Updating a Value

Part of the Logic & Flow section of Coddy's Rust journey — lesson 32 of 66.

Sometimes you need to update values in a hash map in different ways. You've already learned that .insert() will overwrite an existing value, but what if you only want to add a value when the key doesn't already exist?

Rust provides the .entry() API for more sophisticated value updates. The most useful method in this API is .or_insert(), which only inserts a value if the key is not already present:

let mut scores = HashMap::new();
scores.insert("Alice", 85);

// This will NOT overwrite Alice's score
scores.entry("Alice").or_insert(90);

// This WILL add Bob's score since he's not in the map
scores.entry("Bob").or_insert(90);

The .entry() method returns an Entry enum that represents either an occupied or vacant spot in the hash map. When you call .or_insert() on it, it only inserts the new value if the entry is vacant (the key doesn't exist).

Challenge

Easy

You will receive an integer n indicating the number of player names to process. Then you will receive n player names as inputs. Create a hash map to track player scores (type HashMap<String, i32>). For each player name you receive, use .entry() with .or_insert() to add them to the map with an initial score of 100 only if they don't already exist. After processing all names, print each player and their score.

Requirements:

Import HashMap from std::collections
Create a mutable hash map with types HashMap<String, i32>
Read the first input and convert it to i32 to get the number of player names
Use a loop to read n player names
For each player name, use .entry(name).or_insert(100) to add them with a score of 100 if they're not already in the map
After processing all names, iterate over the hash map and print each player's information in the format: [name]: [score]

Input:

First line: An integer n (e.g., 5)
Next n lines: Player names (e.g., Alice, Bob, Alice, Charlie, Bob)

Output:

One line for each unique player in the format: [name]: [score]
The order of output lines may vary between test runs

Cheat sheet

The .entry() API provides sophisticated ways to update hash map values. The .or_insert() method only inserts a value if the key doesn't already exist:

let mut scores = HashMap::new();
scores.insert("Alice", 85);

// This will NOT overwrite Alice's score
scores.entry("Alice").or_insert(90);

// This WILL add Bob's score since he's not in the map
scores.entry("Bob").or_insert(90);

The .entry() method returns an Entry enum representing either an occupied or vacant spot in the hash map. When you call .or_insert(), it only inserts the new value if the entry is vacant (the key doesn't exist).

Try it yourself

use std::collections::HashMap;
use std::io::{self, BufRead};

fn main() {
    let stdin = io::stdin();
    let mut lines = stdin.lock().lines();
    
    // Read the number of player names
    let n: i32 = lines.next().unwrap().unwrap().trim().parse().unwrap();
    
    // Create a mutable HashMap to store player scores
    let mut player_scores: HashMap<String, i32> = HashMap::new();
    
    // TODO: Write your code below
    // Read n player names and use .entry().or_insert(100) to add them to the map
    
    
    
    // Print each player and their score in the format: [name]: [score]
    
}