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Tuesday, 8 March 2016

Excel Tip: Using Index to calculate a cumulative sum (e.g. a Year to Date total)

Let me start with an apology. My current workload has meant that my posts have been a lot less frequent in recent weeks. This may continue for a while, but I will still be aiming to post at least once a month during this busy spell.

In this post I want to show you how to do something I have shown before using OFFSET, but this time using the more efficient INDEX.

Let's look at the example of a 12 month budget spreadsheet with the monthly sales figures in cells B9 to M9 (month 1 in B9, Month 2 in C9, etc.). And let's say that the current month number is entered in cell B6.

Using OFFSET we could return the current month's sales using:

=OFFSET($B9,0,$B$6-1)

and the cumulative sales using:

=SUM(OFFSET($B9,0,0,1,$B$6))

See this earlier post, if you don't understand why.

The problem with OFFSET though, is that it is what is known as a "volatile" function. This means that it always has to be recalculated when any cell in the spreadsheet changes - as it does not specify a range that it is dependent on, therefore does not know whether a change might affect its result.

INDEX however looks at a defined range, making it significantly more efficient, and in its simplest form can be used to replace both of these examples of OFFSET.

For a range the width of a single cell (as in our example), INDEX only needs two arguments to return the current month's sales:

=INDEX($B9:$M9,$B$6)

This returns the value of the cell in position B6 in the range B9:M9.

If $B$6 is 3, this will return the value in cell D9, being the third cell in the range B9:M9.

To do the cumulative calculation, we can use the same INDEX function to return the end of  a SUM range, while fixing the start:

=SUM($B9:INDEX($B9:$M9,$B$6))

Again, if B6 contains 3, then this returns the sum of the range B9:D9.

In both cases, these will only recalculate if a cell in the range B9:M9 is edited. In a complex spreadsheet with many calculations, this can make a huge difference to calculation times.





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