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

User · Answer

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 lt String gt  list1   new ArrayList lt String gt  Arrays asList str1         ListIterator lt String gt  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

User · Answer

Use your own variable and increment it in the loop

User · Answer

Here s a way to do it using your own variable and keeping it concise   List lt String gt  list   Arrays asList  zero    one    two     int i   0  for  Iterator lt String gt  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

User · Answer

Use an int and increment it within your loop

User · Answer

See here   iterator nextIndex   would provide index of element that would be returned by subsequent call to next

User · Answer

just do something like this           ListIterator lt String gt  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

User · Answer

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

User · Answer

This would be the simplest solution   std  vector lt double gt  v  5    for auto itr   v begin   itr    v end     itr     auto current loop index   itr - v begin       std  cout  lt  lt  current loop index  lt  lt  std  endl       Tested on gcc-9 with -std c  11 flag  Output      0   1   2   3   4

User · Answer

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

User · Answer

I had the same question and found using a ListIterator worked  Similar to the test above  List lt String gt  list   Arrays asList  quot zero quot    quot one quot    quot two quot     ListIterator iter   list listIterator         while  iter hasNext          System out println  quot index   quot    iter nextIndex      quot  value   quot    iter next        Make sure you call the nextIndex   before you actually get the next

User · Answer

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

User · Answer

You can use ListIterator to do the counting   final List lt String gt  list   Arrays asList  zero    one    two    three     for  final ListIterator lt String gt  it   list listIterator    it hasNext           final String s   it next        System out println it previousIndex            s

[java] How to get the current loop index when using Iterator?

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