Vb.net DataGridView repeating - vb.net

Question

I have a problem in my DataGridView, please refer to the image below.
Image 1 shows that I've clicked "Add to Cart" on one of my products, the DataGridView shows the product.
Image 1
The problem is when I want to add another product, the list of products in the DataGridView repeats itself instead of adding another different product.
Image 2 shows what happens when I clicked "Add to Cart" on a new product.
Image 2
Private Sub btn_add_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btn_add.Click
grd_cart.RowCount = grd_cart.RowCount + 1
For i As Integer = 0 To grd_cart.RowCount - 1
Dim product As String = grd_cart(0, i).Value
Dim price As String = grd_cart(1, i).Value
Dim quantity As String = grd_cart(2, i).Value
Dim subtotal As String = grd_cart(3, i).Value
grd_cart(0, i).Value = txt_product_id.Text
grd_cart(1, i).Value = txt_price.Text
grd_cart(2, i).Value = num_quantity.Value
grd_cart(3, i).Value = grd_cart(1, 0).Value * grd_cart(2, 0).Value
Next
End Sub

Answer 1

You don't need to cicle through the grid.
(Of course, this may depend on how you calculate the sub total.)
You just need to add a new Row to the grid, using the current values.
This can be a way to do it.
'Check whether the Price value is a number or not (to be safe)
'If can't be used as a number, return
Dim _Price As Long
If Long.TryParse(txt_price.Text, _Price) = False Then Return
'Calculate the subtotal value based on quantity and unit price
'I know not everyone calculates the sub total this way. In case, just add a comment.
Dim _subtotal As Long = CLng(num_quantity.Value) * _Price
'Add the new Row values
grd_cart.Rows.Add(New String() {txt_product_id.Text,
_Price.ToString,
num_quantity.Value.ToString,
_subtotal.ToString})
This is the result:
You might also present the values in Currency format:
'The output can also be formatted using the local Currency format
grd_cart.Rows.Add(New String() {txt_product_id.Text,
_Price.ToString("C"),
num_quantity.Value.ToString,
_subtotal.ToString("C")})
This is the result: