Argument parsing

Matching can be used to parse simple arguments:

  1. use std::env;
  3. fn increase(number: i32) {
  4.     println!("{}", number + 1);
  5. }
  7. fn decrease(number: i32) {
  8.     println!("{}", number - 1);
  9. }
  11. fn help() {
  12.     println!("usage:
  13. match_args <string>
  14.     Check whether given string is the answer.
  15. match_args {{increase|decrease}} <integer>
  16.     Increase or decrease given integer by one.");
  17. }
  19. fn main() {
  20.     let args: Vec<String> = env::args().collect();
  22.     match args.len() {
  23.         // no arguments passed
  24.         1 => {
  25.             println!("My name is 'match_args'. Try passing some arguments!");
  26.         },
  27.         // one argument passed
  28.         2 => {
  29.             match args[1].parse() {
  30.                 Ok(42) => println!("This is the answer!"),
  31.                 _ => println!("This is not the answer."),
  32.             }
  33.         },
  34.         // one command and one argument passed
  35.         3 => {
  36.             let cmd = &args[1];
  37.             let num = &args[2];
  38.             // parse the number
  39.             let number: i32 = match num.parse() {
  40.                 Ok(n) => {
  41.                     n
  42.                 },
  43.                 Err(_) => {
  44.                     eprintln!("error: second argument not an integer");
  45.                     help();
  46.                     return;
  47.                 },
  48.             };
  49.             // parse the command
  50.             match &cmd[..] {
  51.                 "increase" => increase(number),
  52.                 "decrease" => decrease(number),
  53.                 _ => {
  54.                     eprintln!("error: invalid command");
  55.                     help();
  56.                 },
  57.             }
  58.         },
  59.         // all the other cases
  60.         _ => {
  61.             // show a help message
  62.             help();
  63.         }
  64.     }
  65. }
  1. $ ./match_args Rust
  2. This is not the answer.
  3. $ ./match_args 42
  4. This is the answer!
  5. $ ./match_args do something
  6. error: second argument not an integer
  7. usage:
  8. match_args <string>
  9.     Check whether given string is the answer.
  10. match_args {increase|decrease} <integer>
  11.     Increase or decrease given integer by one.
  12. $ ./match_args do 42
  13. error: invalid command
  14. usage:
  15. match_args <string>
  16.     Check whether given string is the answer.
  17. match_args {increase|decrease} <integer>
  18.     Increase or decrease given integer by one.
  19. $ ./match_args increase 42
  20. 43