Rust Conditional Statements

The conditional statements in Rust language are in this format:

fn main() {
　　　　let number =　3;　
　　　　if number <　5　{　
　　　　　　　　println!("Condition is true");　
　　　　} else {　
　　　　　　　　println!("Condition is false");　
　　　　}　
}

In the above program, there is a conditional if statement, which is very common in many other languages, but there are also some differences: first, the conditional expression is number < 5 It is not necessary to use parentheses to enclose (note that it is not forbidden, not not allowed); However, Rust does not have the rule that a single statement does not need to be enclosed in {} without adding {}, and it is not allowed to use a single statement to replace a block. Despite this, Rust still supports traditional else-if statement syntax:

fn main() {　
　　　　let a =　12;　
　　　　let b;　
　　　　if a > 0 {　
　　　　　　　　b =　1;　
　　　　}　　
　　　　else if a < 0 {　
　　　　　　　　b =　-1;　
　　　　}　　
　　　　else {　
　　　　　　　　b = 0;　
　　　　}　
　　　　println!("b is {}", b);　
}

Running Result:

b is　1

Conditional expressions in Rust must be of bool type, for example, the following program is incorrect:

fn main() {　
　　　　let number =　3;　
　　　　if number {　　　//　Error, expected `bool`, found integer (rustc(E0308)
　　　　　　　　println!("Yes");
　　　　}　
}

Although C/C++ In the language, the conditional expression is represented by integers, 0 is false, 1 is true, but this rule is prohibited in many languages that pay attention to code safety.

Combining the function body expressions learned in the previous chapter, we can associate them:

if <condition> { block　1　} else { block　2　}

the syntax of this is { block 1 } and { block 2 } Can it be a function body expression?

The answer is affirmative! That is to say, in Rust, we can use if-else structure implementation is similar to the ternary conditional expression (A ? B : C) Effect:

fn main() {　
　　　　let a =　3;　
　　　　let number = if a > 0 {　1　} else {　-1　};　
　　　　println!("number is {}", number);　
}

Running Result:

number is　1

NoteThe types of two function body expressions must be the same! And there must be an else and its subsequent expression block.