Borrowing within a vector of structures - rust

Question

I have a vector of structures and i would like to update one structure with values in another. For my use case, I prefer to do it in a loop. I'm hitting the borrow-checker but it seems like there must be a simple solution to this type of problem.
#[derive(Debug)]
struct Column {
header: String,
amount: i32,
}
fn main() {
let mut spreadsheet: Vec<Column> = Vec::new();
spreadsheet.push(Column {
header: "Car".to_string(),
amount: 30300,
});
spreadsheet.push(Column {
header: "House".to_string(),
amount: 210800,
});
spreadsheet.push(Column {
header: "Total".to_string(),
amount: 0,
});
for column in &mut spreadsheet {
//mutable borrow here
if column.header == "Total" {
column.amount = spreadsheet[0].amount //immutable borrow here
+ spreadsheet[1].amount;
} else {
column.amount -= 300;
}
}
for column in spreadsheet {
println!("{:?}", column);
}
}

Answer 1

You are trying to set spreadsheet vector element while iterating inside of it. Since you always wanting to use spreadsheet[0].amount and spreadsheet[1].amount you can clone this values into another variable and work with them instead of using them inside of spreadsheet.
Here is the working code:
#[derive(Debug)]
struct Column {
header: String,
amount: i32,
}
fn main() {
let mut spreadsheet: Vec<Column> = Vec::new();
spreadsheet.push(Column {
header: "Car".to_string(),
amount: 30300,
});
spreadsheet.push(Column {
header: "House".to_string(),
amount: 210800,
});
spreadsheet.push(Column {
header: "Total".to_string(),
amount: 0,
});
let car_amount = spreadsheet[0].amount;
let header_amount = spreadsheet[1].amount;
spreadsheet.iter_mut().for_each(|column| {
if column.header == "Total" {
column.amount = car_amount + header_amount;
} else {
column.amount -= 300;
}
});
for column in spreadsheet {
println!("{:?}", column);
}
}
Playground with using iter()
Since you want to do these operations in a for loop instead of iterator you can change the spreadsheet.iter_mut()... code block to the following:
for column in &mut spreadsheet {
if column.header == "Total" {
column.amount = car_amount + header_amount;
} else {
column.amount -= 300;
}
}
Playground with using for loop