wiki/content/20200915153033-hash_maps.md

---
date: 20200915
id: 2b38c21d-4971-42fb-9b87-5d68468e95e0
title: Rust Hash Maps
---

# Description

The type `HashMap<K, V>` stores a mapping of keys of type `K` to values
of type `V`. It does this via a hashing function, which determines how
it places these keys and values into memory. Many programming languages
support this kind of data structure, but they often use a different
name, such as hash, map, object, hash table, dictionary, or associative
array, just to name a few.

## Creating a New Hash Maps

``` rust
fn main() {
    use std::collections::HashMap;

    let mut scores = HashMap::new();

    scores.insert(String::from("Blue"), 10);
    scores.insert(String::from("Yellow"), 50);

    println!("{}", scores["Blue"])
}
```

### collect

Another way of constructing a hash map is by using iterators and the
[collect](https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.collect)
method on a vector of tuples, where each tuple consists of a key and its
value.

``` rust
fn main() {
    use std::collections::HashMap;

    let teams = vec![String::from("Blue"), String::from("Yellow")];
    let initial_scores = vec![10, 50];

    let scores: HashMap<_, _> = teams.into_iter().zip(initial_scores.into_iter()).collect();

    println!("{}", scores["Blue"])
}
```

## Ownership

For types that implement the `Copy` trait, like `i32`, the values are
copied into the hash map. For owned values like `String`, the values
will be moved and the hash map will be the owner of those values:

``` rust
fn main() {
    use std::collections::HashMap;

    let field_name = String::from("Favorite color");
    let field_value = String::from("Blue");

    let mut map = HashMap::new();
    map.insert(field_name, field_value);
    // field_name and field_value are invalid at this point
}
```

## Accessing values

``` rust
fn main() {
    use std::collections::HashMap;

    let mut scores = HashMap::new();

    scores.insert(String::from("Blue"), 10);
    scores.insert(String::from("Yellow"), 50);

    for (key, value) in &scores {
        println!("{}: {}", key, value);
    }
}
```

## Updating

### Overwriting a Value

``` rust
fn main() {
    use std::collections::HashMap;

    let mut scores = HashMap::new();

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

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

### Inserting a value only if the Key has no value

``` rust
fn main() {
    use std::collections::HashMap;

    let mut scores = HashMap::new();
    scores.insert(String::from("Blue"), 10);

    scores.entry(String::from("Yellow")).or_insert(50);
    scores.entry(String::from("Blue")).or_insert(50);

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

### Updating a value based on the old value

``` rust
fn main() {
    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);
}
```