如何从 hashmap 打印多个元素的随机唯一值

Question

I am working on a program where I have a list of tasks each with a predicted time duration. For example: {task: laundry, time: 1000}, {task: cooking, time: 2000}, {task: cleaning, time: 3000} , etc. I want to pick multiple items and increment an integer variable while printing out the task to the console. My approach is to first make a HashMap with the task being the key and time being value. I then put the time values in an ArrayList and iterate through the number of times I need and increment the time variable.

The issue I run into is I need unique numbers, so I opted to use a hashset to only keep track of unique values. However, I cannot print the task with this approach and if I iterate a certain number of times, I might not get enough values in the set if the key is duplicated.

Here is my code for this part.

int time = 0; 
    Random rand = new Random();
    ArrayList<Integer> eta = new ArrayList<Integer>(tasks.values());
    HashSet <Integer> set = new HashSet<>();
    
    for (int i = 0; i<=5; i++) {
        int randomIndex = rand.nextInt(eta.size());
        int tim = eta.get(randomIndex);
        set.add(tim);
    }
    Iterator<Integer> it = set.iterator();
     while(it.hasNext()){
        System.out.println(it.next());
        time += it.next();
     }
    return time;}

I printed the time values to the console and got 3 values before it crashed with a NoSuchElementException error. Is there a way I can fix this, or is there a better approach to do this problem. It would be best if I can pick the 5 values at random and print the task and increment the time variable.

Thank You.

Answer 1

Two Lists

Starting from your approach, you could add also the keys (task names) in a List so you can retrieve them by the random index.

Once you get an index, remove from the two lists the respective element so the next time you are gonna find another valid element and not a repeated one.

int time = 0;
Random rand = new Random();

List<Integer> eta = new ArrayList<>();
List<String> taskNames = new ArrayList<>();
    
tasks.forEach((k,v) -> {
    eta.add(v);
    taskNames.add(k);
});

for (int i = 0; i < 5; i++) {
    int randomIndex = rand.nextInt(eta.size());

    Integer taskTime = eta.remove(randomIndex);
    String taskName = taskNames.remove(randomIndex); 

    System.out.println(taskName  + ": " + taskTime);

    time += taskTime;
}

System.out.println("Total time: " + time);

One List

You actually don't need the value list ( eta ) since you can retrieve the values from the HashMap efficiently with the keys.

So you actually need only the keys list:

int time = 0;
Random rand = new Random();

List<String> taskNames = new ArrayList<>(tasks.keySet());

for (int i = 0; i < 5; i++) {
    int randomIndex = rand.nextInt(taskNames.size());

    String taskName = taskNames.remove(randomIndex);
    Integer taskTime = tasks.get(taskName);

    System.out.println(taskName  + ": " + taskTime);

    time += taskTime;
}

System.out.println("Total time: " + time);

Shuffle

Instead of finding a random index you could shuffle the list with Collections.shuffle and consider the first 5 elements:

int time = 0;

List<String> taskNames = new ArrayList<>(tasks.keySet());

Collections.shuffle(taskNames);

for (int i = 0; i < 5; i++) {

    String taskName = taskNames.get(i);
    Integer taskTime = tasks.get(taskName);

    System.out.println(taskName  + ": " + taskTime);

    time += taskTime;
}

System.out.println("Total time: " + time);

Shuffle + Stream

You could use the streaming API to get the total time.

To print the task name you have to use peek , which should be used only for debugging purposes but seems appropriate in your case:

List<String> taskNames = new ArrayList<>(tasks.keySet());

Collections.shuffle(taskNames);

int time = taskNames.stream()
        .limit(5)
        .peek(System.out::println)
        .mapToInt(tasks::get)
        .sum();

System.out.println("Total time: " + time);

Stream with Random

Instead of Collections.shuffle we could generate the random indexes using the streaming API:

List<String> taskNames = new ArrayList<>(tasks.keySet());

int time = new Random().ints(0, taskNames.size())
        .distinct()
        .limit(5)
        .mapToObj(taskNames::get)
        .peek(System.out::println)
        .mapToInt(tasks::get)
        .sum();

System.out.println("Total time: " + time);

Answer 2

The reason for the NoSuchElementException is that you run off the end of the iterator.

 while(it.hasNext()){
    System.out.println(it.next());
    time += it.next();
 }

Each time you call 'next' you advance the iterator, so you are consuming two elements. Given an odd number of elements, last time around the loop hasNext() is true, you read the last element (with the call to next() in the println call) and then you'll call next() again, and fail.

Call next() once after hasNext().

 while(it.hasNext()){
    Integer n = it.next();
    System.out.println(n);
    time += n;
 }