0x1p-3) // 1/8 (0.125) 
print(0x1p-2) // 1/4 (0.25) 
print(0x1p-1) // 1/2 (0.5) 
print(0x1p1) // 2.0 
print(0x1p2) // 4.0 
print(0x1p3) // 8.0 
// ... 

---

最後，這也是Apple`s Language Reference - Lexical Types: Floating-Point Literals解釋說：

Hexadecimal floating-point literals consist of a 0x prefix, followed by an optional hexadecimal fraction, followed by a hexadecimal exponent. The hexadecimal fraction consists of a decimal point followed by a sequence of hexadecimal digits. The exponent consists of an upper- or lowercase p prefix followed by a sequence of decimal digits that indicates what power of 2 the value preceding the p is multiplied by. For example, 0xFp2 represents 15 x 2^2, which evaluates to 60. Similarly, 0xFp-2 represents 15 x 2^-2, which evaluates to 3.75.