Summary
The challenge - calculate the difference from the current and last entry, even when previous entries are skipped. This is one of those problems that seems simple at first glance, but is tricky when you look closely.
The context
A couple weeks ago, I had an interesting question from a reader about tracking weight gain or loss in a simple table.
The idea is to enter a new weight each day, and calculate the difference from the previous day. When every day has an entry, the formula is straightforward:
The difference is calculated with a formula like this, entered in D6, and copied down the table:
=IF(C6<>"",C6-C5,"")
However, when one or more days are missed, things go awry, and the calculated result doesn't make sense:
No, you did not gain 157 pounds in one day
The problem is the formula uses the blank cell in the calculation, which evaluates to zero. What we need is a way to locate and use the last weight recorded in column C.
The challenge
What formula will calculate a difference from the last entry, even when days have been skipped?
Desired result - difference using last previous entry
Assumptions
- A single formula is entered in D6 and copied down (i.e. same formula in all cells)
- The formula must handle one or many previous blank entries
- Removing blank entries (rows) is not allowed
- No helper columns allowed
Note: one obvious path is to use a Nested IF formula. I would discourage this, since it won't scale well to handle an unknown number of consecutive blank entries.
Got a solution? Leave a comment with your proposed formula below.
I hacked together a formula myself, and I'll share my solution after I give the smart readers of Exceljet some time to submit their own formulas.
Extra credit
Looking for more of a challenge? Here's the same result, with a custom number format applied. What's the number format? I swiped this from Mike Alexander on his now-defunct Bacon Bits blog.
