How do you find the last loop in a For Each (VB.NET)?

Question

How can I determine if I'm in the final loop of a For Each statement in VB.NET?

Answer 1

Check if the element is the last element of the container.

Why do you want to do that?
You could just place instructions after the loop. (That you execute on the last element)

Answer 2

It probably would be easier to just use a For loop instead of ForEach. But, similarly, you could keep a counter inside your ForEach loop and see if its equal to yourCollection.Count - 1, then you are in the last iteration.

Answer 3

If you are tied to IEnumerable. Do you have to be inside the foreach loop? If not you could declare a variable just before the foreach loop. Set it during the loop. Then, use it after the loop (if its not null)

Answer 4

With a foreach, you cannot know this until it is too late (ie. you're out of the loop).

Note, I'm assuming you're using something where you only have an IEnumerable interface. If you have a list, array, etc. then follow the other answers here which uses .Count or similar to find out how many items there are, and thus you can keep track of where you are in the collection.

However, with just IEnumerable/IEnumerator, there is no way to know for sure wether or not there are more, if you use foreach.

If you need to know this, use IEnumerable yourself, which is what foreach does.

The below solution is for C# but should translate easily to VB.NET:

List<Int32> nums = new List<Int32>();
nums.Add(1);
nums.Add(2);
nums.Add(3);

IEnumerator<Int32> enumerator = nums.GetEnumerator();
if (enumerator.MoveNext())
{
    // at least one value available
    while (true)
    {
        // make a copy of it
        Int32 current = enumerator.Current;

        // determine if it was the last value
        // if not, enumerator.Current is now the next value
        if (enumerator.MoveNext())
        {
            Console.Out.WriteLine("not last: " + current);
        }
        else
        {
            Console.Out.WriteLine("last: " + current);
            break;
        }
    }
}
enumerator.Dispose();

This will print:

not last: 1
not last: 2
last: 3

The trick is to take a copy of the current value, then ask the enumerator to attempt to move on to the next one. If that fails, the copy you made was indeed the last value, otherwise there is more.

Answer 5

Short answer: You can't

Long answer: There's nothing in the semantics of a For Each statement that allows you to identify whether you're running the first, last or any particular iteration.

For Each is built in the IEnumerable and IEnumerable<> interfaces, and the behavior is dependent on the implementation you're calling. It's valid, though confusing, for the collection you're iterating to return elements in a different order every time. Fortunately, List<> doesn't do this.

If you really need to know that a particular iteration is the last, you could identify the last element (in advance) and then take different action when you encounter that element.

Easier would be to detect the first iteration (say, through a boolean) and then do something different.

An example (C#, but the VB will be similar):

StringBuilder result = new StringBuilder();
bool firstTime = true;
foreach(string s in names)
{
    if (!firstTime)
    {
        result.Append(", ");
    }

    result.Append(s);
    firstTime = false;
}

Answer 6

It would be easier to use a For loop instead of a ForEach, however you could do something like

If item.Equals(itemCollection(itemCollection.Count)) Then
    ...
End If

inside of your ForEach loop... Assuming the object has properly overridden the Equals method.

This is probably much more resource intensive than just using a For loop or keeping track of the current index in a separate variable.

I'm not sure if this is the correct syntax, it's been a long time since I've used VB.

Answer 7

Using a standard "For Each" loop you can't. The intent of a "for Each" loop is to allow you to concentrate on the data in lieu of the underlying collection.

Answer 8

The generally, collections on which you can perform For Each on implement the IEnumerator interface. This interface has only two methods, MoveNext and Reset and one property, Current.

Basically, when you use a For Each on a collection, it calls the MoveNext function and reads the value returned. If the value returned is True, it means there is a valid element in the collection and element is returned via the Current property. If there are no more elements in the collection, the MoveNext function returns False and the iteration is exited.

From the above explanation, it is clear that the For Each does not track the current position in the collection and so the answer to your question is a short No.

If, however, you still desire to know if you're on the last element in your collection, you can try the following code. It checks (using LINQ) if the current item is the last item.

For Each item in Collection
    If item Is Collection.Last Then
        'do something with your last item'
    End If
Next