从字符串数组获取不同的随机字符串

Question

Hi well i have a string array with like 50 strings and i want it to select only 10 randon results and display them on a table layout i get everything right except it only shows one result. heres my code:

private String[] list;
private static final Random rgenerator = new Random();


@Override
protected void onCreate(Bundle savedInstanceState) {
    // TODO Auto-generated method stub
    super.onCreate(savedInstanceState);
    this.setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_LANDSCAPE);
    requestWindowFeature(Window.FEATURE_NO_TITLE);
    getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,
            WindowManager.LayoutParams.FLAG_FULLSCREEN);
    setContentView(R.layout.layout1);

Resources res = getResources();
    list = res.getStringArray(R.array.names);
    String q = list[rgenerator.nextInt(list.length)];
int total = 10;

    for (int current = 0; current < total; current++) {
        // Create a TableRow and give it an ID
        TableRow tr = new TableRow(this);
        tr.setId(100 + current);
        tr.setLayoutParams(new LayoutParams(LayoutParams.FILL_PARENT,
                LayoutParams.WRAP_CONTENT));

        // Create a TextView to house the name of the province
        TextView labelTV = new TextView(this);
        labelTV.setId(200 + current);
        labelTV.setText(q);
        labelTV.setTextSize(14);
        labelTV.setGravity(Gravity.CENTER);
        labelTV.setTextColor(Color.WHITE);
        labelTV.setLayoutParams(new LayoutParams(LayoutParams.FILL_PARENT,
                LayoutParams.WRAP_CONTENT));
        tr.addView(labelTV);

        tablelayout.addView(tr, new TableLayout.LayoutParams(
                LayoutParams.FILL_PARENT, LayoutParams.WRAP_CONTENT));
}

whats wrong?

Answer 1

Quite simple by using a set, that will take care about avoiding duplicates:

Set<Integer> uniques = new HashSet<Integer>();

while (uniques.size() < 10)
  uniques.add(rgenerator.nextInt(list.length);

for (Integer i : uniques)
{
  String q = list[i];

  ..
}

Answer 2

This is called "random sampling". Two common techniques are:

• simply shuffle the entire list/array, making sure you use a fair shuffling algorithm such as that offered by Collections.shuffle(), then take the first n elements;
• go through the list, each time deciding whether to include the next item based on whether a random number exceeds a threshold representing number of items still needed/number of items left.

If you go for the first method, just make sure that your shuffle is genuinely a fair algorithm. Many programmers if asked to write a shuffle algorithm from scratch get it wrong. Java's Collections.shuffle() is an example of how to do it.

You may also be interested in an article about random sampling in Java that I wrote a while ago which includes example skeleton code for the second case.

For the sake of knowledge, you may also wish to research something called "reservoir sampling", although it is overkill for selecting 10 items from a possible 50.

Answer 3

Thats what I came up with..

import java.util.*;
public class RandomStringFromArray
{
    public static void main(String[] args)
    {
        Random rnd = new Random();
        Scanner scan = new Scanner (System.in);
        System.out.println("Enter how many string in the array");
        int x = scan.nextInt();
        ArrayList<String> arraystr = new ArrayList<String>();

        for (int i=0; i < x ; i++)
            {
                System.out.println("Enter elements of the array");
                arraystr.add(scan.next());
            }
        System.out.println("\nI picked: ");

        for ( int t = 0; t < 3; t++) // 3 is how many elements you want to pick
            {

                    int random = rnd.nextInt(x);
                    System.out.println(arraystr.get(random));
                    arraystr.remove(random);
            }

    }
}

Answer 4

The reason you only get one answer is you get the string outside of your loop so you only get it once. It should look more like it is below.

Also if you want to avoid duplicates you are going to have to do something to take care of that. java.util.Collections.shuffle() might be good for your purposes, then you just take the first 10 items.

    int total = 10;

    for (int current = 0; current < total; current++) {
        String q = list[rgenerator.nextInt(list.length)];
        // Create a TableRow and give it an ID