Java writes wrong data into array

Question

I'm implementing algorithm which will count the appearance of unique numbers in linked list. The problem is in the for loop in which I'm counting the appearance, when the input is something like 1 1 1 2 2 2

Instead of getting for output

1 appearance 3 times
2 appearance 3 times

I'm getting

1 appearance 3
0 appearance 0

Wiht the input 1 2 3 4 5 6 7 8 9 I got array out of index exception. Here's the code

public static void main(String[] args) {
    LinkedList<Integer> list = new LinkedList<Integer>();
    Scanner user_input = new Scanner(System.in);
    String input = user_input.nextLine();
    //Getting user input, if the user enter an empty line(enter, enter) the loop will die
    while(input.length() > 0){
        list.add(Integer.parseInt(input));
        input = user_input.nextLine();
    }
    Collections.sort(list); //sorting the input
    //Getting the number of unique numbers
    int count_of_unique_numbers = 1;// There will be atleast one unique number

    if(!list.isEmpty()){
        int temp = list.get(0);
        for(int i = 1;i < list.size(); i++){
            if(temp != list.get(i)){
                count_of_unique_numbers++;
                temp = list.get(i);
            }
        }
    }
    else{
        System.out.println("The list is empty"); 
        return;
    } 
    //Counting how many times the unique numbers apper;
    int number_appearance[][] = new int[count_of_unique_numbers][2];
    int temp = list.get(0);
    int counter = 1;
    int j = 0;
    for(int i = 1;i < list.size();i++){
        if(temp == list.get(i)){ counter++; }
        else{
            number_appearance[j][0] = temp;
            number_appearance[j][1] = counter;
            counter = 1;
            temp = list.get(i);
            j++;
        }
    }
    //Printing the number_appearance array
    for(int i = 0; i < count_of_unique_numbers; i++){
        System.out.println("The number: " + number_appearance[i][0] + " appearece " + number_appearance[i][1] + " times");
    }
}

Where are you getting the exception, and have you debugged through it? — Jon Skeet
– Jon Skeet, Commented Jan 14, 2015 at 16:26
(You should also consider that you never write the final number into your list of appearances, because you only write that when you spot a different number.) — Jon Skeet
– Jon Skeet, Commented Jan 14, 2015 at 16:27
In the third for loop, in the if condition when I try to get the ith element [ temp == list.get(i) ] — justNoob
– justNoob, Commented Jan 14, 2015 at 16:27
How do you know that "there will be atleast one unique number" before checking your list for being empty? — Sergey Kalinichenko
– Sergey Kalinichenko, Commented Jan 14, 2015 at 16:29

Tom · Accepted Answer · 2015-01-14 17:23:13Z

1

You could simply all of that down to this.

Map < String, Integer > numMap = new HashMap < String, Integer > ();
Scanner user_input = new Scanner(System. in );
String input = user_input.nextLine();

String[] inputArray = input.split(" ");
for (String s: inputArray) {
    if (numMap.containsKey(s)) {
        numMap.put(s, numMap.get(s) + 1);
    } else {
        numMap.put(s, 1);
    }
}

for (Map.Entry < String, Integer > entry: numMap.entrySet()) {
    System.out.println("number: " + entry.getKey() + " appeared: " + entry.getValue() + " times");
}

edited Jan 14, 2015 at 17:23

Tom

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answered Jan 14, 2015 at 16:35

austin wernli

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2 Comments

Tom Over a year ago

Well, if you change the key of that Map to type String, then you can avoid the conversion to integer and you don't need to care if the user really entered a number or not.

austin wernli Over a year ago

Very true, i have updated the answer to reflect that. Thank you.

Eran · Accepted Answer · 2015-01-14 16:39:55Z

You can fix the problem by adding the number of occurences of the final number, which you currently miss :

for(int i = 1;i < list.size();i++){
    if(temp == list.get(i)){ counter++; }
    else{
        System.out.println(temp+":"+counter);
        number_appearance[j][0] = temp;
        number_appearance[j][1] = counter;
        counter = 1;
        temp = list.get(i);
        j++;
    }
}
number_appearance[j][0] = temp; // added
number_appearance[j][1] = counter; // added

Input :

1 1 1 2 2 2

Output :

The number: 1 appearece 3 times
The number: 2 appearece 3 times

Input :

1 2 3 4 5 6 7 8 9

Output :

The number: 1 appearece 1 times
The number: 2 appearece 1 times
The number: 3 appearece 1 times
The number: 4 appearece 1 times
The number: 5 appearece 1 times
The number: 6 appearece 1 times
The number: 7 appearece 1 times
The number: 8 appearece 1 times
The number: 9 appearece 1 times

Jean-François Savard · Accepted Answer · 2015-01-14 16:31:58Z

0

You could simply do

List<Integer> foundNumbers = new ArrayList<Integer>();
for(int i = 0; i < list.size(); i++)
{
    if(foundNumbers.contains(list.get(i))
    {
        System.out.println(list.get(i) + " appearance " + Collections.frequency(list, list.get(i) + " times.");
        foundNumbers.add(list.get(i));
    }
}

Note that Collections.frequency is available since JDK 1.6

answered Jan 14, 2015 at 16:31

Jean-François Savard

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1 Comment

justNoob Over a year ago

This is exercices i don't need it for any software. And i want to make the algorithm myself. But thanks for your help.

Predrag Maric · Accepted Answer · 2015-01-14 16:34:03Z

0

I know it's not what you originally were trying to do, but it would be easier to do it using Map

public static void main(String[] args) {
    LinkedList<Integer> list = new LinkedList<Integer>();
    Scanner user_input = new Scanner(System.in);
    String input = user_input.nextLine();
    // Getting user input, if the user enter an empty line(enter, enter) the loop will die
    while (input.length() > 0) {
        list.add(Integer.parseInt(input));
        input = user_input.nextLine();
    }
    Collections.sort(list); // sorting the input
    Map<Integer, Integer> count = new HashMap<Integer, Integer>();
    for (int i : list) {
        if (count.containsKey(i)) {
            count.put(i, count.get(i) + 1);
        }
        else {
            count.put(i, 1);
        }
    }

    for (Map.Entry<Integer, Integer> num : count.entrySet()) {
        System.out.println("The number: " + num.getKey() + " appearece " + num.getValue() + " times");
    }
}

answered Jan 14, 2015 at 16:34

Predrag Maric

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2 Comments

justNoob Over a year ago

I'm still unfamiliar with maps. But thank you I will keep this piece of code until I read the theory about them.

Predrag Maric Over a year ago

@austinwernli Not the same, you corrected some spelling errors :)

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