Strings and Slices

🎯 In Python, strings are sequences and slicing is simple:

s = "hello world"
first_word = s.split()[0]      # "hello"
last_three = s[-3:]            # "rld"

Rust strings work differently because of its ownership model. There are two main string types:

&str — a string slice. A borrowed, read-only view into string data. This is what string literals ("hello") give you.
String — an owned, heap-allocated string. Like Python's str, but you're responsible for when it's created and dropped.

Slicing strings

Rust slices use byte indices with range syntax:

let s = "hello world";
let first_five = &s[0..5];   // "hello"
let from_six = &s[6..];      // "world"

This is similar to Python's s[0:5], but Rust uses .. instead of :. The ranges are half-open (end is excluded), just like Python.

Important: Rust string slices are byte-indexed, not character-indexed. For ASCII strings this doesn't matter. For Unicode, slicing at the wrong byte offset causes a panic. We'll keep inputs ASCII in this exercise.

Returning slices

When a function takes &str and returns &str, it returns a view into the input — no allocation, no copy:

fn get_domain(url: &str) -> &str {
    // returns a slice of the input string
}

This is like Python's memoryview concept — a zero-cost window into existing data. The Rust compiler ensures the returned slice doesn't outlive the input.

Your Task

Implement two functions:

first_word(s: &str) -> &str — return the substring up to the first space. If there's no space, return the whole string.
last_n(s: &str, n: usize) -> &str — return the last n characters. If n exceeds the string length, return the whole string. (Assume ASCII input.)

Example

assert_eq!(first_word("hello world"), "hello");
assert_eq!(first_word("rustacean"), "rustacean");

assert_eq!(last_n("abcdef", 3), "def");
assert_eq!(last_n("hi", 5), "hi");

Dive deeper: In our Rust Developer Cohort, your tokenizer will slice into the JSON input string to extract tokens — string values, number literals, keywords — all as &str views with zero copying.

Slicing strings

Returning slices

Your Task

Example

Further Reading