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?

Solution 1

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

---

It is important to know that calling Last() on a collection will enumerate the entire collection. It is therefore not recommended to call Last() on the following types of collections:

• Streaming collections
• Computationally expensive collections
• Collections with high tendency for mutation

For such collections, it is better to get an enumerator for the collection (via the GetEnumerator() function) so you can keep track of the items yourself. Below is a sample implementation via an extension method that yields the index of the item, as well as whether the current item is the first or last item in the collection.

<Extension()>
Public Iterator Function EnumerateEx(Of T)(collection As IEnumerable(Of T))
    As IEnumerable(Of (value As T, index As Integer, isFirst As Boolean, isLast As Boolean))

    Using e = collection.GetEnumerator()
        Dim index = -1
        Dim toYield As T

        If e.MoveNext() Then
            index += 1
            toYield = e.Current
        End If

        While e.MoveNext()
            Yield (toYield, index, index = 0, False)
            index += 1
            toYield = e.Current
        End While

        Yield (toYield, index, index = 0, True)
    End Using
End Function

Here is a sample usage:

Sub Main()
    Console.WriteLine("Index   Value   IsFirst   IsLast")
    Console.WriteLine("-----   -----   -------   ------")

    Dim fibonacci = {0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89}

    For Each i In fibonacci.EnumerateEx()
        Console.WriteLine(String.Join("   ", $"{i.index,5}",
                                             $"{i.value,5}",
                                             $"{i.isFirst,-7}",
                                             $"{i.isLast,-6}"))
    Next

    Console.ReadLine()
End Sub

Output

Index   Value   IsFirst   IsLast
-----   -----   -------   ------
    0       0   True      False
    1       1   False     False
    2       1   False     False
    3       2   False     False
    4       3   False     False
    5       5   False     False
    6       8   False     False
    7      13   False     False
    8      21   False     False
    9      34   False     False
   10      55   False     False
   11      89   False     True

Solution 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.

Solution 3

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.

Solution 4

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;
}

Solution 5

here is a simple thing i dont know if it is politically correct but it works

for each item in listOfThings
   if(item = listOfThings.last)then 
       'you found the last loop
   end if
next 

View more solutions