Rust - Multiple Calls to Iterator Methods

Question

I have this following rust code:

fn tokenize(line: &str) -> Vec<&str> { let mut tokens = Vec::new(); let mut chars = line.char_indices(); for (i, c) in chars { match c { '"' => { if let Some(pos) = chars.position(|(_, x)| x == '"') { tokens.push(&line[i..=i+pos]); } else { // Not a complete string } } // Other options... } } tokens }

I am trying to elegantly extract a string surrounded by double quotes from the line, but since chars.position takes a mutable reference and chars is moved into the for loop, I get a compilation error - "value borrowed after move". The compiler suggests borrowing chars in the for loop but this doesn't work because an immutable reference is not an iterator (and a mutable one would cause the original problem where I can't borrow mutably again for position).

I feel like there should be a simple solution to this. Is there an idiomatic way to do this or do I need to regress to appending characters one by one?

gfv · Accepted Answer · 2020-07-06 09:11:30Z

Because a for loop will take ownership of chars (because it calls .into_iter() on it) you can instead manually iterate through chars using a while loop:

fn tokenize(line: &str) -> Vec<&str> { let mut tokens = Vec::new(); let mut chars = line.char_indices(); while let Some((i, c)) = chars.next() { match c { '"' => { if let Some(pos) = chars.position(|(_, x)| x == '"') { tokens.push(&line[i..=i+pos]); } else { // Not a complete string } } // Other options... } } }

Sebastian Redl · Accepted Answer · 2020-07-06 09:12:28Z

It works if you just desugar the for-loop:

fn tokenize(line: &str) -> Vec<&str> { let mut tokens = Vec::new(); let mut chars = line.char_indices(); while let Some((i, c)) = chars.next() { match c { '"' => { if let Some(pos) = chars.position(|(_, x)| x == '"') { tokens.push(&line[i..=i+pos]); } else { // Not a complete string } }, _ => {}, } } tokens }

The normal for-loop prevents additional modification of the iterator because this usually leads to surprising and hard-to-read code. Doing it as a while-loop has no such protection.

If all you want to do is find quoted strings, I would not, however, go with an iterator at all here.

fn tokenize(line: &str) -> Vec<&str> { let mut tokens = Vec::new(); let mut line = line; while let Some(pos) = line.find('"') { line = &line[(pos+1)..]; if let Some(end) = line.find('"') { tokens.push(&line[..end]); line = &line[(end+1)..]; } else { // Not a complete string } } tokens }

Thanks! I do other things as well, not just find quoted strings - that was just what caused the problem.

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