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hainuo authored 2015-07-08 22:38 . 语法变量引用

% if if语句

Rust’s take on if is not particularly complex, but it’s much more like the if you’ll find in a dynamically typed language than in a more traditional systems language. So let’s talk about it, to make sure you grasp the nuances.

Rust对if的处理并不是特别的复杂,但是比起传统的系统语言,你会发现它更像动态类型语言中的if。所以我们来讨论它吧,以确保你掌握细节。

if is a specific form of a more general concept, the ‘branch’. The name comes from a branch in a tree: a decision point, where depending on a choice, multiple paths can be taken.

if 是一个广泛概念“分支”的一个明确的形式。这个名字来自于一个树状分支,依赖于一个选择点,有多条路径可以选择。

In the case of if, there is one choice that leads down two paths:

if案例中,每一个选择都会引出亮条路径

let x = 5;

if x == 5 {
    println!("x is five!");
}

If we changed the value of x to something else, this line would not print.More specifically, if the expression after the if evaluates to true, then the block is executed. If it’s false, then it is not.

如果我们改变了x的值,这一行就不会被打印。更确切的说,如果if后面的表达式等于true,代码块就被执行,如果它是false,就不会执行。

If you want something to happen in the false case, use an else:

如果你你想在false情况下发生一些事情,请使用else

let x = 5;

if x == 5 {
    println!("x is five!");
} else {
    println!("x is not five :(");
}

If there is more than one case, use an else if:

如果多余一个条件,请使用else if

let x = 5;

if x == 5 {
    println!("x is five!");
} else if x == 6 {
    println!("x is six!");
} else {
    println!("x is not five or six :(");
}

This is all pretty standard. However, you can also do this:

这是最完美的标准的,然而你也可以这样做:

let x = 5;

let y = if x == 5 {
    10
} else {
    15
}; // y: i32

Which we can (and probably should) write like this:

我们可以(并且可能应该)这样写:

let x = 5;

let y = if x == 5 { 10 } else { 15 }; // y: i32

This works because if is an expression. The value of the expression is the value of the last expression in whichever branch was chosen. An if without an else always results in () as the value.

这样做是因为if是一个表达式。表达式的值是被选择的分支的最后的表达式的值。一个没有elseif总是会导致以()作为值。

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