Is this a bug in the Log functions?

Hi all

My first post here, as I came across this site while trying to understand what may be a bug in Excel 2010 with Logarithmic functions.

The attached excel file shows the behaviour. In short, log calculations for the value of 1 do not give 0 as a result when the value has been calculated by repeated additions from previous cells. The number of cells added up before the beahviour starts is dependent on the amount being added.

For instance, getting to 1 by adding 0.1 repeatedly from 0.7 will give the erroneous result, but not by adding from 0.8 (more than 2 additions and it goes wrong). If you're adding 0.05, then you need to do more than 3 additions to get the erroneous behaviour.

I suppose it's something to do with floating arithmetic and the way Excel handles real values, but haven't got the knowledge of the inners of Excel to know if this is a true "bug" or just "expected behaviour".

Hope I've given enough information for you all to understand the issue and reply.

Afzal