Optimization of array function that calculates products - arrays

Question

I have the following array formula that calculates the returns on a particular stock in a particular year:
=IF(AND(NOT(E2=E3),H2=H3),PRODUCT(IF($E$2:E2=E1,$O$2:O2,""))-1,"")
But since I have 500,000 row entries as soon as I hit row 50,000 I get an error from Excel stating that my machine does not have enough resources to compute the values.
How shall I optimize the function so that it actually works?
E column refers to a counter to check the years and ticker values of stocks. If year is different from the previous value the function will output 1. It will also output 1 when the name of stock has changed. So for example you may have values for year 1993 and the next value is 1993 too but the name of stock is different, so clearly the return should be calculated anew, and I use 1 as an indication for that.
Then I have another column that runs a cumulative sum of those 1s. When a new 1 in that previous column is encountered I add 1 to the running total and keep printing same number until I observe a new one. This makes possible use of the array function, if the column that contains running total values (E column) has a next value that is different from previous I use my twist on SUMIF but with PRODUCT IF. This will return the product of all the corresponding running total E column values.

Answer 1

The source of the inefficiency, I believe, is in the steady increase with row number of the number of cells that must be examined in order to evaluate each successive array formula. In row 50,000, for example, your formula must examine cells in all the rows above it.
I'm a big fan of array formulas, so it pains me to say this, but I wouldn't do it this way. Instead, use additional columns to compute, in each row, the pieces of your formula that are needed to return the desired result. By taking that approach, you're exploiting Excel's very efficient recalculation engine to compute only what's needed.
As for the final product, compute that from a cumulative running product in an auxiliary column, and that resets to the value now in column O when column P in the row above contains a number. This approach is much more "local" and avoids formulas that depend on large numbers of cells.
I realize that text is not the best language for describing this, and my poor writing skills might be adding to the challenge, so please let me know if more detail is needed.
Interesting problem, thanks.

Answer 2

Could I suggest a really quick and [very] dirty vba? Something like the below. Obviously, have a backup of your file before running this. This assumes you want to start calculating from row 13.
Sub calculateP()
'start on row 13, column P:
Cells(13, 16).Select
'loop through every row as long as column A is populated:
Do
If ActiveCell(1, -14).Value = "" Then Exit Do 'column A not populated so exit loop
'enter formula:
Selection.FormulaR1C1 = _
"=IF(AND(NOT(RC[-11]=R[1]C[-11]),RC[-8]=R[1]C[-8]),PRODUCT(IF(R[-11]C5:RC[-11]=R[-1]C[-11],R2C15:RC[-1],""""))-1,"""")"
'convert cell value to value only (remove formula):
ActiveCell.Value = ActiveCell.Value
'select next row:
ActiveCell(2, 1).Select
Loop
End Sub
Sorry, this is definitely not a great answer for you... in fact, even this method could be achieved more elegantly using range... but, the quick and dirty approach may help you in the interim ??