Nth element

In VB.NET we often use Lists of elements. Sometimes it can be useful to generate a List of every Nth element from our Lists.

With some modulo logic, we can test for every second element, third element or other distance. The Mod keyword, which is modulo division in VB.NET can be used here.

Example

We introduce a function EveryNthElement, which receives a List of strings, and returns another List of strings. The result is a collection containing the specified elements.

Step 1 We loop over all the indexes in the input List. It is important that we can access the indexes, as we test them.

Step 2 Here we use an if-statement with the Mod keyword to determine if the element index is evenly divisible by our "n" value.

Step 3 We return the list, which now contains all the elements at indexes that were evenly divisible by the "n" value.

Module Module1
    Function EveryNthElement(list As List(Of String), n As Integer) As List(Of String)
        Dim result = New List(Of String)
        ' Step 1: loop over all indexes.
        For i As Integer = 0 To list.Count - 1
            ' Step 2: use a modulo expression, and add values if the result is 0.
            If i Mod n = 0
                result.Add(list(i))
            End If
        Next
        ' Step 3: return the list.
        Return result
    End Function

    Sub Main()
        ' Use the function on this list.
        Dim test = New List(Of String)({ "a", "b", "c", "d", "e", "f", "g", "h", "i" })
        
        Dim result = EveryNthElement(test, 2)
        Console.WriteLine(string.Join(",", result))
        
        Dim result2 = EveryNthElement(test, 3)
        Console.WriteLine(string.Join(",", result2))
    End Sub
End Modulea,c,e,g,i
a,d,g

Results

It is easy to determine that the function has correct output on the values 2 and 3. With 2, we skip over every other element. And with 3, we skip over 2 elements at a time.

Using modulo division (with the Mod keyword in VB.NET) allows us to test indexes with a distance between them. This allows us to sample all values separated with a specified distance.