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Q&A How can I create and modify a struct over iterations of a loop?

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 som...

2 answers · posted 3mo ago by GeraldS‭ · last activity 2mo ago by GeraldS‭

Question rust loops mutability ownership

#2: Post edited by

GeraldS‭ · 2024-07-29T06:42:59Z (3 months ago)
added additional code for a (somewhat) working example.

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~~How can I create and modify variables over iterations of a loop?~~

How can I create and modify a struct over iterations of a loop?

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.
```ini
[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.
~~sample pseudocode:~~
```php
$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](https://doc.rust-lang.org/rust-by-example/std/option.html).
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:
```rust
let mut current_section: Option<Section> = None;
for line in read_to_string("input.ini").unwrap().lines() {
if line.trim().ends_with("]") {
let section = Section::new(line);
~~current_section = Some(section);~~
// --------------- this reinitialization might get skipped
}
else {
~~let item: str = line.unwrap();~~
current_section.unwrap().add_entry(line);
~~// ^^^^^^^^^^^^^^^ -------- `current_section` moved due to this method call, in previous iteration of loop~~
}
}
```
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?

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.
```ini
[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.
<details>
<summary>sample pseudocode</summary>
```php
$sections = [];
$currentSection = null;
for line in ini_files {
if ($line is sectionheader) {
if ($currentSection is array) {
$sections[] = $currentSection;
}
$currentSection = [];
}
else {
$currentSection[] = $line;
}
}
```
</details>
Now in Rust it's not that easy of course. If I understood correctly, the **$var = null/None** and assign as needed schema is done [using Options instead](https://doc.rust-lang.org/rust-by-example/std/option.html).
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:
```rust
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
}
}
```
<details>
<summary>additional file: section.rs</summary>
```rust
#[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));
}
}
```
</details>
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?

#1: Initial revision by

GeraldS‭ · 2024-07-28T14:47:02Z (3 months ago)

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How can I create and modify variables over iterations of a loop?

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.

```ini
[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.

sample pseudocode:

```php
$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](https://doc.rust-lang.org/rust-by-example/std/option.html).

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:

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

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?

rust loops mutability ownership

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