Rust Programming: Update a Hash Map

To update a HashMap in Rust, you can use the insert() method. This method takes two arguments: the key and the value. If the key already exists in the map, the old value will be replaced with the new value. Otherwise, a new key-value pair will be inserted into the map.

Overwriting a Value

When you insert a key and value into a hash map in Rust, the hash map will only contain one key/value pair for that key. If you insert the same key again with a different value, the existing value will be overwritten.

This is because a hash map is a data structure that stores key/value pairs in a table. The keys are used to lookup the values. When you insert a new key/value pair, the hash map will calculate a hash value for the key and use that to store the pair in the table. If you insert the same key again, the hash map will find the existing pair and overwrite the value.

Example

The following code shows how to overwrite a value in a hash map:

Rust

use std::collections::HashMap;

fn main() {
    let mut scores = HashMap::new();

    scores.insert(String::from("Blue"), 10);
    scores.insert(String::from("Blue"), 25);

    println!("{:?}", scores);
}

This code will print the following output:

{"Blue": 25}

The original value of 10 has been overwritten by the new value of 25.

Use Cases for Overwriting Values in Hash Maps

Overwriting values in hash maps is a common operation in many programming tasks. For example, you might use a hash map to store a cache of values that are calculated on demand. If you need to update a value in the cache, you can simply overwrite the existing value.

Another common use case is to use a hash map to store the state of a game or simulation. As the game or simulation progresses, you can update the state by overwriting the values in the hash map.

Adding a Key and Value Only If a Key Isn’t Present

Adding a Key and Value Only If a Key Isn’t Present (Rust Common Collection)

The Entry API in Rust is a convenient way to check whether a key already exists in a hash map and insert it with a value if it doesn't. This is useful in cases where you want to avoid overwriting existing values in the map.

To use the Entry API, you first call the entry() method on the hash map, passing in the key that you want to check. This returns an Entry enum value, which represents the value for the given key if it exists, or a vacant entry if it doesn't.

To insert a value for the key if it doesn't exist, you can call the or_insert() method on the Entry value. This method takes a default value as a parameter, which will be inserted into the map if the key doesn't already exist.

Here is an example of how to use the Entry API to add a key and value to a hash map only if the key isn't present:

Rust

use std::collections::HashMap;

fn main() {
    let mut scores = HashMap::new();

    // Insert a value for the "Blue" team.
    scores.insert(String::from("Blue"), 10);

    // Add a value for the "Yellow" team if it doesn't exist.
    scores.entry(String::from("Yellow")).or_insert(50);

    // Add a value for the "Blue" team if it doesn't exist.
    // This will not change the value for the "Blue" team because it already exists.
    scores.entry(String::from("Blue")).or_insert(50);

    println!("{:?}", scores);
}

Output:

{"Yellow": 50, "Blue": 10}

The Entry API is a powerful tool for working with hash maps in Rust. It allows you to write concise and efficient code for checking and updating the values in a map.

Updating a Value Based on the Old Value

To update a value in a hash map based on the old value, we can use the entry method. The entry method takes a key and returns an Entry object. The Entry object has two methods that we can use to update the value:

• or_insert: This method returns a mutable reference to the value for the specified key, or inserts a new value if the key does not exist.
• and_modify: This method takes a closure as an argument, and the closure is applied to the mutable reference to the value for the specified key.

The following example shows how to use the entry method to count how many times each word appears in some text:

Rust

use std::collections::HashMap;

let text = "hello world wonderful world";

let mut map = HashMap::new();

for word in text.split_whitespace() {
    let count = map.entry(word).or_insert(0);
    *count += 1;
}

println!("{:?}", map);

This code will print the following output:

{
    "world": 2,
    "hello": 1,
    "wonderful": 1,
}

The entry method is a powerful tool for updating values in hash maps, and it can be used to implement a variety of different algorithms.

Here is an example of how to use the entry method to implement a simple cache:

Rust

use std::collections::HashMap;

struct Cache {
    map: HashMap<String, String>,
}

impl Cache {
    fn new() -> Self {
        Self { map: HashMap::new() }
    }

    fn get(&mut self, key: &str) -> Option<&str> {
        self.map.get(key)
    }

    fn set(&mut self, key: &str, value: &str) {
        self.map.insert(key.to_string(), value.to_string());
    }

    fn update(&mut self, key: &str, updater: impl FnOnce(&str) -> &str) {
        let entry = self.map.entry(key.to_string());
        let mut value = entry.or_insert("");
        *value = updater(value);
    }
}

fn main() {
    let mut cache = Cache::new();

    cache.set("key1", "value1");

    let value = cache.get("key1").unwrap();
    assert_eq!(value, "value1");

    cache.update("key1", |value| {
        format!("{} updated", value)
    });

    let value = cache.get("key1").unwrap();
    assert_eq!(value, "value1 updated");
}

The update method takes a closure as an argument, and the closure is applied to the mutable reference to the value for the specified key. In the example above, the closure simply adds the string " updated" to the value.

The entry method is a versatile tool for updating values in hash maps, and it can be used to implement a variety of different algorithms.