Rustlings Topic: Collections

24 Jan 2022 in Study Log on Rust, Rustlings

Rust’s standard library includes a number of very useful data structures called collections. Most other data types represent one specific value, but collections can contain multiple values. Unlike the built-in array and tuple types, the data these collections point to is stored on the heap, which means the amount of data does not need to be known at compile-time and can grow or shrink as the program runs.

This exercise will get you familiar with two fundamental data structures that are used very often in Rust programs:

A vector allows you to store a variable number of values next to each other. A hash map allows you to associate a value with a particular key. You may also know this by the names unordered map in C++, dictionary in Python, or an associative array in other languages.

You may find solution code for the topic from my repo.

1. vec1.rs
2. vec2.rs
3. hashmap1.rs
4. hashmap2.rs

vec1.rs

We can declare vector in many ways. The basic way is, of course, using Vec::new(). Or we can use vec! macro. Helper methods such as to_vec() or into_vec() also helps. Rust supports various methods/functions for std structs. So it is better to visit the rust API document.

In this exercise, I’ve used slice::to_vec() to copy contents of the a to v.

/* file: "exercises/collections/vec1.rs" */
fn array_and_vec() -> ([i32; 4], Vec<i32>) {
    let a = [10, 20, 30, 40]; // a plain array
    let v = a.to_vec();
    // let v = vec![10, 20, 30, 40];

    (a, v)
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_array_and_vec_similarity() {
        let (a, v) = array_and_vec();
        assert_eq!(a, v[..]);
    }
}

vec2.rs

This exercise gives us the taste of iterator().
fn iter_mut(&mut self)-> IterMut<'_, T> returns an iterator that allows modifying each value. Combining it with the for loop, we can modify all values in the vector.

/* file: "exercises/collections/vec2.rs" */
fn vec_loop(mut v: Vec<i32>) -> Vec<i32> {
    for i in v.iter_mut() {
        *i *= 2;
    }

    // At this point, `v` should be equal to [4, 8, 12, 16, 20].
    v
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_vec_loop() {
        let v: Vec<i32> = (1..).filter(|x| x % 2 == 0).take(5).collect();
        let ans = vec_loop(v.clone());

        assert_eq!(ans, v.iter().map(|x| x * 2).collect::<Vec<i32>>());
    }
}

hashmap1.rs

This exercise gives an example usage of HashMap.
Take a look at the rust API document if you are not familiar with it.

/* file: "exercises/collections/hashmap1.rs" */
use std::collections::HashMap;

fn fruit_basket() -> HashMap<String, u32> {
    let mut basket = HashMap::<String, u32>::new();

    // Two bananas are already given for you :)
    basket.insert(String::from("banana"), 2);

    basket.insert("apple".to_string(), 1);
    basket.insert("mango".to_string(), 3);

    basket
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn at_least_three_types_of_fruits() {
        let basket = fruit_basket();
        assert!(basket.len() >= 3);
    }

    #[test]
    fn at_least_five_fruits() {
        let basket = fruit_basket();
        assert!(basket.values().sum::<u32>() >= 5);
    }
}

hashmap2.rs

Take a look at TODO from the fruit_basket().

TODO: Put new fruits if not already present. Note that you are not allowed to put any type of fruit that’s already present!

So, we have to do 2 things here.

1. Search HashMap to find out what Fruit is missing.
2. Insert one quantity if the fruit doesn’t exist.

We may use get() & insert(), but Rust also supports entry() & or_insert() methods which suits better in such situation.

/* file: "exercises/collections/hashmap2.rs" */
use std::collections::HashMap;

#[derive(Hash, PartialEq, Eq)]
enum Fruit {
    Apple,
    Banana,
    Mango,
    Lychee,
    Pineapple,
}

fn fruit_basket(basket: &mut HashMap<Fruit, u32>) {
    let fruit_kinds = vec![
        Fruit::Apple,
        Fruit::Banana,
        Fruit::Mango,
        Fruit::Lychee,
        Fruit::Pineapple,
    ];

    for fruit in fruit_kinds {
        hashmap.entry(fruit).or_insert(1);
        // if let None = basket.get(&fruit) {
        //     basket.insert(fruit, 1);
        // }
    }
}

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