具有分支的if-else语句类似于其他语言。 不同于其它，布尔条件不需要由括号包围，并且每个条件后跟一个块。if-else 条件等都是表达式，以及所有分支必须返还相同种类。
fn main() {
let n = 5;
if n < 0 {
print!("{} is negative", n);
} else if n > 0 {
print!("{} is positive", n);
} else {
print!("{} is zero", n);
}
let big_n =
if n < 10 && n > -10 {
println!(", and is a small number, increase ten-fold");
// This expression returns an `i32`.
10 * n
} else {
println!(", and is a big number, reduce by two");
// This expression must return an `i32` as well.
n /2
// TODO ^ Try suppressing this expression with a semicolon.
};
// ^ Don't forget to put a semicolon here! All `let` bindings need it.
println!("{} -> {}", n, big_n);
}
