Is there a way to access an iteration-counter in Java's for-each loop?

Question

Is there a way in Java's for-each loop

for(String s : stringArray) {
  doSomethingWith(s);
}

to find out how often the loop has already been processed?

Aside from using the old and well-known for(int i=0; i < boundary; i++) - loop, is the construct

int i = 0;
for(String s : stringArray) {
  doSomethingWith(s);
  i++;
}

the only way to have such a counter available in a for-each loop?

Answer 1

No, but you can provide your own counter.

The reason for this is that the for-each loop internally does not have a counter; it is based on the Iterable interface, i.e. it uses an Iterator to loop through the "collection" - which may not be a collection at all, and may in fact be something not at all based on indexes (such as a linked list).

Answer 2

There is another way.

Given that you write your own Index class and a static method that returns an Iterable over instances of this class you can

for (Index<String> each: With.index(stringArray)) {
    each.value;
    each.index;
    ...
}

Where the implementation of With.index is something like

class With {
    public static <T> Iterable<Index<T>> index(final T[] array) {
        return new Iterable<Index<T>>() {
            public Iterator<Index<T>> iterator() {
                return new Iterator<Index<T>>() {
                    index = 0;
                    public boolean hasNext() { return index < array.size }
                    public Index<T> next() { return new Index(array[index], index++); }
                    ...
                }
            }
        }
    }
}

Answer 3

The easiest solution is to just run your own counter thus:

int i = 0;
for (String s : stringArray) {
    doSomethingWith(s, i);
    i++;
}

The reason for this is because there's no actual guarantee that items in a collection (which that variant of for iterates over) even have an index, or even have a defined order (some collections may change the order when you add or remove elements).

See for example, the following code:

import java.util.*;

public class TestApp {
  public static void AddAndDump(AbstractSet<String> set, String str) {
    System.out.println("Adding [" + str + "]");
    set.add(str);
    int i = 0;
    for(String s : set) {
        System.out.println("   " + i + ": " + s);
        i++;
    }
  }

  public static void main(String[] args) {
    AbstractSet<String> coll = new HashSet<String>();
    AddAndDump(coll, "Hello");
    AddAndDump(coll, "My");
    AddAndDump(coll, "Name");
    AddAndDump(coll, "Is");
    AddAndDump(coll, "Pax");
  }
}

When you run that, you can see something like:

Adding [Hello]
   0: Hello
Adding [My]
   0: Hello
   1: My
Adding [Name]
   0: Hello
   1: My
   2: Name
Adding [Is]
   0: Hello
   1: Is
   2: My
   3: Name
Adding [Pax]
   0: Hello
   1: Pax
   2: Is
   3: My
   4: Name

indicating that, rightly so, order is not considered a salient feature of a set.

There are other ways to do it without a manual counter but it's a fair bit of work for dubious benefit.

Answer 4

Using lambdas and functional interfaces in Java 8 makes creating new loop abstractions possible. I can loop over a collection with the index and the collection size:

List<String> strings = Arrays.asList("one", "two","three","four");
forEach(strings, (x, i, n) -> System.out.println("" + (i+1) + "/"+n+": " + x));

Which outputs:

1/4: one
2/4: two
3/4: three
4/4: four

Which I implemented as:

   @FunctionalInterface
   public interface LoopWithIndexAndSizeConsumer<T> {
       void accept(T t, int i, int n);
   }
   public static <T> void forEach(Collection<T> collection,
                                  LoopWithIndexAndSizeConsumer<T> consumer) {
      int index = 0;
      for (T object : collection){
         consumer.accept(object, index++, collection.size());
      }
   }

The possibilities are endless. For example, I create an abstraction that uses a special function just for the first element:

forEachHeadTail(strings, 
                (head) -> System.out.print(head), 
                (tail) -> System.out.print(","+tail));

Which prints a comma separated list correctly:

one,two,three,four

Which I implemented as:

public static <T> void forEachHeadTail(Collection<T> collection, 
                                       Consumer<T> headFunc, 
                                       Consumer<T> tailFunc) {
   int index = 0;
   for (T object : collection){
      if (index++ == 0){
         headFunc.accept(object);
      }
      else{
         tailFunc.accept(object);
      }
   }
}

Libraries will begin to pop up to do these sorts of things, or you can roll your own.

Answer 5

Java 8 introduced the Iterable#forEach() / Map#forEach() method, which is more efficient for many Collection / Map implementations compared to the "classical" for-each loop. However, also in this case an index is not provided. The trick here is to use AtomicInteger outside the lambda expression. Note: variables used within the lambda expression must be effectively final, that is why we cannot use an ordinary int.

final AtomicInteger indexHolder = new AtomicInteger();
map.forEach((k, v) -> {
    final int index = indexHolder.getAndIncrement();
    // use the index
});

Answer 6

I'm afraid this isn't possible with foreach. But I can suggest you a simple old-styled for-loops:

    List<String> l = new ArrayList<String>();

    l.add("a");
    l.add("b");
    l.add("c");
    l.add("d");

    // the array
    String[] array = new String[l.size()];

    for(ListIterator<String> it =l.listIterator(); it.hasNext() ;)
    {
        array[it.nextIndex()] = it.next();
    }

Notice that, the List interface gives you access to it.nextIndex().

(edit)

To your changed example:

    for(ListIterator<String> it =l.listIterator(); it.hasNext() ;)
    {
        int i = it.nextIndex();
        doSomethingWith(it.next(), i);
    }

Answer 7

Idiomatic Solution:

final Set<Double> doubles; // boilerplate
final Iterator<Double> iterator = doubles.iterator();
for (int ordinal = 0; iterator.hasNext(); ordinal++)
{
    System.out.printf("%d:%f",ordinal,iterator.next());
    System.out.println();
}

this is actually the solution that Google suggests in the Guava discussion on why they did not provide a CountingIterator.

Answer 8

One of the changes Sun is considering for Java7 is to provide access to the inner Iterator in foreach loops. the syntax will be something like this (if this is accepted):

for (String str : list : it) {
  if (str.length() > 100) {
    it.remove();
  }
}

This is syntactic sugar, but apparently a lot of requests were made for this feature. But until it is approved, you'll have to count the iterations yourself, or use a regular for loop with an Iterator.

Answer 9

Though there are soo many other ways mentioned to achieve the same, I will share my way for some unsatisfied users. I am using the Java 8 IntStream feature.

1. Arrays

Object[] obj = {1,2,3,4,5,6,7};
IntStream.range(0, obj.length).forEach(index-> {
    System.out.println("index: " + index);
    System.out.println("value: " + obj[index]);
});

2. List

List<String> strings = new ArrayList<String>();
Collections.addAll(strings,"A","B","C","D");

IntStream.range(0, strings.size()).forEach(index-> {
    System.out.println("index: " + index);
    System.out.println("value: " + strings.get(index));
});

Answer 10

If you need a counter in an for-each loop you have to count yourself. There is no built in counter as far as I know.

Answer 11

For situations where I only need the index occasionally, like in a catch clause, I will sometimes use indexOf.

for(String s : stringArray) {
  try {
    doSomethingWith(s);
  } catch (Exception e) {
    LOGGER.warn("Had some kind of problem with string " +
      stringArray.indexOf(s) + ": " + s, e);
  }
}

Answer 12

There is a "variant" to pax' answer... ;-)

int i = -1;
for(String s : stringArray) {
    doSomethingWith(s, ++i);
}

Answer 13

The best and optimized solution is to do the following thing:

int i=0;

for(Type t: types) {
  ......
  i++;
}

Where Type can be any data type and types is the variable on which you are applying for a loop.

Answer 14

I'm a little surprised no-one suggested the following (I admit it's a lazy approach...); If stringArray is a List of some sort, you could use something like stringArray.indexOf(S) to return a value for the current count.

Note: this assumes that the elements of the List are unique, or that it doesn't matter if they are non-unique (because in that case it will return the index of the first copy found).

There are situations in which that would be sufficient...

Answer 15

Here is an example of how I did this. This gets the index at the for each loop. Hope this helps.

public class CheckForEachLoop {

    public static void main(String[] args) {

        String[] months = new String[] { "JANUARY", "FEBRUARY", "MARCH", "APRIL", "MAY", "JUNE", "JULY", "AUGUST",
                "SEPTEMBER", "OCTOBER", "NOVEMBER", "DECEMBER" };
        for (String s : months) {
            if (s == months[2]) { // location where you can change
              doSomethingWith(s); // however many times s and months
                                  // doSomethingWith(s) will be completed and 
                                  // added together instead of counter
            }

        }
        System.out.println(s); 


    }
}