C# - Populate The Values Of Multiple Rows In Single Row In Gridview?

I am trying to populate the values of multiple rows in single row in gridview using c sharp in rowdatabound section of code behind page like:

Col1 Col6

a 1
a 2
a 3
a 4

I have changed this to

Col1 Col6

a 1
2
3
4

Now I want to achieve this?

Col1 Col6

a 1 2 3 4

What wrong with the code below?

if (e.Row.Cells[0].Text == "" && e.Row.Cells[5].Text != "")
{
for (int a = 0; a<GridView1.Rows.Count; a++)
{
string s = GridView1.Rows[a].Cells[5].Text;
GridView1.Rows[e.Row.RowIndex - 1].Cells[5].Text += s;
}
}

What mistake I am making?

Cell[0] is Col1
Cell[5] is Col6