add some origin

source/chapter2/02_Basic_Operators.md

@@ -8,4 +8,84 @@ Unlike C, Swift lets you perform remainder (%) calculations on floating-point nu
 
 This chapter describes the common operators in Swift. Advanced Operators[…]”
 
-Excerpt From: Apple Inc. “The Swift Programming Language.” iBooks. https://itun.es/cn/jEUH0.l
+Excerpt From: Apple Inc. “The Swift Programming Language.” iBooks. https://itun.es/cn/jEUH0.l
+
+“Terminology
+Operators are unary, binary, or ternary:
+
+Unary operators operate on a single target (such as -a). Unary prefix operators appear immediately before their target (such as !b), and unary postfix operators appear immediately after their target (such as i++).
+Binary operators operate on two targets (such as 2 + 3) and are infix because they appear in between their two targets.
+Ternary operators operate on three targets. Like C, Swift has only one ternary operator, the ternary conditional operator (a ? b : c).”
+
+Excerpt From: Apple Inc. “The Swift Programming Language.” iBooks. https://itun.es/cn/jEUH0.l
+
+“The values that operators affect are operands. In the expression 1 + 2, the + symbol is a binary operator and its two operands are the values 1 and 2.
+
+‌
+Assignment Operator
+The assignment operator (a = b) initializes or updates the value of a with the value of b:
+
+let b = 10
+var a = 5
+a = b
+// a is now equal to 10
+If the right side of the assignment is a tuple with multiple values, its elements can be decomposed into multiple constants or variables at once:
+
+let (x, y) = (1, 2)
+// x is equal to 1, and y is equal to 2
+Unlike the assignment operator in C and Objective-C, the assignment operator in Swift does not itself return a value. The following statement is not valid:
+
+if x = y {
+    // this is not valid, because x = y does not return a value
+}
+This feature prevents the assignment operator (=) from being used by accident when the equal to operator (==) is actually intended. By making if x = y invalid, Swift helps you to avoid these kinds of errors in your code.
+
+‌
+Arithmetic Operators
+Swift supports the four standard arithmetic operators for all number types:
+
+Addition (+)
+Subtraction (-)
+Multiplication (*)
+Division (/)
+1 + 2[…]”
+
+Excerpt From: Apple Inc. “The Swift Programming Language.” iBooks. https://itun.es/cn/jEUH0.l
+
+“Unlike the arithmetic operators in C and Objective-C, the Swift arithmetic operators do not allow values to overflow by default. You can opt in to value overflow behavior by using Swift’s overflow operators (such as a &+ b). See Overflow Operators.
+
+The addition operator is also supported for String concatenation:
+
+"hello, " + "world"  // equals "hello, world"
+Two Character values, or one Character value and one String value, can be added together to make a new String value:
+
+let dog: Character = "🐶"
+let cow: Character = "🐮"
+let dogCow = dog + cow
+// dogCow is equal to "🐶🐮"
+See also Concatenating Strings and Characters.
+
+‌
+Remainder Operator
+The remainder operator (a % b) works out how many multiples of b will fit inside a and returns the value that is left over (known as the remainder).
+
+NOTE
+
+The remainder operator (%) is also known as a modulo operator in other languages. However, its behavior in Swift for negative numbers means that it is, strictly speaking, a remainder rather than a modulo operation.
+
+Here’s how the remainder operator works. To calculate 9 % 4, you first work out how many 4s will fit inside 9:
+
+ 
+You can fit two 4s inside 9, and the remainder is 1 (shown in orange).
+
+In Swift, this be written as:
+
+9 % 4    // equals 1
+To determine the answer for a % b, the % operator calculates the following equation and returns remainder as its output:
+
+a = (b × some multiplier) + remainder
+
+where some multiplier is the largest number of multiples of”
+
+Excerpt From: Apple Inc. “The Swift Programming Language.” iBooks. https://itun.es/cn/jEUH0.l
+