Welcome to Software Development on Codidact!

Will you help us build our independent community of developers helping developers? We're small and trying to grow. We welcome questions about all aspects of software development, from design to code to QA and more. Got questions? Got answers? Got code you'd like someone to review? Please join us.

Post History

60%

+1 −0

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 5mo ago by GeraldS‭ · last activity 4mo ago by GeraldS‭

Question rust loops mutability ownership

#2: Post edited by

GeraldS‭ · 2024-07-29T06:42:59Z (5 months ago)
added additional code for a (somewhat) working example.

Copy Link

Raw

Markdown

~~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 (5 months ago)

Copy Link

Raw

Markdown

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

Communities

Post History