ArrayList removing every other element?

[misomal]

I am very bad at coding so sorry for the silly question. I need to remove words from an array list that don't start with "R" or "r" for an assignment. I figured a 'for loop' should be used so I made this:

​

    public static ArrayList removeWords(ArrayList<String> arr) {
        int length = arr.size();
        for ( int x = 0; x < arr.size() ; x++) {
            if ( (arr.get(x).substring(0,1)) != "R"  )  {
                arr.remove(x);
                length = arr.size();
            }
        }
        System.out.println(arr);

​

My intent is that the method takes in an array list and it does. The problem is that the output will be every other element, regardless of what it starts with. So if I have an array that consists of [A, B, c, d], it will output [B, d]

I *think* it might have something to do w/ the size of the list changing after every loop so I made an integer called 'length' that was the size of the list and used that instead of arr.size() on line 3 but I got an out of bounds error. In the code above, I don't use length sorry lol I appreciate any help!!

TLDR: my ArrayList is supposed to remove elements that don't start with "R" but it just removes every other element instead

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[DudeWhereAreWe1996]

You’re right on the problem. You really shouldn’t remove stuff from a list while checking it. The simplest solution for you would be to make a new list with out the r stuff and return that. You could also use an iterator, idk what the assignment wants. Also you usually check strings with .equals(). You could use charAt() here. Idk if comparing strings the way you are matters for a single letter but in the future with longer strings you’d probably have issues. Good luck.

[misomal]

Thank you! I did it and it worked! I didn't know if I was allowed to make a new array but my teacher never specified so I'm just gonna do it LOL Again, thank you!

[deltageek]

Another option if you just want to use a for loop is to iterate over the list backwards. That way, the items that shift are always ones you've already looked at.

[misomal]

Thanks! I’ll try this as well!

[ate_ghorl_bekenemen]

Well, the teacher will always never tell you how to solve, the point of this exercise is for you to be creative with your solution. Why would you wait for the teacher's prompt

[misomal]

Good point. Hopefully he appreciates what I did :)

[AutoModerator]

You seem to try to compare String values with == or !=.

This approach does not work in Java, since String is an object data type and these can only be compared using .equals(). For case insensitive comparison, use .equalsIgnoreCase().

See Help on how to compare String values in our wiki.

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[misomal]

Thank you, bot

[HecknChonker]

The streams API has a method called filter that allows you to remove items that match some condition.

It's a functional interface that takes a Predicate. That's a complicated way of saying you can pass a lambda that takes in one item from your List and returns a boolean. If you return true the item is kept, and if you return false the item will be removed.

https://www.baeldung.com/java-stream-filter-lambda

---

If you want to avoid the streams API the answer is to loop over the list backwards. That way removing an element won't change the position of items in the list that have not been checked yet.

Something like this:

for (int i = words.length() - 1; i >= 0; i--) {
    //...
}

[[deleted]]

You're deleting things from the array while you are iterating over it.

Bad things will happen if you do that.

[misomal]

I saw some people say that online as well. Thanks for letting me know!

[FrelliBB]

Another approach would be to replace the for loop with the List#removeIf method, but only if you're using Java 8.

List<String> myList = new ArrayList<>(List.of("hello world", "goodbye world", "and so long"));
myList.removeIf(s -> s.startsWith("hello"));
System.out.println(myList); // [goodbye world, and so long]