CODEWITHSUNDEEP
vb.netarrays
Zosimas
Zosimas

Reputation: 508

Combine parts of an array in VB.NET

Is there a fast way to get all the items of an array that have even indexes and put them into a new array in VB.NET?

Upvotes: 1

Views: 994

Answers (2)

dbasnett
dbasnett

Reputation: 11773

Whenever I see questions like this, and answers that use LINQ I always want to know how big, and how often. How big is the array and how often the code will be used. If this is something that will be used often LINQ isn't the answer.

Here is a test with a comparison of LINQ and an alternative

Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
    Debug.WriteLine("")
    prng.NextBytes(oldArray)
    useLinq()
    useOther()
End Sub

Dim prng As New Random
Dim oldArray(131072) As Byte
Dim stpw As New Stopwatch

Private Sub useLinq()
    stpw.Restart()
    Dim newArray() As Byte = Enumerable.Range(0, oldArray.Length).Where(Function(i) i Mod 2 = 0).Select(Function(i) oldArray(i)).ToArray()
    stpw.Stop()
    Debug.WriteLine("{0} {1:n0} {2:n0} {3} {4} {5}", "L", stpw.ElapsedMilliseconds, newArray.Length, newArray(0), newArray(1), newArray(newArray.Length - 1))
End Sub

Private Sub useOther()
    stpw.Restart()
    Dim foo As New List(Of Byte)
    For x As Integer = 0 To oldArray.Length - 1 Step 2
        foo.Add(oldArray(x))
    Next
    Dim newArray() As Byte = foo.ToArray
    stpw.Stop()
    Debug.WriteLine("{0} {1:n0} {2:n0} {3} {4} {5}", "O", stpw.ElapsedMilliseconds, newArray.Length, newArray(0), newArray(1), newArray(newArray.Length - 1))
End Sub

Upvotes: 2

Bala R
Bala R

Reputation: 109017

The easiest thing to do is to do it with a for-loop but you could do something like this with linq.

Dim newArray = Enumerable.Range(0, oldArray.Length) _
                         .Where(Function(i) i Mod 2 = 0) _
                         .Select(Function(i) oldArray(i)) _
                         .ToArray()

EDIT: here's an example with byte array

Dim oldArray As Byte() = {1, 2, 3, 4, 5, 6}
Dim newArray As Byte() = Enumerable.Range(0, oldArray.Length).Where(Function(i) i Mod 2 = 0).Select(Function(i) oldArray(i)).ToArray()

Upvotes: 0

Related Questions