I'm getting strange output from java program

Question

I've some strange situation here and i thought that you may help me. I have an int array populated with numbers from 1 to 10. I want to generate random number from this array and save it to another int array. I used class Random to pick any number and since random throws 0 also i modify it like that ( so it throws numbers from 1 to 10 )

randNum = rand.nextInt(numbers.length-min+1)+min;

Following code makes sure that if it generates same random number, it skips it. Program is actually working and i'm getting in another array randomly positioned numbers from 1 to 10. That's what i wanted. But sometimes i'm missing one number from 1 - 10 AND iam Getting ZERO instead. Why??

int[] numbers = {1,2,3,4,5,6,7,8,9,10};
int[] usednum = new int[10];
Random rand = new Random();
int randNum;
int min = 1;

for (int x = 0; x<numbers.length; x++) {        
  for (int i = 0; i<usednum.length; i++) { 
    randNum = rand.nextInt(numbers.length-min+1) + min;
    for (int f = 0; f<usednum.length; f++) {
      if (usednum[f] == randNum) {
        break;
      } else if (usednum[f] == 0) { 
        usednum[x] = randNum;   
      }
    }
  } 
}

for (int c = 0; c<usednum.length; c++) {
  System.out.println(usednum[c]);
}

What is the purpose of g? you assign it, but do not use it. — Colin D
– Colin D, Commented Aug 31, 2012 at 14:20

Jon Lin · Accepted Answer · 2012-08-31 14:22:52Z

You're inner-most for loop only checks if the current random number is in the usednum[] array. And the for loop immediately outer of that only checks 10 times total. It gives up too quickly because it only tries 10 random numbers. If all 10 are already used, nothing will get stored in that slot of usednum[] (thus it will be 0), try adding a while loop around that and get rid of the extraneous outer-most for loop:

        for(int i = 0; i<usednum.length; i++) {
           while(usednum[i]==0) {
              randNum = rand.nextInt(numbers.length-min+1)+min;
              for(int f = 0; f<usednum.length; f++) {
                 if(usednum[f] == randNum) {
                    break;
                 } //if                                                                                                                                        
                 else if (usednum[f] == 0) {
                    usednum[i] = randNum;
                 }
              }
           }
        }

Also note that the assignment is for usednum[i] = randNum;.

This is essentially replacing the middle for loop (the one that goes from i=0 to 9) with the while loop.

Community · Accepted Answer · 2017-05-23 12:20:10Z

2

If your goal is simply to shuffle an array of numbers, try this instead:

Integer[] numbers = {1,2,3,4,5,6,7,8,9,10};
Collections.shuffle(Arrays.asList(numbers));

It will have the same effect. Unless you are completing a homework assignment that forces you to solve the issue in a more manual fashion, just make use of the standard Java libraries.

The shuffle method writes changes through to the underlying Integer array, thanks to the special type of List returned by Arrays.asList(...). Note you have to use an array of Integer not int (see Why does Collections.shuffle() fail for my array?).

edited May 23, 2017 at 12:20

CommunityBot

11 silver badge

answered Aug 31, 2012 at 14:19

Duncan Jones

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2 Comments

Firefoxx Pwnass Over a year ago

Thank you for your answer. But as you said i must complete my homework in hard way, lol. It's stupid tho..

Duncan Jones Over a year ago

Well, it's difficult enough to have challenged you, which is a good thing! In the future, please mark such questions with the "homework" tag so that people answer your question in a more constructive manner.

Sign · Accepted Answer · 2012-08-31 14:17:52Z

0

You are generating used numbers through an entire pass, so it doesn't generate a zero is just fails to generate a value it should.

answered Aug 31, 2012 at 14:17

Sign

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2 Comments

Firefoxx Pwnass Over a year ago

I think this makes some sense yes. And why exactly fails to generate a value?

Sign Over a year ago

It generates 10 values all of which have been used before. Try using randNum = 4; in your code it will give you 4 then 9 zeros

Minion91 · Accepted Answer · 2012-08-31 14:23:13Z

0

you have one for loop too much.

remove the loop with the i iterator and the program should do what you want.

Oh and remove the -min+1 from the random generator, -1+1=0

answered Aug 31, 2012 at 14:23

Minion91

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1 Comment

Colin D Over a year ago

the -min+1 is probably there to be used in the future.

Miljen Mikic · Accepted Answer · 2012-08-31 14:25:26Z

0

Your array usednum is consisted of zeros at the beginning. In some cases, your program doesn't change that initial value, but breaks before at the line:

if(usednum[f] == randNum)

and does that during all iterations with same value x. X increments and there goes your chance to change the zero value.

answered Aug 31, 2012 at 14:25

Miljen Mikic

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Comments

Dan · Accepted Answer · 2012-08-31 14:28:02Z

0

Edit - followed it and re-wrote it:

List<Integer> numbers = new LinkedList<Integer>(Arrays.asList(new Integer[]{ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 }));
        int[] usednum = new int[10];
        Random rand = new Random();
        int n = numbers.size();
        for (int i = 0; i < n; i++) {
            int randNum = rand.nextInt(numbers.size());
            usednum[i]=numbers.get(randNum);
            numbers.remove(randNum);
        }
        for (int c:usednum) {
            System.out.println(c); 
        }

edited Aug 31, 2012 at 14:28

answered Aug 31, 2012 at 14:13

Dan

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1 Comment

Dan Over a year ago

Edited - please review again - sorry about that

Community · Accepted Answer · 2017-05-23 12:27:36Z

Actually, you are never using the content of the array numbers. Try changing the array to something like int[] numbers = { 10, 22, 23, 42, 53, 18, 7, 8, 93, 10 };. You will get similar output.

Jon Lin's answer describe why your code is not working but does not address this issue. I think you will want to change your code to something like:

    int[] numbers = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
    int[] usednum = new int[10];
    Random rand = new Random();

    int selectedCount = 0;

    while (selectedCount < numbers.length) {
        int randNum = numbers[rand.nextInt(numbers.length)];
        boolean contains = false;
        for (int x = 0; x < selectedCount; x++) {
            if (usednum[x] == randNum) {
                contains = true;
                break;
            }
        }

        if (!contains) {
            usednum[selectedCount] = randNum;
            selectedCount++;
        }
    }


    for (int c = 0; c < usednum.length; c++) {
        System.out.println(usednum[c]);
    }

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