Excel Formula Optimisation

Question

I am no excel expert and after some research have come up with this formula to look at two sets of the same data from different times. It then displays new entries that are

Answer 1

Array formula is slow. When you have thousands of array formula, it will make the speed very slow. Thus the key will be to avoid any array formula.

The following will be my way to achieve it, using only simple formula. It should be fast enough if you only have 2500 rows.

• Column F and H are "Keys", created by concatenating your 2 columns (E and F in your original formula)
• Assuming the first line of data is on row 3.

Data:

|   A   |      B      |    |  D |       E       |     F     |      |     H     |
| index | final value |    | ID | exist in Old? | Key (New) |      | Key (Old) |
--------------------------------------------------------------------------------
|   1   |    XXX-33   |    |  0 |      3        | OOD-06    |      | OOC-01    |
|   2   |    ZZZ-66   |    |  0 |      1        | OOC-01    |      | OOC-02    |
|   3   |    ZZZ-77   |    |  1 |     N/A       | XXX-33    |      | OOD-06    |
|   4   |             |    |  1 |      4        | OOE-01    |      | OOE-01    |
|   5   |             |    |  1 |      2        | OOC-02    |      | OOF-03    |
|   6   |             |    |  2 |     N/A       | ZZZ-66    |      |           |
|   7   |             |    |  3 |     N/A       | ZZZ-77    |      |           |

Column E "exist in Old?": test if the new key (Column F) exists in the old list (Column H)

=MATCH(F3, $H$3:$H$2500, 0)

Column D "ID": to increment by one whenever a new item is found

=IF(ISNA(E3), 1, 0)+IF(ISNUMBER(D2), D2, 0)  

the 2nd part of ISNUMBER is just for the first row, where just using D2 can cause an error

Column A "index": just a plain series starting from 1 (until the length of new list Column F)

Column B "final value": to find the new key by matching column A to Column D.

=IF(A3>MAX($D$3:$D$2500), "", INDEX($F$3:$F$2500, MATCH(A3, $D$3:$D$2500, 0))

This column B will be the list you want.

If it is still too slow, there exists some dirty tricks to speed up the calculation, e.g. by utilizing a sorted list with MATCH( , , 1) instead of MATCH( , , 0).