How can I try reading data from a file or else return empty string?

Question

I would like to read a specific line of a file, if the:

file does not exist
file cannot be open
the number of lines is less that the one that I want

just return an empty string.

Consider the following examples:

let wifi = "".to_string();
if let Ok(wifi_file) = fs::read_to_string("/proc/net/wireless") {
    if let Some(wifi_level) = wifi_file.lines().nth(2) {
        let wifi = format!("{:.0} dBa ", wifi_level
            .split_whitespace()
            .collect::<Vec<&str>>()[3]
            .parse::<f32>()
            .unwrap()
        );
    }
}
// wifi is out of scope

let wifi = match fs::read_to_string("/proc/net/wireless") {
    Ok(s) => format!("{:.0} dBA", s.lines()
        .nth(2)
        .expect("Unable to open")
        .split_whitespace()
        .collect::<Vec<&str>>()[3]
        .parse::<f32>()
        .unwrap()),
    Err(_) => "".to_string(),
};
// thread 'main' panicked at 'Unable to open', ...

Am I missing something obvious about match?

kmdreko · Accepted Answer · 2022-09-24 02:41:30Z

2

In your first code, you are declaring wifi variable again instead of using the already declared variable.

Working code:

let mut wifi = "".to_string();
if let Ok(wifi_file) = fs::read_to_string("/proc/net/wireless") {
    if let Some(wifi_level) = wifi_file.lines().nth(2) {
        wifi = format!("{:.0} dBa ", wifi_level
            .split_whitespace()
            .collect::<Vec<&str>>()[3]
            .parse::<f32>()
            .unwrap()
        );
    }
}

Second code is working without any changes.

edited Sep 24, 2022 at 2:41

kmdreko

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answered Sep 21, 2022 at 17:17

Aditya Supugade

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4 Comments

yellowhat Over a year ago

Thanks for your reply. The second code crashes when the file contains only 2 lines instead of 3.

Jmb Over a year ago

@yellowhat actually it crashes if the third line doesn't contain a valid f32. To handle this case don't call unwrap, but use another if let Ok (float_level) = … instead.

yellowhat Over a year ago

For my understanding: does the match(Ok / Err) just covers fs::read_to_string("/proc/net/wireless") not s.lines().nth(2).expect("Unable to open") ...?

Jmb Over a year ago

@yellowhat Rust doesn't have exceptions like C++/Java/Python that can break the control flow. In Rust, match is just the same as switch … case in those languages. OTOH, expect (like unwrap) means: "crash if the expectation is not met" (e.g. if the value is Err for a Result or None for an Option).

kmdreko · Accepted Answer · 2022-09-24 02:36:06Z

1

You already have an answer, but this code can be simplified if you encapsulate it in a function returning an Option and use ? for none-propagation:

fn get_wifi(path: &str, n: usize) -> Option<String> {
    let file = std::fs::read_to_string(path).ok()?;
    let level = file.lines().nth(n)?;
    let value = level.split_whitespace().nth(3)?.parse::<f32>().ok()?;
    
    Some(format!("{:.0} dBa ", value))
}

You can then call it like this:

let wifi = get_wifi("/proc/net/wireless", 2).unwrap_or_default();

answered Sep 24, 2022 at 2:36

kmdreko

65.4k6 gold badges106 silver badges178 bronze badges

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