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--------------------------------------------------------------------------
Mon May 7 14:38:32 IDT 2007
============================
Thanks to Andrew Schorr for some of the wording here.
<quote>
The Open Group awk specification says that a string value shall
be converted to a numeric value using the ISO C standard atof()
function. The awk spec is here:
http://www.opengroup.org/onlinepubs/009695399/utilities/awk.html
And the ISO C standard says that atof() shall work the same way
as strtod(), and strtod() must recognize the IEEE special values
for NaN (not-a-number) and Infinity.
</quote>
As a result, more than one gawk user has come to the conclusion that
gawk is not standard compliant since it does not support converting
these special values.
Similar reasoning has led other users to conclude that gawk must also
support input data in the format of C99-style hexadecimal floating point
numbers.
I, Arnold Robbins, the gawk maintainer, feel that while these interpretations
of the standard can be made, they are _unintended_ side effects of the
change in the POSIX standard to refer to the ISO C99 standard everywhere.
With resepct to hexadecimal floating point, historically, AWK has always
supported the input of only decimal numbers. Adding such a facility is
likely to break many programs, very badly, and is a very big departure
from historical practice.
With respect to "NaN" and "Inf" values, many people point out that Unix
awk and mawk "support the standard" and print such values in their output.
This is _accidental_ support, since these programs are relying on the
system library version of strtod/atof to do the work. If compiled on a
system without such library support, those programs do NOT support the
special IEEE values.
These special interpretations introduce a large departure from existing
practice:
# Thanks to Brian Kernighan for this great example
$ nawk 'BEGIN { print "Nancy Reagan" + 0 }'
nan
This is just plain wrong. String values with no leading digits must
continue to convert to numeric zero.
The POSIX people have been asked to clarify these issues, and not recently,
but have not, to my knowledge, done so. Further discussion is provided
in the node "POSIX Floating Point Problems" in the gawk manual.
---------- Stuff below this line is of historical interest only ----------
March 2001:
It looks like the revised 1003.2 standard will actually follow the
rules given below. Hallelujah!
October 1998:
The 1003.2 work has been at a stand-still for ages. Who knows if or
when a new revision will actually happen...
August 1995:
Although the published 1003.2 standard contained the incorrect
comparison rules of 11.2 draft as described below, no actual implementation
of awk (that I know of) actually used those rules.
A revision of the 1003.2 standard is in progress, and in the May 1995
draft, the rules were fixed (based on my submissions for interpretation
requests) to match the description given below. Thus, the next version
of the standard will have a correct description of the comparison
rules.
June 1992:
Right now, the numeric vs. string comparisons are screwed up in draft
11.2. What prompted me to check it out was the note in gnu.bug.utils
which observed that gawk was doing the comparison $1 == "000"
numerically. I think that we can agree that intuitively, this should
be done as a string comparison. Version 2.13.2 of gawk follows the
current POSIX draft. Following is how I (now) think this
stuff should be done.
1. A numeric literal or the result of a numeric operation has the NUMERIC
attribute.
2. A string literal or the result of a string operation has the STRING
attribute.
3. Fields, getline input, FILENAME, ARGV elements, ENVIRON elements and the
elements of an array created by split() that are numeric strings
have the STRNUM attribute. Otherwise, they have the STRING attribute.
Uninitialized variables also have the STRNUM attribute.
4. Attributes propagate across assignments, but are not changed by
any use. (Although a use may cause the entity to acquire an additional
value such that it has both a numeric and string value -- this leaves the
attribute unchanged.)
When two operands are compared, either string comparison or numeric comparison
may be used, depending on the attributes of the operands, according to the
following (symmetric) matrix:
+----------------------------------------------
| STRING NUMERIC STRNUM
--------+----------------------------------------------
|
STRING | string string string
|
NUMERIC | string numeric numeric
|
STRNUM | string numeric numeric
--------+----------------------------------------------
So, the following program should print all OKs.
echo '0e2 0a 0 0b
0e2 0a 0 0b' |
$AWK '
NR == 1 {
num = 0
str = "0e2"
print ++test ": " ( (str == "0e2") ? "OK" : "OOPS" )
print ++test ": " ( ("0e2" != 0) ? "OK" : "OOPS" )
print ++test ": " ( ("0" != $2) ? "OK" : "OOPS" )
print ++test ": " ( ("0e2" == $1) ? "OK" : "OOPS" )
print ++test ": " ( (0 == "0") ? "OK" : "OOPS" )
print ++test ": " ( (0 == num) ? "OK" : "OOPS" )
print ++test ": " ( (0 != $2) ? "OK" : "OOPS" )
print ++test ": " ( (0 == $1) ? "OK" : "OOPS" )
print ++test ": " ( ($1 != "0") ? "OK" : "OOPS" )
print ++test ": " ( ($1 == num) ? "OK" : "OOPS" )
print ++test ": " ( ($2 != 0) ? "OK" : "OOPS" )
print ++test ": " ( ($2 != $1) ? "OK" : "OOPS" )
print ++test ": " ( ($3 == 0) ? "OK" : "OOPS" )
print ++test ": " ( ($3 == $1) ? "OK" : "OOPS" )
print ++test ": " ( ($2 != $4) ? "OK" : "OOPS" ) # 15
}
{
a = "+2"
b = 2
if (NR % 2)
c = a + b
print ++test ": " ( (a != b) ? "OK" : "OOPS" ) # 16 and 22
d = "2a"
b = 2
if (NR % 2)
c = d + b
print ++test ": " ( (d != b) ? "OK" : "OOPS" )
print ++test ": " ( (d + 0 == b) ? "OK" : "OOPS" )
e = "2"
print ++test ": " ( (e == b "") ? "OK" : "OOPS" )
a = "2.13"
print ++test ": " ( (a == 2.13) ? "OK" : "OOPS" )
a = "2.130000"
print ++test ": " ( (a != 2.13) ? "OK" : "OOPS" )
if (NR == 2) {
CONVFMT = "%.6f"
print ++test ": " ( (a == 2.13) ? "OK" : "OOPS" )
}
}'
