计算Rust中的素数

Question

我试图计算Rust中的素数，但有一些问题。我收到两个错误。我不明白值是如何返回主函数的。

fn main() {
    let x = is_prime(25); //function calling
    println!("{}", x);
}

fn is_prime(n: u32) -> bool {
    let mut result: bool = for a in 2..n {
        result = if n % a == 0 { false } else { true };
    };
    result
}

error[E0425]: cannot find value `result` in this scope
 --> src/main.rs:8:9
  |
8 |         result = if n % a == 0 { false } else { true };
  |         ^^^^^^ not found in this scope
help: possible candidates are found in other modules, you can import them into scope
  |
1 | use futures::future::result;
  |
1 | use tokio::prelude::future::result;
  |

error[E0308]: mismatched types
 --> src/main.rs:7:28
  |
6 |   fn is_prime(n: u32) -> bool {
  |                          ---- expected `bool` because of return type
7 |       let mut result: bool = for a in 2..n {
  |  ____________________________^
8 | |         result = if n % a == 0 { false } else { true };
9 | |     };
  | |_____^ expected bool, found ()
  |
  = note: expected type `bool`
             found type `()`

Answer 1

您的代码的问题在于您在定义时使用变量result

...
let mut result: bool = for a in 2..n { // declared here
    result = if n % a == 0 { // used here, but it is still not initialized
...

没有result变量你可以很容易地做到，没有必要：

fn is_prime(n: u32) -> bool {
    for a in 2..n {
        if n % a == 0 {
            return false; // if it is not the last statement you need to use `return`
        }
    }
    true // last value to return
}

Playground link

Answer 2

您的代码中有几个问题（忽略它不能编译）：

• 你覆盖了结果 - >想象n = 4。除以2得到result = true，但在下一次迭代中除以3得到result = false
• 如果n<=2你的循环永远不会被执行，那么结果会是什么

不要尝试使用任何新的语法，而是尝试将其编写为尽可能可读：

fn is_prime(n: u32) -> bool {
    let limit = (n as f64).sqrt() as u32;

    for i in 2..=limit {
        if n % i == 0 {
            return false;
        }
    }

    true
}