Remove consecutive duplicates from an arraylist in Java

Question

I have an integer arraylist that has consecutive duplicate lines. I want to output each number that is not the same as the one before.

Sample: 1 3 3 3 2 2 1 2 2 3 3 3

Desired output: 1 3 2 1 2 3

How should I do this?

I am essentially creating an inverted index of words to any input of text files. I have a file/buffered reader reading from multiple text files line by line and placing each line into a 2d array with the first column being a .split word and the second column being the filename from which the word was obtained. I then have a for loop to get the frequency a particular word occurs in all of the files and add it to another column. I then copied each column into their own arraylist. I added the words arraylist to a hashset to remove the duplicates. but I want to remove only consecutive duplicates of the frequency arraylist.

Questions should include a summary of the work you've done so far to solve the problem, and a description of the difficulty you are having solving it. Please edit your question and include your attempt to solve this problem. More info here — Pshemo
– Pshemo, Commented Feb 12, 2015 at 1:14

ישו אוהב אותך · Accepted Answer · 2017-05-24 10:08:42Z

7

Possible idea comparing elements to each other:

public static ArrayList<Integer> noConsecutiveDups(ArrayList<Integer> input) {

  ArrayList<Integer> newList = new ArrayList<Integer>();

  // Always add first value
  newList.add(input.get(0));

  // Iterate the remaining values
  for(int i = 1; i < input.size(); i++) {
    // Compare current value to previous
    if(input.get(i-1) != input.get(i)) {
       newList.add(input.get(i));
    }
  }

  return newList;
}

edited May 24, 2017 at 10:08

ישו אוהב אותך

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answered Feb 12, 2015 at 1:25

shirrine

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2 Comments

Randy Jones Over a year ago

You're a miracle worker. i kept trying to compare input.get(i) with input.get(i+1) and it kept going out of bounds. I didn't think to add one and then compare to the previous.

shirrine Over a year ago

LOL! Just being creative. :-)

Ritveak · Accepted Answer · 2021-04-02 09:27:15Z

I saw two answers here, one was using extra space and other was using extra time(~n^2) so here's a simple reverse iteration and removal that would help to remove consecutive duplicates in o(n) time without using extra space:

import java.util.*;
public class HelloWorld{

     public static void main(String []args){
        ArrayList<String> list = new ArrayList<String>();
        list.add("a");
        list.add("b");
        list.add("a");
        list.add("a");
        list.add("b");
        list.add("a");
        list.add("a");
        list.add("a");
        System.out.println("Before:"+list);
        
        for(int i =list.size()-1;i>=1;i--){
            if(list.get(i).equals(list.get(i-1))){
                list.remove(i);
            }
        }
        System.out.println("After:"+list);
     }
}

quant · Accepted Answer · 2018-11-14 13:47:19Z

-1

The above logic is by taking a new list and making changes.But the below code helps you to make changes for the same list.

public class AvoidConsecutiveDups {
    public static void main(String args[]) {
        ArrayList<String> list = new ArrayList<String>();
        list.add("a");
        list.add("b");
        list.add("a");
        list.add("a");
        list.add("b");
        list.add("a");
        list.add("a");
        list.add("a");

        for (int i = 1; i <= list.size() - 1; i++) {
            if (list.get(i - 1) == list.get(i)) {
                list.remove(i - 1);
                for (int j = 1; j <= list.size() - 1; j++) {
                    if (list.get(j - 1) == list.get(j)) {
                        list.remove(j - 1);
                    }
                }
            }
        }

        Iterator itr = list.iterator();

        while (itr.hasNext()) {
            System.out.println(itr.next());
        }
    }
}

edited Nov 14, 2018 at 13:47

quant

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answered Nov 14, 2018 at 12:49

sandeep358

1

1 Comment

Ritveak Over a year ago

I don't like the idea of having a loop inside another loop when a simple reverse traversal and removal can do the trick....

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