CODEWITHSUNDEEP
java
qwertz
qwertz

Reputation: 325

Is it possible to change the counter variable inside a for loop?

I am trying to sort a LinkedList in Java. I need to go through mylist from back to front. The elements in the list are objects from my class CustomElement. If they match a certain pattern I want to put them up front.

My problem is that if I detect that the element in my list with index 5 for example matches my pattern and I move it to index 0, the previous element with index 4 has index 5 now, right? That is why I want the for loop to check the element with index 5 again: i++. But that's causing an infinite loop, whreas the method's working fine without i++, but not the way that I want it, because it's skipping the element with index 4 (now 5).

Is it gernerally possible to raise the variable i inside the for loop? And if yes, what am I doing wrong.

    for (int i = mylist.size() - 1; i >= 0; i--) {

        if (mylist.get(i) matches a certain pattern) {


            CustomElement helper = mylist.get(i);                
            mylist.remove(i);
            mylist.add(0, helper);
            i++;
        }
    }

Upvotes: 1

Views: 1031

Answers (2)

user3392484
user3392484

Reputation: 1919

It is, but in your case using get(i) for a linked list will give quadratic performance.

If you don't mind your "matching" items being reversed in order then you'd be better creating a new list:

final LinkedList<CustomElement> newList = new LinkedList<> ();

for (final CustomElement e: myList)
{
  if (e matches your pattern) { newList.addFirst (e); }
  else { newList.addLast (e); }
}

myList = newList;

Then all problems with index variables disappear...

(You could also achieve linear performance whilst modifying the existing list, but it's a little more complicated.)

Upvotes: 1

Sam I am says Reinstate Monica
Sam I am says Reinstate Monica

Reputation: 31194

Yes, it is possible to modify i inside your for loop, if it weren't possible, you wouldn't be getting this infinite loop.

What must be happening, is that if (mylist.get(i) matches a certain pattern) continues to be true after a certain point, and you never get to a point where i >= 0 is not true.

So, if myList.get(0) matches your pattern, you'll just put it back at index 0, and keep checking it forever.

Upvotes: 4

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