How can I have a mutable object (for example a vector) that is created inside a loop iteration and needs to be updated in later iterations of said loop?

As a concrete example, consider parsing something similar to an ini file.

[section1]
entry1
entry2

[section2]
entry3
entry4

In a simple scripting language like PHP I would just create an array when a new section starts, append items and store it away when the next section starts.

$sections = [];
$currentSection = null;
for line in ini_files {
   if ($line is sectionheader) {
       if ($currentSection is array) {
           $sections[] = $currentSection;
       }
       $currentSection = [];
   }
   else {
       $currentSection[] = $line;
   }
}

Now in Rust it's not that easy of course. If I understood correctly, the $var = null/None and assign as needed schema is done using Options instead.

I'm currently trying to wrap my head around that. Let's say I have the following loop, and Section is a custom struct that is supposed to store the related entries:

let mut sections: Vec<Section> = Vec::new();
let mut current_section: Option<Section> = None;
for line in read_to_string("input.ini").unwrap().lines() {
    if line.trim().ends_with("]") {
        if current_section.is_none() {
            sections.push(current_section.unwrap());
        }
        current_section = Some(Section::new(line));
     // --------------- this reinitialization might get skipped
    }
    else {
        current_section.unwrap().add_entry(line);
     // ^^^^^^^^^^^^^^^ ------- `current_section` moved due to this method call,
     //                         in previous iteration of loop
    }
}

#[derive(Debug)]
pub struct Section {
    pub name: String,
    pub entries: Vec<String>,
}

impl Section {
    pub fn new(name: &str) -> Section {
        Section {
            name: String::from(name),
            entries: Vec::new()
        }
    }

    pub fn add_entry(&mut self, line: &str) {
        self.entries.push(String::from(line));
    }
}

I get that current_section is moved and that I can't access it anymore due to the changed ownership. But how can I store it so that I can access it on further iterations? I tried every combination of referencing and dereferencing with & and * I could think of.

How can I store a variable from a loop iteration and still be able to modify it at later iterations?

posted 3 months ago

CC BY-NC-SA 4.0

3mo ago

GeraldS‭

61 reputation 4 3 10 5