Identifying last loop when using for each

Question

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.

Answer 1

Ruby also has each_index method:

list = ['A','B','C']
list.each_index{|i|
  if i < list.size - 1
    puts "Looping:"+list[i]
  else
    puts "Last one:"+list[i]
}

EDIT:

Or using each (corrected TomatoGG and Kirschstein solution):

list = ['A', 'B', 'C', 'A']
list.each { |i|
   if (i.object_id != list.last.object_id)
      puts "Looping:#{i}"
   else
      puts "Last one:#{i}"
   end
}

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

Or

list = ['A', 'B', 'C', 'A']
list.each {|i| 
  i.object_id != list.last.object_id ? puts "Looping:#{i}" : puts "Last one:#{i}"       
}

Answer 2

The foreach construct (in Java definitely, probably also in other languages) is intended to represent the most general kind if iteration, which includes iteration over collections that have no meaningful iteration order. For example, a hash-based set does not have an ordering, and therefore there is no "last element". The last iteration may yield a different element each time you iterate.

Basically: no, the foreach construct is not meant to be used that way.

Answer 3

C# 3.0 or newer

Firstly, I would write an extension method:

public static void ForEachEx<T>(this IEnumerable<T> s, Action<T, bool> act)
{
    IEnumerator<T> curr = s.GetEnumerator();

    if (curr.MoveNext())
    {
        bool last;

        while (true)
        {
            T item = curr.Current;
            last = !curr.MoveNext();

            act(item, last);

            if (last)
                break;
        }
    }
}

Then using the new foreach is very simple:

int[] lData = new int[] { 1, 2, 3, 5, -1};

void Run()
{
    lData.ForEachEx((el, last) =>
    {
        if (last)
            Console.Write("last one: ");
        Console.WriteLine(el);
    });
}

Answer 4

Similar to kgiannakakis's answer:

list.first(list.size - 1).each { |i| puts "Looping: " + i }
puts "Last one: " + list.last

Answer 5

In Ruby I'd use each_with_index in this situation

list = ['A','B','C']
last = list.length-1
list.each_with_index{|i,index|
  if index == last 
    puts "Last one: "+i 
  else 
    puts "Looping: "+i # if not last loop iteration
  end
}

Answer 6

I notice a number of suggestions assume that you can find the last item in the list before beginning the loop, and then compare every item to this item. If you can do this efficiently, then the underlying data structure is likely a simple array. If that's the case, why bother with the foreach at all? Just write:

for (int x=0;x<list.size()-1;++x)
{
  System.out.println("Looping: "+list.get(x));
}
System.out.println("Last one: "+list.get(list.size()-1));

If you cannot retrieve an item from an arbitrary position efficiently -- like it the underlying structure is a linked list -- then getting the last item probably involved a sequential search of the entire list. Depending on the size of the list, that may be a performance issue. If this is a frequently-executed function, you might want to consider using an array or ArrayList or comparable structure so you can do it this way.

Sounds to me like you're asking, "What's the best way to put a screw in using a hammer?", when of course the better question to ask is, "What's the correct tool to use to put in a screw?"