条件语句 (Conditionals)

An if expression specifies that some code should only be evaluated if a certain condition is true. For example:

if 语句可以用来指定一块代码块，但只在判断条件(condition)为true时才会被执行。例如:

if (x > 5) x = x - 5

The condition must be an expression of type bool.

An if expression can optionally include an else clause to specify another expression to evaluate when the condition is false.

条件语句(condition)必须是 bool 类型的表达式。

if 语句可选包含 else 子句，以指定当条件(condition)为 false 时要执行的另一个代码块。

if (y <= 10) y = y + 1 else y = 10

Either the "true" branch or the "false" branch will be evaluated, but not both. Either branch can be a single expression or an expression block.

The conditional expressions may produce values so that the if expression has a result.

无论是"true"分支还是"false"分支都会被执行，但不会同时执行.其中任何一个分支都可以是单行代码或代码块。条件表达式会产生值，所以 if 表达式会有一个结果。

let z = if (x < 100) x else 100;

The expressions in the true and false branches must have compatible types. For example:

true 和 false 分支的表达式类型必须是一致的,例如:

// x和y必须是u64整型
// x and y must be u64 integers
let maximum: u64 = if (x > y) x else y;

// 错误！分支的类型不一致
// (ERROR! branches different types)
let z = if (maximum < 10) 10u8 else 100u64;

// 错误！分支的类型不一致,false-branch默认是()不是u64
// ERROR! branches different types, as default false-branch is () not u64
if (maximum >= 10) maximum;

If the else clause is not specified, the false branch defaults to the unit value. The following are equivalent:

如果else子句未定义，false分支默认为 unit 。下面的例子是相等价的:

if (condition) true_branch // implied default: else ()
if (condition) true_branch else ()

Commonly, if expressions are used in conjunction with expression blocks.

一般来说, if 表达式与多个表达式块结合使用.

let maximum = if (x > y) x else y;
if (maximum < 10) {
    x = x + 10;
    y = y + 10;
} else if (x >= 10 && y >= 10) {
    x = x - 10;
    y = y - 10;
}

条件语句的语法 (Grammar for Conditionals)

if-expression → if ( expression ) expression else-clause_opt else-clause → else expression