Java – Identifying last loop when using for each

c++foreachjavaloopsruby

I want to do something different with the last loop iteration when performing 'foreach' on an object. I'm using Ruby but the same goes for C#, Java etc.

  list = ['A','B','C']
  list.each{|i|
    puts "Looping: "+i # if not last loop iteration
    puts "Last one: "+i # if last loop iteration
  }

The output desired is equivalent to:

  Looping: 'A'
  Looping: 'B'
  Last one: 'C'

The obvious workaround is to migrate the code to a for loop using 'for i in 1..list.length', but the for each solution feels more graceful. What is the most graceful way to code a special case during a loop? Can it be done with foreach?

Best Solution

I see a lot of complex, hardly readable code here... why not keep it simple:

var count = list.Length;

foreach(var item in list)
    if (--count > 0)
        Console.WriteLine("Looping: " + item);
    else
        Console.Writeline("Lastone: " + item);

It's only one extra statement!

Another common situation is that you want to do something extra or less with the last item, like putting a separator between the items:

var count = list.Length;

foreach(var item in list)
{
    Console.Write(item);
    if (--count > 0)
        Console.Write(",");
}

Related Solutions

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.

Java – Iterating through a Collection, avoiding ConcurrentModificationException when removing objects in a loop

Iterator.remove() is safe, you can use it like this:

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

// This is a clever way to create the iterator and call iterator.hasNext() like
// you would do in a while-loop. It would be the same as doing:
//     Iterator<String> iterator = list.iterator();
//     while (iterator.hasNext()) {
for (Iterator<String> iterator = list.iterator(); iterator.hasNext();) {
    String string = iterator.next();
    if (string.isEmpty()) {
        // Remove the current element from the iterator and the list.
        iterator.remove();
    }
}

Note that Iterator.remove() is the only safe way to modify a collection during iteration; the behavior is unspecified if the underlying collection is modified in any other way while the iteration is in progress.

Source: docs.oracle > The Collection Interface

And similarly, if you have a ListIterator and want to add items, you can use ListIterator#add, for the same reason you can use Iterator#remove — it's designed to allow it.

In your case you tried to remove from a list, but the same restriction applies if trying to put into a Map while iterating its content.

Best Solution

Related Solutions

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

Java – Iterating through a Collection, avoiding ConcurrentModificationException when removing objects in a loop

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