4.Operators-操作符(Dart中文文檔)
阿新 • • 發佈：2018-12-21
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Dart有如下操作符：
| Description | Operator |
|---|---|
| unary postfix | expr++ expr-- () [] . ?. |
| unary prefix | -expr !expr ~expr ++expr --expr |
| multiplicative | * / % ~/ |
| additive | + - |
| shift | << >> |
| bitwise AND | & |
| bitwise XOR | ^ |
| bitwise OR | | |
| relational and type test | >= > <= < as is is! |
| equality | == != |
| logical AND | && |
| logical OR | || |
| if null | ?? |
| conditional | expr1 ? expr2 : expr3 |
| cascade | .. |
| assignment | = *= /= ~/= %= += -= <<= >>= &= ^= |= ??= |
在操作符表中，優先級是從上到下遞減。例如，即便&&在==之前，優先級是求余（%）>判斷相等（==）> 且(&&)。下面代碼中，計算順序是一致的
// Parentheses improve readability. if ((n % i == 0) && (d % i == 0)) ... // Harder to read, but equivalent. if (n % i == 0 && d % i == 0) ...
註意：操作符左右各有一個對象，操作符具體行為是由左邊的對象決定的，這個涉及到操作符重載，比如Vector和Point對象重載了+號操作符的行為，Vector+Point的+號具體行為由Vector決定。
Arithmetic operators 算術運算符
Dart supports the usual arithmetic operators, as shown in the following table.
Dart支持常見的幾種算術運算符
| Operator | Meaning |
|---|---|
| + | Add |
| – | Subtract |
| -expr | Unary minus, also known as negation (reverse the sign of the expression) |
| * | Multiply |
| / | Divide |
| ~/ | Divide, returning an integer result |
| % | Get the remainder of an integer division (modulo) |
assert(2 + 3 == 5);
assert(2 - 3 == -1);
assert(2 * 3 == 6);
assert(5 / 2 == 2.5); // Result is a double
assert(5 ~/ 2 == 2); // Result is an int
assert(5 % 2 == 1); // Remainder
assert(‘5/2 = ${5 ~/ 2} r ${5 % 2}‘ == ‘5/2 = 2 r 1‘);Dart也支持自增自減運算符
| Operator | Meaning |
|---|---|
| ++var | var = var + 1 (expression value is var + 1) |
| var++ | var = var + 1 (expression value is var) |
| --var | var = var – 1 (expression value is var – 1) |
| var-- | var = var – 1 (expression value is var) |
var a, b;
a = 0;
b = ++a; // Increment a before b gets its value.
assert(a == b); // 1 == 1Equality and relational operators 關系運算符
| Operator | Meaning |
|---|---|
| == | Equal; see discussion below |
| != | Not equal |
| > | Greater than |
| < | Less than |
| >= | Greater than or equal to |
| <= | Less than or equal to |
驗證兩個對象是否相同，用==操作符,（在極少數情況下，你要判斷兩個對象是否完全相同，需要用identical()函數）
下面是==的判斷邏輯：
步驟1.如果x,y都是null,返回true ,x,y只有一個是null,返回false
步驟2.返回x.==(y)的結果，這種寫法類似a.method(params),可以將==看做x的方法。
assert(2 == 2);
assert(2 != 3);
assert(3 > 2);
assert(2 < 3);
assert(3 >= 3);
assert(2 <= 3);
a = 0;
b = a++; // Increment a AFTER b gets its value.
assert(a != b); // 1 != 0a = 0;
b = --a; // Decrement a before b gets its value.
assert(a == b); // -1 == -1
a = 0;
b = a--; // Decrement a AFTER b gets its value.
assert(a != b); // -1 != 0
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4.Operators-操作符(Dart中文文檔)