Conditionals

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

条件句

if 表达式指定仅当某个条件为真时才应评估某些代码。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.

条件必须是布尔类型的表达式。

if 表达式可以选择包含 else 子句，以指定另一个表达式在条件为假时进行评估。

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. 将评估“真”分支或“假”分支，但不会同时评估两者。任何一个分支都可以是单个表达式或表达式块。

条件表达式可以产生值，以便 if 表达式有结果。

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 分支中的表达式必须具有兼容的类型。例如：

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

// ERROR! branches different types
let z = if (maximum < 10) 10u8 else 100u64;

// 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:

如果没有指定 else 子句，则 false 分支默认为单位值。以下是等价的：

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

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

通常，if 表达式与表达式块一起使用。

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;
}

Grammar for Conditionals

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

条件语法

if 表达式 → if ( 表达式 ) 表达式 else-clauseopt else-clause → else 表达式