I have questions about the two Excel commands listed in the title of this thread as well as a general question about Excel 2008 for the Mac.

Suppose I have a list of 6000 items (numbers or text) in column C. I can determine how many times each item appears in the list by inserting the following command into the first 6000 cells of another column (say, column D).
=COUNTIF($C$1:$C$6000,$C$1:$C$6000&"")
Specifically, let's say that the above command is in D1 . . . and it is identical with the command in D2, which is identical with the one in D3, and so on. Yet, the number in D1 clearly refers to the value of C1, the number in D2 clearly refers to the value of C2, etc.

My questions about the above command are as follows:
  1. How does the command work?
  2. What does the &"" do?

I can also determine how many unique values are in the column-C list by using the following command:
=SUMPRODUCT((C1:C6000<>"")/COUNTIF(C1:C6000,C1:C6000&""))
Here are my questions about this command:
  1. How does it work?
  2. What does the forward slash (/) do?
  3. What does the &"" do?
I also have another unrelated question: Does anyone know why Excel 2008 is so slow?

Thank you very much!

Greg