Rust Speedrun - Generate Verification Code Example

1. Reference materials

• Rust Course

2. Function function

Input: Verification code length length

Output: length length string consisting of letters and numbers

3. Implement the code

use rand::Rng;
use rand::rngs::ThreadRng;

fn generate_verify_code(length: u8) -> String {

    // 使用动态数组保存验证码结果
    let mut verify_code_vec: Vec<char> = Vec::new();

    // 验证码组成库
    const CHAR_ARRAY: [char; 62] = [
        '1', '2', '3', '4', '5', '6', '7', '8', '9', '0',
        'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j',
        'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't',
        'u', 'v', 'w', 'x', 'y', 'z',
        'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J',
        'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T',
        'U', 'V', 'W', 'X', 'Y', 'Z'
    ];

    // 随机生成 N 位验证码
    let mut rng: ThreadRng = rand::thread_rng();
    for _i in 0..length {
        verify_code_vec.push(CHAR_ARRAY[rng.gen_range(0..CHAR_ARRAY.len())]);
    }

    // 返回生成的验证码
    String::from_iter(verify_code_vec.iter())
}

fn main() {
    println!("generate_verify_code [ {:?} ]", generate_verify_code(4))
}

4. Operation results

5. Grammar analysis

1. Function declaration

Image from Rust Course

a. Parameters and return values

fn generate_verify_code(length: u8) -> String {...}

• fn is the declaration keyword
• generate_verify_code is the function name
• length: u8 specifies parameter name and parameter type
• -> String specifies the return value type

The return value can be returned early using the return keyword, or using the last expression as the return.

// 特别注意，以 ";" 结尾的是语句，没有 ";" 结尾的才是表达式

// 1、表达式返回
String::from_iter(verify_code_vec.iter())

// 2、return 返回
return String::from_iter(verify_code_vec.iter())

b. No return value()

The main function main is a function with no parameters and no return value . No return value is also a value. In fact, it will implicitly return a unit type () .

fn main() {

// 等同于

fn main() -> () {
   
   

c. Never return

When used as a function return type, it means that the function will never return (diverge function). In particular, this syntax is often used as a function that will cause the program to crash. ! 

——Rust Course

// 永不返回的发散函数，这种语法往往用做会导致程序崩溃的函数
fn diverge_function() -> ! {
   // panic!("ERROR!!!");
    loop {
        println!("ERROR")
    };
}

fn main() {
    println!("generate_verify_code [ {:?} ]", generate_verify_code(4));
    // 永不返回的发散函数，程序止步于此，无法继续往下走
    diverge_function();
    // Unreachable code
    println!("generate_verify_code [ {:?} ]", generate_verify_code(4));
}

As you can see, there is a compile-time warning that the statements below the divergent function cannot be executed.

2. Variables and constants

// 由 let 关键字定义变量，但是变量是不可变的，需要 mut 关键字修饰才能改变它的值
// 这里定义了一个 char 类型的动态数组，数组的值可变
let mut verify_code_vec: Vec<char>

// 由 const 关键字定义常量，这里定义了一个 char 类型长度为 62 的数组
const CHAR_ARRAY: [char; 62] 

But how come the i variable in the for loop has a _ prefix?

for _i in 0..length {
   
   

Generally, variables must be used after they are defined, and unused variables can be written with an underscore _ to tell the compiler to ignore the variable, otherwise the compiler will prompt an alarm. Both of the following writing methods are acceptable.

// 1、忽略变量 i
for _i in 0..length {

// 2、用 _ 忽略循环中的值
for _ in 0..length {
   
   

3. Dynamic arrays and static arrays

//TODO