Computer Capers: Tales of Electronic Thievery, Embezzlement, and Fraud

Author: Thomas Whiteside
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Comments

by danbruc   2017-08-19
That does not solve the problem in general, you still may end up with fractional quantities if you, for example, calculate a 1% fee. The correct way to do it is to use a decimal floating point format AND carefully pay attention to rounding.

Also note the the way you round usually depends on the context, for some calculations you want to round up, for some down, and for some in yet another way. So you perform explicit rounding with an explicit rounding mode everywhere it is required and don't assume that implicit rounding or the default rounding mode will do the right thing because they won't in general.

EDIT: To expand a bit on the context. If you, for example, charge a percentage fee you can choose between rounding up getting at least the fee you are asking for or rounding down not charging the customer more than you are asking for. Or you can try to be less biased and round either up or down depending on the fractional part. This may still require tie-breaking, either in a direction of your choice, or randomized, or by some rule like round half to even. Different choices will obviously have different biases.

While this may almost seem somewhat pedantic, such things have been exploited, see for example Ā»Computer Capers: Tales of Electronic Thievery, Embezzlement & FraudĀ«. [1][2] Also note that there may be even legal regulations prescribing how you can and can not round, for example between the Euro and the replaced national currencies. [3] I also have some faint memories of a case against someone systematically exploiting rounding errors but I can not come up with a reference and may even misremember this.

[1] https://www.amazon.com/Computer-Capers-Electronic-Thievery-E...

[3] http://www.sysmod.com/eurofaq.htm#ROUNDING