Vector first() with length check before

[andre-dasilva]

I am new to rust and started to write some code a couple of days ago.

Question:
Lets say i have this function:

fn get_text() -> Result<String> {
    let texts = vec!["Hello"];

    return match texts.len() {
        0 => Err("text not found"),
        1 => texts.first().cloned().ok_or("text can not be none"),
        _ => Err("Too many texts found")
    }

in the match where I check for 1, I need to .ok_or() the Option and handle the None case. Is there a way to simply ignore None? Or a better way in general?

Because the check for texts.len() == 1 already asserts that the vector has at least one element. Its kinda redundant to also have to check for None, because its impossible that first() returns None.

[RedDocMD]

You can simply use unwrap() since it is impossible for the Option to be null.

[andre-dasilva]

ahh makes sense. but a way to simply ignore None is not possible? you have to handle the none case? in this case with unwrap() to just silence it

[Bruecki]

You can match on slices

fn get_text() -> Result<String, &'static str> {
    let texts = vec!["Hello"];

    return match texts[..] {
        [] => Err("text not found"),
        [text] => Ok(text.to_string()),
        _ => Err("Too many texts found")
    }
}

[andre-dasilva]

I am glad I asked the question in this forum. thats a nice way, that I didn't thought about.

[erelde]

If you have control over the data structures, using a queue and pop_front could be more ergonomic.

Edit: If the vec only has 1 element a pop will work just as well

[scottmcm]

Since you're only trying to handle exactly one element, you can also try_into an array:

pub fn get_text(texts: Vec<String>) -> Result<String, String> {
    if texts.is_empty() { return Err("text not found".into()); }
    
    let [text]: [_; 1] = texts.try_into().map_err(|_| "Too many texts found")?;
    Ok(text)
}

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=943e8168e3f1739707c01631d906e512

I moved texts to a parameter (and Strings instead of &strs) to demonstrate that this doesn't need to copy anything -- it moves the one element of the vector into the array pattern.

---

Actually, the error type there gives another way to write it that avoids the pre-check:

pub fn get_text(texts: Vec<String>) -> Result<String, String> {
    let [text]: [_; 1] = texts.try_into().map_err(|v: Vec<String>| {
        if v.is_empty() {
            "text not found"
        } else {
            "Too many texts found"
        }
    })?;
    Ok(text)
}

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=b6f6006004fdbeb6b01de5725ad59b88

[andre-dasilva]

thanks everyone for the quick help