Borrow In Loop

Part of the Fundamentals section of Coddy's Rust journey — lesson 67 of 75.

When we write a loop in Rust, we have two main ways to work with our data: we can look at it (using &) or take it. For example consider the following data:

let numbers = vec![1, 2, 3, 4, 5];
let mut sum = 0;

Method 1: Looking at the data (using &):

for number in &numbers {
    sum += number;
}
println!("I can use it here: {:?}", numbers);

Method 2: Taking the data (without &):

for number in numbers {
    sum += number;
}
// println!("Can't use {:?} anymore!", numbers);
// This would cause an error

Think of it like this:

Using & is like looking at a photo album: you can see all the photos, but the album stays intact

Not using & is like taking the photos out: you can use them, but the album will be empty afterward

Now this style:

for num in numbers.iter() {}

Is similar to this:

for num in &numbers {}

And a mutation loop:

for num in numbers.iter_mut() { 
    *num *= 2;
}

Can also be written this way:

for num in &mut numbers { 
    *num *= 2;
}

Cheat sheet

In Rust loops, you can either look at data (using &) or take ownership of it:

Looking at data (borrowing):

let numbers = vec![1, 2, 3, 4, 5];
for number in &numbers {
    sum += number;
}
// numbers can still be used after the loop

Taking ownership:

let numbers = vec![1, 2, 3, 4, 5];
for number in numbers {
    sum += number;
}
// numbers cannot be used after the loop

Iterator methods:

// These are equivalent:
for num in numbers.iter() {}
for num in &numbers {}

// For mutation:
for num in numbers.iter_mut() { *num *= 2; }
for num in &mut numbers { *num *= 2; }

Try it yourself

This lesson doesn't include a code challenge.