Solution

// Copyright 2023 Google LLC
// SPDX-License-Identifier: Apache-2.0

/// Determine the length of the collatz sequence beginning at `n`.
fn collatz_length(mut n: i32) -> u32 {
    let mut len = 1;
    while n > 1 {
        n = if n % 2 == 0 { n / 2 } else { 3 * n + 1 };
        len += 1;
    }
    len
}

fn main() {
    println!("Length: {}", collatz_length(11)); // should be 15
}

This solution demonstrates a few key Rust features:

mut arguments: The n argument is declared as mut n. This makes the local variable n mutable within the function scope. It does not affect the caller’s value, as integers are Copy types passed by value.
if expressions: Rust’s if is an expression, meaning it produces a value. We assign the result of the if/else block directly to n. This is more concise than writing n = ... inside each branch.
Implicit return: The function ends with len (without a semicolon), which is automatically returned.

Note that n must be strictly greater than 0 for the Collatz sequence to be valid. The function signature takes i32, but the problem description implies positive integers. A more robust implementation might use u32 or return an Option or Result to handle invalid inputs (0 or negative numbers), but panic or infinite loops are potential outcomes here if n <= 0.
The overflow is a potential issue if n grows too large, similar to the Fibonacci exercise.

Comprehensive Rust 🦀

Solution