Limit the number of values for a vector argument

---

• A different approach is to accept multiple values with a single mention of the argument. We can achieve this by setting the num_args.
• It can accept either a single number or a range such as 2..=3 (2 or 3), ..=3 (same as 0..=3), or 3.. (3 or more).
• We can also supply required=true to make sure the argument must be supplied.
• See num_args

examples/clap/limit-number-of-args/src/main.rs

use clap::Parser;

#[derive(Parser, Debug)]
struct Cli {
    #[arg(long, num_args=2..=3, required=true)]
    animal: Vec<String>,

    #[arg(long, num_args = 3)]
    sisters: Vec<String>,
}

fn main() {
    let args = Cli::parse();
    println!("{args:?}");
}

$ cargo run -q
error: the following required arguments were not provided:
  --animal <ANIMAL> <ANIMAL>...

Usage: limit-number-of-args --animal <ANIMAL> <ANIMAL>...

For more information, try '--help'.


$ cargo run -q -- --animal Cat Dog
Cli { animal: ["Cat", "Dog"], sisters: [] }


$ cargo run -q -- --animal Cat Dog Crab
Cli { animal: ["Cat", "Dog", "Crab"], sisters: [] }


$ cargo run -q -- --animal Cat Dog Crab Snake
error: unexpected argument 'Snake' found

Usage: limit-number-of-args [OPTIONS] --animal <ANIMAL> <ANIMAL>...

For more information, try '--help'.