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Hello,

It seems that Excel does not recognize a zero when the equation leading to zero involves numbers that are not integers and when the equation involves an empty cell (or a cell equal to zero). it is difficult to describe without an example.

I enclose a screen shot of simple calculations (row 2 shows the formulae entered in row 3). The total of each equation should be equal to zero, but it is not (see True, false).

Do you know if it is a well known documented problem in Excel? I cannot find any trace of this anywhere. I thought that considering your experience, you might know more about this.

i would really appreciate if you could let me know if there is some documentation about it somewhere.

thanks a lot.

Caroline

Caroline,

It is a well known problems that computers in general have problem with representing floating point numbers. Just like decimal numbers have problems representing some numbers exactly such as 1/3 and 1/7. Most applications represent these numbers using the IEEE standard as does excel. So those problems are inherent in non-integer numerical representation and storage in a computer and these problems evident in Excel. Over the various excel versions, microsoft has tried to overcome this problem by doing some internal rounding - but i tis not perfect. In you first example, where you get True, I suspect excel has intervened and corrected the result. In your other two renditions, I suspect the adding of the third term prevents excel from performing the corrective algorithm.

Generally when working with floating point (non-integer) numbers, it is best to do overt rounding or account for these imperfections when doing comparisons. For example, to check for zero you might do

=if(abs(A1)<.000000001,0,A1)

so you are defining any result smaller than that very small number to actually be zero.

Here is a microsoft support article that gives a good explanation

http://support.microsoft.com/default.aspx?scid=kb;en-us;78113

Floating-point arithmetic may give inaccurate results in Excel

a much larger article.

http://support.microsoft.com/default.aspx?scid=kb;en-us;42980

(Complete) Tutorial to Understand IEEE Floating-Point Errors

another

http://support.microsoft.com/default.aspx?scid=kb;en-us;48606

XL: Comparison of Values Does Not Return Correct Result

one on how to work within the problem

http://support.microsoft.com/kb/214118/

How to correct rounding errors in floating-point arithmetic

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Regards,

Tom Ogilvy

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