Questions regarding what appear to be automatic formula replication.

[Billco]

This title may be a bit confusing but I couldn't figure any other way to state my question.

I have a spreadsheet that keeps track of my investments. I simply list the number of shares of a particular stock in cell 1A, the value of that stock at the end of each week in cell 1B, and a formula (1A*1B) goes in cell 1C. Each row then, represents the status or value of a stock at the end of any given week (row one = week one, row two = week two, etc.)

Strangely, even though I do not copy the formula all the way down column C (because it created a #VALUE statement in those future week cells), excel sometimes (but not always) magically knows the formula and applies it as soon as I enter the number of shares and the stock value for a given week. I have no idea how this works.
Does anyone know why this happens, and more importantly how I can get excel to do this for all my entries? It's pretty cool when it happens, but it doesn't always happen.

Bill

[etaf]

use an if statement

IF( A1="", "" A1*B1)
and copy down

now if no entry in the column A - in C will be blank
when you enter a value into A it will then do the calc

[Billco]

Had a problem at first but, if I put a comma behind the second set of quotation marks, it works perfectly. Thanks for your help

[etaf]

excellent , sorry about that - missed the syntax