Curriculum Ownership & borrowing Strings and slices exercise 3 · mcq-explain

Strings and slices

Given the source on the side, what is the type of first and what does the slicing operation cost?

first is &str — a borrowed string slice of s covering bytes 0..2 ("pa"). Zero allocation; first borrows from s and is invalidated if s goes out of scope or is mutated.

first is str — slicing a String returns the unsized str type by value, which is why the & prefix is required.

first is &[u8] — String indexing produces a byte-array slice; you'd need std::str::from_utf8 to get a string back.

first is String — &s[0..2] allocates a new owned String containing "pa"; modifying it does not affect s.

About this theme

Rust has owned strings (String) and borrowed string slices (&str). The same borrowed-view idea applies to slices like &[T]. This theme teaches the signature reflex: accept &str or &[T] when you only read, return String or Vec<T> when you create owned data. Planned exercises: 1. choose &str for a read-only parameter. 2. choose String for owned output. 3. recognise slice syntax. 4. fill .to_string(). 5. fill a slice parameter. 6. write a string-reading helper. 7. write a slice-summing helper. 8. translate TS string code. 9. fix a function that returns a reference to a temporary string.