Question

I’ve got a very large array of data that I’m writing to a range. However, sometimes only a few elements of the array change. I believe that since I am writing the entire array to the range, all of the cells are being re-calculated. Is there any way to efficiently write a subset of the elements – specifically, those that have changed?

Update: I’m essentially following this method to save on write time:

http://www.dailydoseofexcel.com/archives/2006/12/04/writing-to-a-range-using-vba/

In particular, I have a property collection that I populate with all of the objects (they are cells) with data that I need. Then, I loop through all of the properties and write the values to an array, indexing the array so it matches the dimensions of the range that I want to write to. Finally, with TheRange.Value = TempArray I write the data in the array to she sheet. This last step overwrites the full range, I believe causing recalculations even in cells whose actual values didn’t change.

Answer 1

Let me start with a few basics:

• When you write to a range of cells, even if the values are the same, Excel still sees it as a change and will recalculate accordingly. It does not matter if you have calculation turned off, then next time the range/sheet/workbook is calculated it will recalculate everything that is dependent on that range.
• As you’ve discovered, writing an array to a range is much, much faster than writing cell-by-cell. It is also true that reading a range into an array is much faster than reading cell-by-cell.

As to your question of only writing the subset of data that has changed, you need a fast way to identify which data has changed. This is probably obvious, but needs to be taken into account as whatever that method is will also take some time.

To write only the changed data, you can do this two ways: either go back to writing cell-by-cell or break the array into smaller chunks. The only way to know if either of these is faster than writing the whole range is to try all three methods and time them with your data. If 90% of the data is changed, writing the entire block will certainly be faster than writing cell-by-cell. On the other hand, if the changed data only represents 5%, cell-by-cell may be better. The performance is dependent on too many variables to give a one-answer-fits-all solution.