Question 14.4

My floating-point calculations are acting strangely and giving me different answers on different machines.

First, see question 14.2.

If the problem isn't that simple, recall that digital computers usually use floating-point formats which provide a close but by no means exact simulation of real number arithmetic. Underflow, cumulative precision loss, and other anomalies are often troublesome.

Don't assume that floating-point results will be exact, and especially don't assume that floating-point values can be compared for equality. (Don't throw haphazard ``fuzz factors'' in, either; see question 14.5.)

These problems are no worse for C than they are for any other computer language. Certain aspects of floating-point are usually defined as ``however the processor does them'' (see also question 11.34), otherwise a compiler for a machine without the ``right'' model would have to do prohibitively expensive emulations.

This article cannot begin to list the pitfalls associated with, and workarounds appropriate for, floating-point work. A good numerical programming text should cover the basics; see also the references below.

References: Kernighan and Plauger, The Elements of Programming Style Sec. 6 pp. 115-8
Knuth, Volume 2 chapter 4
David Goldberg, ``What Every Computer Scientist Should Know about Floating-Point Arithmetic''

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