1 · Values and control flow · 25 MIN

Borrow strings instead of moving them

Borrowing lets a function inspect data without becoming its owner.

Passing a String by value transfers its ownership, so the caller normally cannot use the old binding afterwards. A shared reference borrows the value and leaves ownership with the caller. &str accepts both slices of owned strings and string literals, making it a flexible read-only input. String length counts bytes, whereas chars counts Unicode scalar values; neither directly counts grapheme clusters such as combined emoji. Choose the measurement that matches the product requirement instead of treating every notion of length as identical.

Use &str when ownership and mutation are unnecessary.

Read the example

fn scalar_count(text: &str) -> usize { text.chars().count() }
fn main() {
    let name = String::from("café");
    println!("{}", scalar_count(&name));
    println!("{}", name.len());
    println!("{}", name);
}

4
5
café

Your challenge

Write a function borrowing a string and returning whether it is empty after trim, then reuse the owned String afterwards.

Solution cost: O(n) scalar traversal; len reads the stored byte length in O(1). time · Account for collection storage separately from the returned result. space

Common trap

Slicing a string at an arbitrary byte index can panic on a non-character boundary.

fn scalar_count(text: &str) -> usize { text.chars().count() }
fn main() {
    let name = String::from("café");
    println!("{}", scalar_count(&name));
    println!("{}", name.len());
    println!("{}", name);
}

Further reading: Official documentation