What does ReDim Preserve do?

Question

I am looking at someone else's vba excel code. they are doing ReDim Preserve dataMatrix(7, i) in both loops. What does this do?

Also, it seems like the second loop just overwrites the data in the first loop, is that correct?

Dim dataMatrix() As String

    Worksheets.Item("ETS").Select
    Do While Trim(Cells(r, 1)) <> ""
       Debug.Print "The line: ", Trim(Cells(r, 1)), r
        r = r + 1
        dataMatrix(1, i) = Trim(Cells(r, 1))    ''file name
        dataMatrix(2, i) = Trim(Cells(r, 2))    ''sample type
        dataMatrix(3, i) = Trim(Cells(r, 3))    ''sample name
        dataMatrix(4, i) = "ETS"    ''
        dataMatrix(5, i) = Trim(Cells(r, 5))    ''Response
        dataMatrix(6, i) = Trim(Cells(r, 6))    ''ISTD Response
        dataMatrix(7, i) = Trim(Cells(r, 10))   ''Calculated Conc
        i = i + 1
        ReDim Preserve dataMatrix(7, i)
    Loop

    r = 5
    Worksheets.Item("ETG").Select
    Do While Trim(Cells(r, 1)) <> ""
       Debug.Print "The line: ", Trim(Cells(r, 1)), r
        r = r + 1
        dataMatrix(1, i) = Trim(Cells(r, 1))    ''file name
        dataMatrix(2, i) = Trim(Cells(r, 2))    ''sample type
        dataMatrix(3, i) = Trim(Cells(r, 3))    ''sample name
        dataMatrix(4, i) = "ETG"
        dataMatrix(5, i) = Trim(Cells(r, 5))    ''Response
        dataMatrix(6, i) = Trim(Cells(r, 6))    ''ISTD Response
        dataMatrix(7, i) = Trim(Cells(r, 10))   ''Calculated Conc
        i = i + 1
        ReDim Preserve dataMatrix(7, i)
    Loop

Answer 1

Redim Preserve allows you to change the dimensions of an array while keeping the contents of the array.

The Redim Preserve at the end of each loop is adding another row to the array.

I think the second loop is appending to the array because the i variable is not changed between the loops.

Answer 2

ReDim preservers a data in an array when you modify its size. Also it shouldn't overwrite the data becasuse i is incremented every iteration of the loop: the first loop won't stop until it encounters an empty cell, changing array length every iteration by one, so that when the lenght is 8 it adds 1 to the i and the new length becomes 9. When the second iteration occurs it writes the new element to the bound of the array with index 9, modifies its length again so that it becomes 1 element longer and iterates until encounters an empty line.