Mutable Iteration

Sometimes you need to do more than just read vector elements - you need to modify them. While the previous lesson showed how to iterate and read values, this lesson teaches you how to change the elements during iteration using mutable references.

To modify elements while iterating, you need to use &mut instead of & in your for loop:

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

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

The &mut numbers creates a mutable reference to the vector, and each iteration gives you a mutable reference to an individual element. The * operator (called the dereference operator) is used to access and modify the actual value that the reference points to.

This pattern is incredibly useful for transforming data in place. Instead of creating a new vector with modified values, you can efficiently update the existing one. The vector must be declared as mut for this to work, since you're changing its contents.

After the loop above, numbers would contain [2, 4, 6, 8] - each original value doubled in place.