Java - Listing the index of duplicate values in an int array

Question

I have to locate and list all duplicate index values in array.

Example: int[] array = { 0, 7, 9, 1, 5, 8, 7, 4, 7, 3};

7 is located in three different locations at index 1, 6, and 8. How would I go about modifying my existing code in order to have outputResults.setText() show the location of the duplicate values? outputResults.setText() is JTextField if that helps.

String tmp1 = getNumbers.getText();
    try {
        int search = Integer.parseInt(tmp1);
        for (p = 0; p < array.length; p++) {
            if(array[p]==search) {
                b = true;
                index = p;
            }
        }   
        if(b==true)
            outputResults.setText(search + " was in the following fields of the array " + index);
         else 
            throw new NumberNotFoundException("Your number was not found.");


    } catch (NumberFormatException ex) {
        JOptionPane.showMessageDialog(getContentPane(), "You can only search for integers.");

    } catch (NumberNotFoundException ex) {
        JOptionPane.showMessageDialog(getContentPane(), ex.getMessage());
    }

At it's current state, it will only list the last time the duplicate number was located which would be index 8 based on my example. The list of numbers in the array is inputted by the user, and I'm not allowed to sort the values. My original guess was to create a nested loop and whenever it located an duplicated number, add p (current index it's searching) to a new array. I would then list the full array in outputResults.setText() but it gave several warnings and errors when I tried.

The full code can be found here if needed: http://pastebin.com/R7rfWAv0 And yes, the full program is a mess but it gets the job done and I was having such a headache with it. Also note, in the full program, the professor asked us to throw an exception if a duplicate value was detected as extra credit. I did it, but I commented it out to finish the original assignment so please disregard it.

Solution 1

I think you should use a List to record the indexs

List<Integer> indexs =new ArrayList<Integer>();
for (p = 0; p < array.length; p++) {
    if(array[p]==search) {
        indexs.add(p);
    }
}
if(p.length()>0){
    //print the result
}

Solution 2

How about just two for loops?

for (int i = 0; i < array.length; i++) {
  for (int j = 0; j < array.length; j++) {
    if (array[i] == array[j]) {
      System.out.println("Duplicate - " + array[i] + " found at index " + i + " and " + j);
    }
  }
}

Solution 3

No need for Hash tables, lists or whatever, you can do this very easily as so:

int [] array = { 0, 7, 9, 1, 5, 8, 7, 4, 7, 3};
int pointer=0;
int currNumber;
while(pointer<array.length)
{   
  currNumber=array[pointer];
  for(int i=0;i<array.length;i++){          
    if(currNumber==array[i] && i>pointer){
        System.out.println("Duplicate for "+currNumber +" in " +i);
        break;
    }
  }   
  pointer++;
}

It will print all duplicates for all the numbers in the array.

Duplicate for 7 in 6
Duplicate for 7 in 8

Obviously, you'd probably have to concatenate a string and display it at the end of the loop by calling outputResults.setText()

Demo here.

Comments

[user1462300]

I'm sorry but I don't know what a HashMap is. We never went over anything related to it in class. Would you be willing to explain it a little? I'll research into it in the meantime.

[user1462300]

We haven't gone over List yet in class but after a reading about it in orcales, it seems like it can get the job done. The only problem is that eclipse is telling me "The type List is not generic; it can not be parameterized with arguments <Integers>". I'll look further into it, thanks for the tip!

[plucury]

@user1462300 Just use <Integer> is OK. <Integers> is invalid.see java.sun.com/j2se/1.5/pdf/generics-tutorial.pdf for more detail.

[user1462300]

Oh wow, cannot believe I forgot about +=. That actually worked perfectly! And yea, I know there's several ways to achieve the results I wanted, but like I mentioned before, when you work on something for a long period of time, you tend to forget the basics and simple methods of doing things. Thank you so much!

[Michael]

docs.oracle.com/javase/6/docs/api/java/util/HashMap.html Basically, it's a hashtable but implemented using the Map Collection interface.

[Donnie]

If this helped solve your issue please mark it as accepted. The tick mark under the number of up votes.

[karlihnos]

Thats wrong! you are always evaluating the same position. E.g. array[0] == array[0]

[Gavin]

@karlihnos Pretend array.length = 3. The loops are nested. It enters first loop at i = 0. Then enters second loop at j = 0. Then i=0, j=1; i=0, j=2. Now j=3 !< array.length, so exit second loop. First loop increments i. i=1, j=0; i=1, j=1, etc.

[karlihnos]

Taking the example from the op: int[] array = { 0, 7, 9, 1, 5, 8, 7, 4, 7, 3}. He says: 7 is repeated at postiitons 1,6 and 8. In your code you evaluate array[0]==array[0], after j is done, it continues comparing array[1] and j will be somewhen arrays[1], it means again array[1]=array[1]. So basically, you are comparing each number with its self