Rust by Example

49.1 Argument parsing

Matching can be used to parse simple arguments:

#![feature(core)]
#![feature(env)]
use std::env;
fn increase(number: i32) {
    println!("{}", number + 1);
}
fn decrease(number: i32) {
    println!("{}", number - 1);
}
fn help() {
    println!("usage:
match_args <string>
    Check whether given string is the answer.
match_args {{increase|decrease}} <integer>
    Increase or decrease given integer by one.");
}
fn main() {
    let args: Vec<String> = env::args().map(|x| x.to_string())
                                       .collect();
    match args.as_slice() {
        // no arguments passed
        [ref name] => {
            println!("My name is '{}'. Try passing some arguments!", name);
        },
        // one argument passed
        [_, ref string] => {
            if string.as_slice() == "42" {
                println!("This is the answer!");
            } else {
                println!("This is not the answer.");
            }
        },
        // one command and one argument passed
        [_, ref cmd, ref num] => {
            // parse the number
            let number: i32 = match num.parse() {
                Ok(n) => {
                    n
                },
                Err(_) => {
                    println!("error: second argument not an integer");
                    help();
                    return;
                },
            };
            // parse the command
            match cmd.as_slice() {
                "increase" => increase(number),
                "decrease" => decrease(number),
                _ => {
                    println!("error: invalid command");
                    help();
                },
            }
        },
        // all the other cases
        _ => {
            // show a help message
            help();
        }
    }
}
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$ ./match_args Rust
This is not the answer.
$ ./match_args 42
This is the answer!
$ ./match_args do something
error: second argument not an integer
usage:
match_args <string>
    Check whether given string is the answer.
match_args {increase|decrease} <integer>
    Increase or decrease given integer by one.
$ ./match_args do 42
error: invalid command
usage:
match_args <string>
    Check whether given string is the answer.
match_args {increase|decrease} <integer>
    Increase or decrease given integer by one.
$ ./match_args increase 42
43

For implementing more complicated, unix-like command line interfaces see the getopts example.