Java – way to access an iteration-counter in Java’s for-each loop

for-loopforeachjavaloops

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?

Best Answer

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).

Related Solutions

C# – How to get the index of the current iteration of a foreach loop

Ian Mercer posted a similar solution as this on Phil Haack's blog:

foreach (var item in Model.Select((value, i) => new { i, value }))
{
    var value = item.value;
    var index = item.i;
}

This gets you the item (item.value) and its index (item.i) by using this overload of LINQ's Select:

the second parameter of the function [inside Select] represents the index of the source element.

The new { i, value } is creating a new anonymous object.

Heap allocations can be avoided by using ValueTuple if you're using C# 7.0 or later:

foreach (var item in Model.Select((value, i) => ( value, i )))
{
    var value = item.value;
    var index = item.i;
}

You can also eliminate the item. by using automatic destructuring:

<ol>
foreach ((MyType value, Int32 i) in Model.Select((value, i) => ( value, i )))
{
    <li id="item_@i">@value</li>
}
</ol>

Java – How does the Java ‘for each’ loop work

for (Iterator<String> i = someIterable.iterator(); i.hasNext();) {
    String item = i.next();
    System.out.println(item);
}

Note that if you need to use i.remove(); in your loop, or access the actual iterator in some way, you cannot use the for ( : ) idiom, since the actual iterator is merely inferred.

As was noted by Denis Bueno, this code works for any object that implements the Iterable interface.

Also, if the right-hand side of the for (:) idiom is an array rather than an Iterable object, the internal code uses an int index counter and checks against array.length instead. See the Java Language Specification.

Best Answer

Related Solutions

C# – How to get the index of the current iteration of a foreach loop

Java – How does the Java ‘for each’ loop work

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