🦀 Rust – HashMap: Efficient Key-Value Storage with Safety and Power

🧲 Introduction – Why Learn HashMap in Rust?

When you need to associate keys with values (like usernames to emails, or words to counts), the HashMap is your go-to tool. Rust’s HashMap is part of the std::collections module and offers constant-time lookups, ownership-aware insertion, and safe access patterns.

🎯 In this guide, you’ll learn:

• How to create and use HashMap
• Insertion, update, and retrieval techniques
• How ownership and borrowing apply to keys and values
• Practical examples with real output and safety insights

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🧰 Creating a HashMap

🔹 Code Example:

use std::collections::HashMap;

fn main() {
    let mut scores = HashMap::new();
    scores.insert("Alice", 50);
    scores.insert("Bob", 75);
    println!("{:?}", scores);
}

🧠 Explanation:

• HashMap::new() creates an empty map
• .insert(key, value) adds key-value pairs
• HashMap must be declared mut to insert

📤 Output:

{"Alice": 50, "Bob": 75}

⚠️ Order is not guaranteed in HashMap

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🔄 Accessing Values with .get()

🔹 Code Example:

fn main() {
    let mut users = HashMap::new();
    users.insert("admin", "root");

    match users.get("admin") {
        Some(role) => println!("Role: {}", role),
        None => println!("User not found"),
    }
}

🧠 Explanation:

• .get(key) returns Option<&V>
• Safe access avoids panics on missing keys

📤 Output:

Role: root

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🧩 Looping Through Key-Value Pairs

🔹 Code Example:

fn main() {
    let mut capitals = HashMap::new();
    capitals.insert("France", "Paris");
    capitals.insert("Japan", "Tokyo");

    for (country, city) in &capitals {
        println!("{} => {}", country, city);
    }
}

📤 Output:

France => Paris
Japan => Tokyo

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📝 Updating and Overwriting Values

🔹 Code Example:

fn main() {
    let mut data = HashMap::new();
    data.insert("key1", 10);
    data.insert("key1", 20); // Overwrites previous value

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

📤 Output:

{"key1": 20}

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🔍 Conditional Insert with entry()

🔹 Code Example:

use std::collections::HashMap;

fn main() {
    let mut scores = HashMap::new();
    scores.entry("Alice").or_insert(50);
    scores.entry("Alice").or_insert(100); // Skipped

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

🧠 Explanation:

• .entry().or_insert() only inserts if key doesn’t exist
• Useful for caching, counting, or default initialization

📤 Output:

{"Alice": 50}

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🔒 Ownership and HashMap

Rust moves both the key and the value into the HashMap.

🔹 Code Example:

fn main() {
    let name = String::from("user1");
    let mut map = HashMap::new();
    map.insert(name, "active");
    // println!("{}", name); // ❌ Error: name was moved
}

✅ Use .clone() if you want to keep using the original variable after insertion.

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🔄 Common Operations Summary

Operation	Method	Behavior
Insert	.insert(k, v)	Moves key and value into map
Get	.get(k)	Returns Option<&V>
Update	.insert() with same key	Overwrites old value
Insert if absent	.entry(k).or_insert(v)	Inserts only if key does not exist
Remove	.remove(k)	Deletes key-value pair and returns Option

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📌 Summary – Recap & Next Steps

Rust’s HashMap gives you powerful, flexible key-value storage—backed by ownership guarantees and safe access. Whether you’re counting frequencies or caching results, it’s a must-have tool in your Rust toolbox.

🔍 Key Takeaways:

• HashMap stores key-value pairs with unique keys
• All insertions move ownership unless cloned
• .get() returns an Option, avoiding panics
• Use .entry() for conditional inserts and mutation

⚙️ Next: Explore Rust – Structs to define your own custom data types and models.

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❓FAQs

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❓ Why is my variable inaccessible after inserting into a HashMap?
✅ Because ownership is moved. Use .clone() if you need to retain access.

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❓ How do I safely check if a key exists?
✅ Use .get() to return Option<&V> or .contains_key() to check existence.

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❓ Does HashMap preserve insertion order?
❌ No. Standard HashMap is unordered. Use IndexMap (from the indexmap crate) if order matters.

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❓ Can I use complex types as keys?
✅ Yes, but the key type must implement Eq and Hash.

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