start error handling

average-speed/Cargo.toml

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+[package]
+name = "average_speed"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]

average-speed/src/main.rs

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+fn main() {
+    use std::collections::HashMap;
+
+    // initialize a hash map of runners to store their average lap timings
+    let mut runners = HashMap::new();
+
+    let joe = "Joe".to_string();
+    let jill = "Jill".to_string();
+    let jane = "Jane".to_string();
+    let jack = "Jack".to_string();
+
+    // insert joe and set his average to 10
+    runners.insert(joe.clone(), 10);
+    println!("DEBUG: there's only Joe");
+    println!("\tthe current values are:\n\t{runners:?}");
+
+    // insert other runners and set their default values
+    runners.entry(jill.clone()).or_insert(5);
+    runners.entry(jane.clone()).or_insert(5);
+    runners.entry(jack.clone()).or_insert(5);
+    println!("\nDEBUG: we initialized Jill, Jane and Jack with 5 minutes per lap");
+    println!("\tthe current values are:\n\t{runners:?}");
+
+    // hard update jill's average to another value
+    println!("\nDEBUG: hard setting Jill's value to 20");
+    println!("\tJill's average before: {:?}", runners.get(&jill).unwrap());
+    runners.insert(jill.clone(), 20);
+    println!("\tJill's average after: {:?}", runners.get(&jill).unwrap());
+
+    // update current values based on old values
+    println!(
+        "\nDEBUG: runners have completed one lap
+       their timings are:
+       \tJill: 4
+       \tJack: 10
+       \tJane: 8
+       \tJoe: 9"
+    );
+    println!("\nDEBUG: updating their values...");
+    println!("\tvalues before updating:\n\t{runners:?}");
+
+    // updating values using another hash map
+    let mut new_values = HashMap::new();
+
+    new_values.entry(jane.clone()).or_insert(8);
+    new_values.entry(jill.clone()).or_insert(4);
+    new_values.entry(jack.clone()).or_insert(10);
+    new_values.entry(joe.clone()).or_insert(9);
+
+    for (key, val) in new_values.iter() {
+        let updater = runners.entry(key.clone()).or_insert(0);
+        *updater = (*updater + val) / 2;
+    }
+
+    println!("\n\tvalues after updating:\n\t{runners:?}");
+}