Day 14: HashMaps

HashMap<K, V> stores key-value pairs with O(1) average lookup time.

Creating HashMaps

use std::collections::HashMap;

let mut scores = HashMap::new();
scores.insert("Alice", 100);
scores.insert("Bob", 80);

Accessing Values

// Returns Option<&V>
if let Some(score) = scores.get("Alice") {
    println!("Alice: {}", score);
}

// Or with default
let score = scores.get("Charlie").unwrap_or(&0);

The Entry API

Powerful pattern for conditional insertion:

// Insert only if key doesn't exist
scores.entry("Alice").or_insert(50);

// Modify existing or insert
let count = scores.entry("Alice").or_insert(0);
*count += 1;

The Entry API is perfect for counting frequencies - a common interview pattern!

Iterating

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

The Task

Implement char_frequency(s) that counts character occurrences.

Only count alphabetic characters
Convert to lowercase
Return HashMap<char, u32>

Requirements

"Hello" → {h: 1, e: 1, l: 2, o: 1}
Ignore non-alphabetic characters

Hints

use std::collections::HashMap;

fn char_frequency(s: &str) -> HashMap<char, u32> {
    let mut freq = HashMap::new();
    
    for c in s.chars() {
        if c.is_alphabetic() {
            let c = c.to_ascii_lowercase();
            *freq.entry(c).or_insert(0) += 1;
        }
    }
    
    freq
}