Update Exercises Directory Names to Reflect Order

exercises/20_threads/threads2.rs

@@ -0,0 +1,39 @@
+// threads2.rs
+//
+// Building on the last exercise, we want all of the threads to complete their
+// work but this time the spawned threads need to be in charge of updating a
+// shared value: JobStatus.jobs_completed
+//
+// Execute `rustlings hint threads2` or use the `hint` watch subcommand for a
+// hint.
+
+// I AM NOT DONE
+
+use std::sync::Arc;
+use std::thread;
+use std::time::Duration;
+
+struct JobStatus {
+    jobs_completed: u32,
+}
+
+fn main() {
+    let status = Arc::new(JobStatus { jobs_completed: 0 });
+    let mut handles = vec![];
+    for _ in 0..10 {
+        let status_shared = Arc::clone(&status);
+        let handle = thread::spawn(move || {
+            thread::sleep(Duration::from_millis(250));
+            // TODO: You must take an action before you update a shared value
+            status_shared.jobs_completed += 1;
+        });
+        handles.push(handle);
+    }
+    for handle in handles {
+        handle.join().unwrap();
+        // TODO: Print the value of the JobStatus.jobs_completed. Did you notice
+        // anything interesting in the output? Do you have to 'join' on all the
+        // handles?
+        println!("jobs completed {}", ???);
+    }
+}