Case Statements

I know the concept of Case statements sounds simple, but trying to write it in my scenario seems a lot more difficult than I thought it would be. I have three columns with an unspecified number of rows (which are generated from another macro). Lets say those colums are C, D, E, and they should be strings because my criteria are based on the number of digits contained in each cell. I want to add a certain number of periods to a given cell, then add that string onto two other cell strings, and form one combined string in a separate cell.

Basically I want to sort of Concatenate the cells into a given cell in column F for each row, but there are many conditions. If the cells in column C contain 3 digits, I want to add one period "." onto the original string. If it has 4 digits, then I add nothing, and then, add that cell to the string in cell D, and then Cell E. [It is like the function Concatenate(C1&".",D1&"..",E1)]. Cells in Column D have 6 criteria, and cells in Column E have zero criteria.

And I need this to loop down until it reaches an empty row or cells.