Chapter 10

Rounding and Exceptions

Rounding is one of the most important but confusing aspects of floating-point. We learned this early in school with problems asking for the nearest whole number when we divided 9 by 4. The pencil and paper result is 2.25, but what whole number do we give in the answer? The solution, we were taught, is to add 0.5 to the result and drop the fraction. So,

9/4 + 0.5 = 2.75,

and dropping the fraction gives 2. What if the problem was 9 divided by 2? We get 4.5, and adding 0.5 gives us 5. Is this the best we can do, since the computed value is exactly halfway between two whole numbers? We have the same issue in floating-point. The result of each operation must be a ...

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