Inserting objects into arraylist

Question

I have an arraylist of 50 RANDOM integers. I ask a user to remove a number and all occurences of that number are removed from the list. I did that using

while (randInts.contains(removeInt) )
{
  if (randInts.get(i) == removeInt)                 
   randInts.remove(randInts.get(i));
   i++;             

 } 

 System.out.println("\n" + randInts.toString());
 System.out.println("\n" + randInts.size());`

The other part of the problem is to prompt the user to enter another number. The removed number from above is inserted after each occurrence of the second prompted number. I am having issues with the second part as I keep getting IndexOutOfBoundsException.

can you post the code that is throwing the out of bounds exception...? — chancea
– chancea, Commented Sep 16, 2013 at 20:46
System.out.println("\n Please enter another integer in 1...10: "); int insertAfterInt = input2.nextInt(); while (randInts.contains(insertAfterInt) ){ if (randInts.get(i) == insertAfterInt) randInts.remove(randInts.get(i)); } System.out.println("\n" + randInts.toString()); System.out.println("\n" + randInts.size()); — dogbern
– dogbern, Commented Sep 17, 2013 at 17:38

chrylis -cautiouslyoptimistic- · Accepted Answer · 2013-09-17 22:23:33Z

1

Use a LinkedList instead; it's a much better choice when you need in-order traversal but not really random access, and when you need to insert and remove elements in the middle of the list.

You can accomplish what you're wanting (removing all instances of removeInt and inserting removeInt after every instance of insertAfterInt) with a simple traversal of the list's iterator:

ListIterator<Integer> li = randInts.listIterator();
while(li.hasNext()) {
    int i = li.next();
    if(removeInt == i)     // assumes removeInt is an int; use equals() for Integer
        li.remove();
    if(insertAfterInt == i)
        li.add(removeInt);  // the iterator will skip this element, so it won't get removed
}

edited Sep 17, 2013 at 22:23

answered Sep 17, 2013 at 21:42

chrylis -cautiouslyoptimistic-

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2 Comments

dogbern Over a year ago

Thanks it works. But I had to make some changes. May be it was a typo int i =li.next();

chrylis -cautiouslyoptimistic- Over a year ago

@dogbern Thanks, yes; I probably got confused by my tendency to use i instead of it for iterators. ;-) Feel free to edit answers for issues like that.

Sam Dozor · Accepted Answer · 2013-09-16 20:53:45Z

1

I see two big issues: You're not bounding i to anything, and you wrote an n^2 loop (you can do this in linear time).

You're shrinking the size of the `List` as you go...take this simple example:

Say you want to remove all instances of 5

Given a list that looks like {1,2,3,5,5}

When i = 3 you will remove the first 5, making the list look like: {1,2,3,5}

then you will attempt to remove the element at i = 4, but that element you want to remove is really now at i = 3, and you'll get the IndexOutOfBoundsException

You don't want to use a `contains`, as this expands the worst case performance of your loop to n^2, this would be faster:

int size = randInts.size() - 1;
for (int i = size; i >= 0; i--){
   if (randInts.get(i).equals(removeInt))
       randInts.remove(i);
}

edited Sep 16, 2013 at 20:53

answered Sep 16, 2013 at 20:47

Sam Dozor

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6 Comments

chrylis -cautiouslyoptimistic- Over a year ago

And this is a textbook case of when to use a LinkedList instead of an ArrayList.

Cruncher Over a year ago

@chrylis I don't think you know enough about the list to make that call

dogbern Over a year ago

@Chrylis I like your approach but the problem gives a hint that says the removeInt part can be accomplished with a very simple while statement.

dogbern Over a year ago

@chrylis, also what approach would you take on the second part of the question for insertAfterInt?

chrylis -cautiouslyoptimistic- Over a year ago

@Cruncher He's doing lots of inserts and removes in the middle of the list. That's pretty much the distinguishing characteristic between use cases.

|

KhAn SaAb · Accepted Answer · 2013-09-16 20:48:37Z

0

while (randInts.contains(removeInt) )
{

   if(i<randInts.size());
    {
  if (randInts.get(i) == removeInt)                 
   randInts.remove(randInts.get(i));
     }//if
   i++;             

 }while

answered Sep 16, 2013 at 20:48

KhAn SaAb

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Comments

Tim · Accepted Answer · 2013-09-16 20:59:25Z

0

I am guessing you are starting with a collection of 50 items (randInts) and removing the items that users enter (i)?

If that is the case, once your remove an item, your collection then only 49 indexes left and get gets by the index. Try something like...

if (randInts.contains(i)){
     randInts.remove(randInts.indexOf(i));
}

edited Sep 16, 2013 at 20:59

answered Sep 16, 2013 at 20:52

Tim

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2 Comments

Aleksander Fular Over a year ago

remove(i) - remove takes index not value as argument.

Tim Over a year ago

Actually remove takes either index or object according to the documentation. In this case without knowing what i is (Integer or int), value could just as easily worked. But to eliminate confusion I edited the answer.

phntmasasin · Accepted Answer · 2013-09-16 21:14:21Z

0

This is n^2, but it should work

int i = 0;

    while(i < loFnumbers.size()){
        if(loFnumbers.get(i) == removeInt){
            loFnumbers.remove(i);
            continue;
        }
        i++;

    }

answered Sep 16, 2013 at 21:14

phntmasasin

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Comments

earldouglas · Accepted Answer · 2013-09-16 21:34:03Z

Here's an approach that avoids any state mutation (i.e. randInts is never modified):

package so;

import java.util.ArrayList;

public class SO_18836900 {

  public static void main(String[] args) {

    // build a collection of random ints
    ArrayList<Integer> randInts = new ArrayList();
    for (int i = 0; i < 50; i ++) {
      randInts.add((int)(Math.random() * 5));
    }

    // create a collection with all 3s filtered out
    ArrayList<Integer> filtered = filterOut(randInts, 3);
    System.out.println(filtered);
    System.out.println(filtered.size());

    // create a collection with a 99 inserted after each 4
    ArrayList<Integer> insertedAfter = insertAfter(randInts, 4, 99);
    System.out.println(insertedAfter);
    System.out.println(insertedAfter.size());

  }

  static ArrayList<Integer> filterOut(Iterable<Integer> xs, int toRemove) {
    ArrayList<Integer> filteredInts = new ArrayList();
    for (int x : xs) {
      if (x != toRemove) filteredInts.add(x);
    }
    return filteredInts;
  }

  static ArrayList<Integer> insertAfter(Iterable<Integer> xs, int trigger, int toInsert) {
    ArrayList<Integer> insertedAfter = new ArrayList();
    for (int x : xs) {
      insertedAfter.add(x);
      if (x == trigger) insertedAfter.add(toInsert);
    }
    return insertedAfter;
  }

}

nanofarad · Accepted Answer · 2013-09-17 19:38:55Z

0

if (randInts.get(i) == removeInt)                 
randInts.remove(randInts.get(i));
i++;

You're never checking stopping conditions. Fix is:

while (randInts.contains(removeInt) )
{
    i=0;
   while(i<randInts.size()){

  if (randInts.get(i) == removeInt)                 
   randInts.remove(randInts.get(i));
   i++;             
}
 }

edited Sep 17, 2013 at 19:38

answered Sep 16, 2013 at 20:47

nanofarad

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6 Comments

Cruncher Over a year ago

while(i<randInts.size()); What does this accomplish exactly? Are you depending on another thread to increment i?

nanofarad Over a year ago

@Cruncher No, I'm making sure the OP doesn't exceed the bound. There's no need to even think about threading here.

Cruncher Over a year ago

My point here is that the loop has no body. If the condition was true the first time, it will be true forever.

nanofarad Over a year ago

@Cruncher Ah, I see what you were talking about. That was actually just a typo.

Cruncher Over a year ago

The dangers of using implicit blocks. I use them sometimes for super trivial if statements. There's a lot of code where I work with 4 levels deep of implicit blocks. It's a nightmare. If you add a line of code anywhere it just breaks.

|

nanofarad · Accepted Answer · 2013-09-16 20:45:55Z

-1

Dont use == on "Integers" you are comparing references.
Either unbox into int or use equals(

edited Sep 16, 2013 at 20:45

nanofarad

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answered Sep 16, 2013 at 20:45

Aleksander Fular

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2 Comments

nanofarad Over a year ago

That's true, but it's not the only issue here.

Aleksander Fular Over a year ago

well, I assumed that he "zeroes" his i just before while - he did not mention i initialization in his snippet. And if he did just that then it caused indexoutofbounds.

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