How to loop through an array and check for duplicates?

Question

I am creating a program that lets you store 10 items in an array. What I haven't been able to get the program to do is give an error if one of the entered items already exists in the array.

So, for example, if the array looks like [banana, potato, 3, 4, yes, ...] and I enter banana again, it should say "Item has already been stored" and ask me to re-enter the value. The code I currently have is:

public static void main(String[] args) {
 Scanner keyboard = new Scanner(System.in);
 int stringNumber = 0;
 String[] stringArray = new String[10];
 for (int i = 0; i <= stringArray.length; i++) {
 out.println("\nEnter a string");
 String input = keyboard.next();
 stringArray[stringNumber] = input;
 out.println("\"" + stringArray[stringNumber] + "\"" + " has been stored.");
 PrintArray(stringArray);
 stringNumber++;

• Ingo: I don't understand the question. WHat exactly prevents you from going through the array and checking for duplicates?
• Philipp Sander: there is missing some code
• Chris: add them to a set, if it already exists, it wont allow you to do so. convert to an array at the end (should you really need an array specifically?)
• Philipp Sander: don't write your own printArray-method. use System.out.println(Arrays.toString(stringArray));
• Gregg1989: lngo, the problem is that I don't know how to loop through the array and see if the input string value already exists. (I'm a beginner)

Answer 1

You can use nested loops to go through the array to see if the new input exists. It would be better to do this in a function. Also when doing this you need to make sure that you are not at the first element or you will get a null pointer exception.

for (int i = 0; i <= stringArray.length; i++) {
 boolean isInArray = false;
 System.out.println("\nEnter a string");
 String input = keyboard.next();
 if (i > 0) {
 for (int j = 0; j < stringArray.length; j++) {
 if (stringArray[j].equalsIgnoreCase(input)) {
 isInArray = true;
 break;
 }
 }
 }
 if (!isInArray) {
 stringArray[stringNumber] = input;
 } else {
 System.out.println("\"" + stringArray[stringNumber-1] + "\""
 + " has been stored.");
 }
 PrintArray(stringArray);
 stringNumber++;
 }

• Matt Penna: I only used loops and arrays assuming this is for a school assignment and you could not use other data types.
• Gregg1989: There's a small bug in this code, but I'm working on fixing it! Thanks Matt, this gave me a much better perspective on the direction I should be taking with this program. :)

Answer 2

It's always better to use a HashSet when you don't want to store duplicates. Then use HashSet#contains() method to check if element is already there. If ordering is important, then use LinkedHashSet.

---

If you really want to use an array, you can write a utility method contains() for an array. Pass the array, and the value to search for.

public static boolean contains(String[] array, String value) {
 // Iterate over the array using for loop
 // For each string, check if it equals to value.
 // Return true, if it is equal, else continue iteration
 // After the iteration ends, directly return false.
}

• For iterating over the array, check enhanced for statement.
• For comparing String, use String#equals(Object) method.

Answer 3

When you got the String input, you can create a method that will :

1. Go through the entire array and check if the string is in it (you can use equals() to check content of Strings)
2. Returns a boolean value wheter the string is in the array or not
3. Then just add a while structure to re-ask for an input

Basically it can look like this :

String input = ""; 
do {
 input = keyboard.next();
}while(!checkString(input))

The checkString method will just go through all the array(using a for loop as you did to add elements) and returns the appropriate boolean value.

---

• Gregg1989: That's exactly what I thought I should do. The thing is that I've been using Java for 2 days and I'm not sure how to write that piece of code :(

Answer 4

Without introducing some order in your array and without using an addition structure for instance HashSet, you will have to look through the whole array and compare the new item to each of the items already present in the array.

For me the best solution is to have a helper HashSet to check the item for presence.

Also have a look at this question.

Answer 5

To avoid you should use an Set instead of an array and loop until size = 10.

If you need to keep an array, you can use the .contains() method to check if the item is already present in the array.

Answer 6

while (no input or duplicated){
 ask for a new string
 if (not duplicated) {
 store the string in the array
 break;
 }
}

Answer 7

You should check the input value in array before inserting into it. You can write a method like exists which accepts String[] & String as input parameter, and find the string into the String array, if it finds the result then return true else false.

public boolean exists(String[] strs, String search){
 for(String str : strs){
 if(str.equals(search))
 return true;
 }
 return false;
}

performance would be O(n) as it searchs linearly.