How to get the current loop index when using Iterator?

Question

I am using an Iterator to iterate through a collection and I want to get the current element's index.

How can I do that?

Answer 1

I had the same question and found using a ListIterator worked. Similar to the test above:

List<String> list = Arrays.asList("zero", "one", "two");

ListIterator<String> iter = list.listIterator();
    
while (iter.hasNext()) {
    System.out.println("index: " + iter.nextIndex() + " value: " + iter.next());
}

Make sure you call the nextIndex() before you actually get the next().

Answer 2

Use your own variable and increment it in the loop.

Answer 3

Here's a way to do it using your own variable and keeping it concise:

List<String> list = Arrays.asList("zero", "one", "two");

int i = 0;
for (Iterator<String> it = list.iterator(); it.hasNext(); i++) {
    String s = it.next();
    System.out.println(i + ": " + s);
}

Output (you guessed it):

0: zero
1: one
2: two

The advantage is that you don't increment your index within the loop (although you need to be careful to only call Iterator#next once per loop - just do it at the top).

Answer 4

You can use ListIterator to do the counting:

final List<String> list = Arrays.asList("zero", "one", "two", "three");

for (final ListIterator<String> it = list.listIterator(); it.hasNext();) {
    final String s = it.next();
    System.out.println(it.previousIndex() + ": " + s);
}

Answer 5

What kind of collection? If it's an implementation of the List interface then you could just use it.nextIndex() - 1.

Answer 6

Use an int and increment it within your loop.

Answer 7

just do something like this:

        ListIterator<String> it = list1.listIterator();
        int index = -1;
        while (it.hasNext()) {
            index++;
            String value = it.next();
            //At this point the index can be checked for the current element.

        }

Answer 8

Use a ListIterator to iterate through the Collection. If the Collection is not a List to start with use Arrays.asList(Collection.toArray()) to turn it into a List first.

Answer 9

All you need to use it the iterator.nextIndex() to return the current index that the iterator is on. This could be a bit easier than using your own counter variable (which still works also).

public static void main(String[] args) {    
    String[] str1 = {"list item 1", "list item 2", "list item 3", "list item 4"};
    List<String> list1 = new ArrayList<String>(Arrays.asList(str1));

    ListIterator<String> it = list1.listIterator();

    int x = 0;

    //The iterator.nextIndex() will return the index for you.
    while(it.hasNext()){
        int i = it.nextIndex();
        System.out.println(it.next() + " is at index" + i); 
    }
}

This code will go through the list1 list one item at a time and print the item's text, then "is at index" then it will print the index that the iterator found it at. :)

Answer 10

See here.

iterator.nextIndex() would provide index of element that would be returned by subsequent call to next().

Answer 11

Though you already had the answer, thought to add some info.

As you mentioned Collections explicitly, you can't use listIterator to get the index for all types of collections.

List interfaces - ArrayList, LinkedList, Vector and Stack.

Has both iterator() and listIterator()

Set interfaces - HashSet, LinkedHashSet, TreeSet and EnumSet.

Has only iterator()

Map interfaces - HashMap, LinkedHashMap, TreeMap and IdentityHashMap

Has no iterators, but can be iterated using through the keySet() / values() or entrySet() as keySet() and entrySet() returns Set and values() returns Collection.

So its better to use iterators() with continuous increment of a value to get the current index for any collection type.

Answer 12

This would be the simplest solution!

std::vector<double> v (5);

for(auto itr = v.begin();itr != v.end();++itr){

 auto current_loop_index = itr - v.begin();

  std::cout << current_loop_index << std::endl;

}

Tested on gcc-9 with -std=c++11 flag

Output:

0
1
2
3
4