Storing Keys with Associated Values in Hash Maps

In this chapter, we'll explore store key-value pairs with hash maps.

Storing Keys with Associated Values in Hash Maps

The HashMap<K, V> type stores a mapping of keys of type K to values of type V using a hashing function to determine how to place these key-value pairs into memory.

Creating a Hash Map

use std::collections::HashMap
 
let mut scores = HashMap::new()
 
scores.insert("Blue", 10)
scores.insert("Yellow", 50)

Accessing Values

Use get to retrieve values:

let team_name = "Blue"
let score = scores.get(team_name)  // Returns Option<int>
 
match score {
    Some(s) => println(team_name, ": ", s)
    None => println("Team not found")
}

Iterating Over Hash Maps

for (key, value) in scores {
    println(key, ": ", value)
}

Updating Values

Overwrite existing value:

scores.insert("Blue", 25)  // Replaces 10 with 25

Insert only if key doesn't exist:

scores.entry("Blue").or_insert(50)  // Won't change (Blue exists)
scores.entry("Red").or_insert(50)   // Inserts Red: 50

Update based on old value:

let text = "hello world wonderful world"
 
let mut map = HashMap::new()
 
for word in text.split(" ") {
    let count = map.entry(word).or_insert(0)
    *count = *count + 1
}
// {"hello": 1, "world": 2, "wonderful": 1}

Hash Map Methods

let mut map = HashMap::new()
map.insert("a", 1)
 
map.len()           // Number of entries
map.is_empty()      // Check if empty
map.contains_key("a")  // Check if key exists
map.remove("a")     // Remove and return value
map.clear()         // Remove all entries

Summary

Vectors, strings, and hash maps provide essential ways to store collections of data. Combined with ownership and borrowing rules, they help you write safe and efficient code.

Next, let's learn about Error Handling in Chapter 9.

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