GAWK: Effective AWK Programming

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304 GAWK: Effective AWK Programming⊣ 0000051582Ctrl-dThis shows that some values can be represented exactly, whereas others are only approximated.This is not a “bug” in awk, but simply an artifact of how computers representnumbers.Another peculiarity of floating-point numbers on modern systems is that they often havemore than one representation for the number zero! In particular, it is possible to represent“minus zero” as well as regular, or “positive” zero.This example shows that negative and positive zero are distinct values when storedinternally, but that they are in fact equal to each other, as well as to “regular” zero:$ gawk ’BEGIN { mz = -0 ; pz = 0> printf "-0 = %g, +0 = %g, (-0 == +0) -> %d\n", mz, pz, mz == pz> printf "mz == 0 -> %d, pz == 0 -> %d\n", mz == 0, pz == 0> }’⊣ -0 = -0, +0 = 0, (-0 == +0) -> 1⊣ mz == 0 -> 1, pz == 0 -> 1It helps to keep this in mind should you process numeric data that contains negativezero values; the fact that the zero is negative is noted and can affect comparisons.D.3.3 Standards Versus Existing PracticeHistorically, awk has converted any non-numeric looking string to the numeric value zero,when required. Furthermore, the original definition of the language and the original POSIXstandards specified that awk only understands decimal numbers (base 10), and not octal(base 8) or hexadecimal numbers (base 16).As of this writing (February, 2007), changes in the language of the current POSIXstandard can be interpreted to imply that awk should support additional features. Thesefeatures are:• Interpretation of floating point data values specified in hexadecimal notation(‘0xDEADBEEF’). (Note: data values, not source code constants.)• Support for the special IEEE 754 floating point values “Not A Number” (NaN), positiveInfinity (“inf”) and negative Infinity (“−inf”). In particular, the format for these valuesis as specified by the ISO C99 standard, which ignores case and can allow machinedependentadditional characters after the ‘nan’ and allow either ‘inf’ or ‘infinity’.The first problem is that both of these are clear changes to historical practice:• The gawk maintainer feels that hexadecimal floating point values, in particular, is ugly,and was never intended by the original designers to be part of the language.• Allowing completely alphabetic strings to have valid numeric values is also a very severedeparture from historical practice.The second problem is that the gawk maintainer feels that this interpretation of thestandard, which requires a certain amount of “language lawyering” to arrive at in the firstplace, was not intended by the standard developers, either. In other words, “we see howyou got where you are, but we don’t think that that’s where you want to be.”Nevertheless, on systems that support IEEE floating point, it seems reasonable to providesome way to support NaN and Infinity values. The solution implemented in gawk, as ofversion 3.1.6, is as follows:
Appendix D: Basic Programming Concepts 3051. With the ‘--posix’ command-line option, gawk becomes “hands off.” String values arepassed directly to the system library’s strtod() function, and if it successfuly returnsa numeric value, that is what’s used. By definition, the results are not portable acrossdifferent systems. 3 They are also a little surprising:$ echo nanny | gawk --posix ’{ print $1 + 0 }’⊣ nan$ echo 0xDeadBeef | gawk --posix ’{ print $1 + 0 }’⊣ 37359285592. Without ‘--posix’, gawk interprets the four strings ‘+inf’, ‘-inf’, ‘+nan’, and ‘-nan’specially, producing the corresponding special numeric values. The leading sign actsa signal to gawk (and the user) that the value is really numeric. Hexadecimal floatingpoint is not supported (unless you also use ‘--non-decimal-data’, which is notrecommended). For example:$ echo nanny | gawk ’{ print $1 + 0 }’⊣ 0$ echo +nan | gawk ’{ print $1 + 0 }’⊣ nan$ echo 0xDeadBeef | gawk ’{ print $1 + 0 }’⊣ 0gawk does ignore case distinction in the four special values. Thus ‘+nan’ and ‘+NaN’ arethe same.3 You asked for it, you got it.
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