added hashmap part 1

Author: hitesh-bhatnagar

examples/07_hashmap_part_1.rs

@@ -0,0 +1,113 @@
+use std::collections::HashMap;
+
+fn main(){
+	let mut scores: HashMap<String, i32> = HashMap::new();
+	println!("empty map: {:?}, scores");
+
+	// PART 2: Insert, Get, Remove
+
+	let mut map: HashMap<String, String> = HashMap::new();
+
+	// insert - add a key-value pair
+	map.insert(String::from("name"), String::from("alice"));
+	map.insert(String::from("city"), String::from("Belarus"));
+	map.insert(String::from("lang"), String::from("Rust"));
+	println!("after inserts: {:?}", map);
+
+	// insert same key- OVERWRITES the old value
+	map.insert(String::from("name"), String::from("Bheem"));
+	println!("after overwrite: {:?}", map);
+
+	// get -- returns Option<&Value>
+	match map.get("name") {
+		Some(v) => println!("name = {}", v),
+		None => println!("key not found"),
+	}
+
+	// get a key that doesn't exist
+	match map.get("age") {
+		Some(v) => println!("age = {}", v),
+		None => println!("age key does not exist"),
+	}
+
+	// contains_key - check if key exists
+	println!("has city: {}", map.contains_key("city"));
+	println!("has age: {}", map.contains_key("age"));
+
+	// remove - delete a key, returns Option<Value>
+	let removed = map.remove("city");
+	println!("removed: {:?}", removed);
+	println!("after remove: {:?}", map);
+
+	// PART 3: entry() - insert only if key doesn't exist
+
+	let mut map: HashMap<String, i32> = HashMap::new();
+	map.insert(String::from("alice"), 10);
+
+	// or_insert - only inserts if key is NOT already there
+	map.entry(String::from("alice")).or_insert(99);		// alice exists -- no change
+	map.entry(String::from("bob")).or_insert(50);	// bob missing -- inserts 50 
+
+	println!("{:?}", map);	// alice = 10, bob = 50
+
+	// or_insert returns a mutable reference -- you can modify it
+	let alice_score = map.entry(String::from("alice")).or_insert(0);
+	*alice_score += 5;		// add 5 to alice's score
+
+	println!("{:?}", map);
+
+	// PART 4: Looping over a hashmap
+
+	let mut ages: HashMap<&str, u32> = HashMap::new();
+	ages.insert("alice", 25);
+	ages.insert("bob", 30);
+	ages.insert("carol", 22);
+
+	// loop over all key-value pairs
+	// Note: Hashmap has NO guranteed order
+
+	for(name, age) in &ages{
+		println!("{} is {}", name, age)
+	}
+
+	// Loop over just keys
+	println!("keys");
+	for key in ages.keys(){
+		println!("	{}", key);
+	}
+
+	// Loop over just values
+	prontln!("values: ");
+	for value in ages.values() {
+		println!("	{}", value);
+	}
+
+	// PART 5 : Useful HashMap Methods
+
+	let mut map: HashMap<&str, i32> = HashMap::new();
+
+	map.insert("a", 1);
+	map.insert("b", 2);
+	map.insert("c", 3);
+
+	println!("length: {}", map.len());
+
+	println!("is_empty ? : {}", map.is_empty());
+
+	let mut temp = map.clone();
+	temp.clear();
+	println!("after clear length: {}", temp.len());
+
+	// PART 6: Word counter -- classic HashMap patter
+
+	let text = "hello world hello rust world hello";
+	let mut word_count: HashMap<&str, i32> = HashMap::new();
+
+	for word in text.split_whitespace(){
+		// get the count for this word, or start at 0
+		let count = word_count.entry(word).or_insert(0);
+		*count += 1;
+	}
+	println!("{:?}", word_count);
+
+}