Letter: It doesn't add up

From Harold Thimbleby and Johan Wideberg, Middlesex University; University of Seville, Spain

Feedback (21 January) seems amused that Microsoft Window’s calculator cannot correctly work out 2.01−2 (it gets 0, not 0.01). I have three calculators that give three different answers for the sum 1 + 5%. Not all of them can be right. I have a calculator that cannot do some sums at all. Several recent calculators claim to use proper algebraic notation; I haven’t found one that actually works correctly. One last example: I have only found one calculator that claims to be able to and can calculate 4×−5 correctly as −1.

I suppose manufacturers would like to think nobody uses calculators for anything important. Their attitude is made pretty clear (I quote from Casio’s fx-P401 manual, p 7) “In no event will Casio and its suppliers be liable to you or any other person for any damages, including any incidental or consequential expenses, lost profits, lost savings or any other damages arising out of the use of this product.” Casio is a market leader, and the disclaimer is pretty typical.

I’ve collected several examples of manufacturers’ adverts showing their calculators doing things that are impossible to recreate when you take them home. It seems even the marketing departments don’t know what is going on.

Feedback wonders why, in comparison, there is all the fuss about small errors in the Pentium chip. I myself wonder why nobody makes a fuss about the inaccurate, difficult to use, and unreliable gadgets that fill our lives and on which we all depend – from passing exams to repaying loans – but which manufacturers refuse to put right.

Feedback is both right and wrong about the Microsoft Windows calculator. If you do 2.01−0.01, it renders 0.00 on the screen but if you invert the result it gives you 100, which is correct. Also, if you take the result 0.00 and subtract 0.01 you will get something like 2.23644783246E-16 which of course, is the error using double precision.

So the conclusion is that the calculator is correct but the interface to Windows is buggy. I hope that you will have some use for these utterly fascinating and important results.