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Solution

{{#include exercise.rs:solution}}

The solution demonstrates the fundamental distinction between shared and exclusive references:

  • Shared References (&): Used in magnitude because the function only reads the vector components.
  • Exclusive References (&mut): Required in normalize to modify the array elements in place.
  • Explicit Dereferencing: Inside normalize, item is an &mut f64. We use *item to access and modify the underlying value.
Details
  • Iterating over References: Iterating over &vector or &mut vector yields references to the elements. This is why coord is &f64 and item is &mut f64.
  • Arrays vs. Slices: The functions are defined using array references (&[f64; 3]), which ensures the length is known at compile time. Using slices (&[f64]) would make the functions more flexible but would introduce a runtime length check or potential for panics if the slice has the wrong size.
  • Method Call Ergonomics: In magnitude, we can call mag_squared.sqrt() directly. In normalize, we pass vector (an &mut [f64; 3]) to magnitude, and Rust automatically downgrades the exclusive reference to a shared reference to match the signature.