Both approaches have pros and cons, its often a matter of personal
preference:
The helper column approach can be easier to debug and understand, usually
calculates faster and tends to make better use of Excel's Smart Recalc
because usually only a few of the helper cells need to be recalculated,
whereas an array formula always has to calculate all of its virtual cells.
The single-cell array formula approach takes up less space in the workbook
(although the actual virtual memory savings are small), and can be
considered more "elegant". If the data volume is small enough then any extra
calculation time will not be significant and debugging using F9 on the
formula bar or the Evaluate Formula button works well. Sometimes the
compactness of an array formula can be easier to understand than a sprawl of
helper cells.
Multi-cell array formulae have some additional advantages in that you can
only alter the entire block of cells, so there is some additional protection
against unwanted accidental copy/paste/drag/overwrite.
There is also a third way: using UDFs. UDFs can often be faster than array
formulae when properly written.
Charles
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Outlines for my Sessions at the Australia Excel Users Group
http://www.decisionmodels.com/OZEUC.htm