Bits.Bytes.

Microsoft Excel 2007 has an interesting, albeit embarrassing bug.

Type the following into a shiny new spreadsheet

Press Enter and you should see the following

Then, type the following

You’d think you’d get the same result. This is where you would be wrong

Let’s try with other factors

The result?

Ok, maybe there’s something special about 77.1, 850 and multiplication.

From what I remember from school, A * B = A / (1/B). Let’s see if Excel 2007 agrees

Type the following

Then type the following formula and fill down

What do you reckon the results will be? Without further ado:

The point of this post is not to gleefully point out the bug. I have my fair share of the same! And so do you! It is to merely speculate

1. What conditions in the algorithm the Excel guys used could lead to such a situation? Does it have anything to do with the number 65535, that just happens to be 2^16?
2. Since the bug manifests itself in the division, it conjures up some food for thought. In the division algorithm does it actually divide or does it reduce the expression directly to multiplication?
3. Where else will this beast rear its head? For instance, some formulas make use of multiplication
4. Are there any other magic numbers that cause this behaviour?
5. How do you test for this, short of writing tests for all permutations of numbers being added/divided/multiplied/subtracted?