Removing items from the list

Question

I have problem when i need to remove some items from list.

for (int i = 0; i < list.size() - 1; i++) {
    if (list.get(i).length() > 1 && list.get(i + 1).length() > 1) {
        list.remove(i+1);
        if(i != 0){
            i--;
        }
    }

I put 6 Strings which lenght are greater than 1 and I recieve in output 1st and 3rd. If I put at beggining string which legnth is 1 I recieve good output.

Answer 1

It will be simpler to write and easier to understand the loop counter's value if you iterate from the end:

for (int i = list.size() - 1; i > 0; i--) {
    if (list.get(i).length() > 1 && list.get(i - 1).length() > 1) {
        list.remove(i);
    }
}

Answer 2

Iterate from the end of the list to the beginning: since you're removing objects the indexes after the index you're removing are changing so if you're iterating from start to end you'll end up not iterating some of the items.

Example:

List has 3 items: [1, 2, 3], size = 3, i = 0, we're removing 1 (new list is: [2, 3]). Now size equals 2 and i is incremented: i = 1. now we've skipped 2!

Iterating in reverse can easily be implemented:

    for (int i = list.size() - 2; i >= 0; i--) {
        if (list.get(i).length() > 1 && list.get(i + 1).length() > 1) {
            list.remove(i + 1);
        }
    }

Update: If you know that you want to get rid of all the elements except one you can either build and new list with that one element and use it instead, or you can do something simpler such as:

list.removeIf(x -> x != 10000) // available from Java 8

Answer 3

Not entirely sure what you are trying to do here, but I think you want to remove elements such that there are no two string with length > 1 in a row. While the general advice to loop the list in reverse order is sound, there might be reasons to loop the list in the right order.

Your idea with using i-- after removing an element was correct. But you have to do it in all cases, not only if you are not on the first element. Remove the if, then it should work.

for (int i = 0; i < list.size() - 1; i++) {
    if (list.get(i).length() > 1 && list.get(i + 1).length() > 1) {
        list.remove(i+1);
        i--;
    }
}

If i == 0, this will set i to -1, which may seem odd, but remember that i++ will set it back to 0 before the next iteration of the loop. So for 11, 22, 33, it will first compare 11 and 22, then remove 22, and continue comparing 11 and 33 (and remove 33 and so forth).

Answer 4

The problem is you're going from 0 to list.size() - 2, but each time you remove an item from the list you're changing the index of the next items. So the next iteration will skip one item

The solution is to go from list.size() - 2 to 0, so that each time you remove an item the next iteration won't care. Your for would look something like this:

for (int i = list.size() - 2; i >= 0; i--) {
    if (list.get(i).length() > 1 && list.get(i + 1).length() > 1) {
        list.remove(i+1);
    }
}

Here you have the working result