In this exercise we will implement a function that returns a bool whether a number (integer) is a Narcissistic number or not.

Specification

A number is Narcissistic number if the sum of each of its digits raised to the power of the number of digits is equal to the number itself. Also called Armstrong number.

e.g. 407 = 4^3 + 0^3 + 7^3 = 407

You can read more about them on Wikipedia

You will learn:

  • How to create a Rust library

  • To get more practice with iterators

Tasks

Step 1

Create a new lib and name it narcheck.

Click to see the solution 
cargo new --lib narcheck

Step 2

Create a function that converts an integer into its digits. The signature of the function is -

fn get_digits(num: u32) -> Vec<u32>
Click to see the hints/solutions for this step 
fn get_digits(num: u32) -> Vec<u32> {
    let digits: Vec<_> = num
        .to_string()
        .chars()
        .map(|d| d.to_digit(10).unwrap())
        .collect();

    digits
}

Step 3

Create a function that uses the get_digits function above to check if a number is Narcissistic number. The signature of the function is -

fn is_narcissistic(num: u32) -> bool
Click to see the hints/solutions for this step 
fn is_narcissistic(num: u32) -> bool {
    let digits = get_digits(num);
    let cubed_digits: Vec<u32> = digits
        .iter()
        .map(|d| u32::pow(*d, digits.len() as u32))
        .collect();
    let sum: u32 = cubed_digits.iter().sum();
    sum == num
}

Step 4

Add tests.

Click to see the hints/solutions for this step 
#[test]
fn nar() {
    assert!(is_narcissistic(407))
}

#[test]
fn not_nar() {
    assert_eq!(false, is_narcissistic(144))
}