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* [Bug manual/31322] New: Section 9.1 should document that the comparison function used in the example is not transitive
@ 2024-01-31 15:15 vincent-srcware at vinc17 dot net
0 siblings, 0 replies; only message in thread
From: vincent-srcware at vinc17 dot net @ 2024-01-31 15:15 UTC (permalink / raw)
To: glibc-bugs
https://sourceware.org/bugzilla/show_bug.cgi?id=31322
Bug ID: 31322
Summary: Section 9.1 should document that the comparison
function used in the example is not transitive
Product: glibc
Version: 2.38
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: manual
Assignee: unassigned at sourceware dot org
Reporter: vincent-srcware at vinc17 dot net
CC: mtk.manpages at gmail dot com
Target Milestone: ---
Section 9.1 gives an example of a comparison function on floating-point numbers
of type "double" that is not transitive due to possible NaN values. This can be
shown by the following program:
#include <stdio.h>
#include <math.h>
int
compare_doubles (const void *a, const void *b)
{
const double *da = (const double *) a;
const double *db = (const double *) b;
return (*da > *db) - (*da < *db);
}
int main (void)
{
double t[3] = { 1.0, NAN, 2.0 };
printf ("%d\n", compare_doubles(t+0, t+1));
printf ("%d\n", compare_doubles(t+1, t+2));
printf ("%d\n", compare_doubles(t+0, t+2));
return 0;
}
which gives
0
0
-1
while the initial 0 0 implies a third 0 with a transitive comparison function.
This should be documented.
Apparently, this is not clear to everyone:
https://sourceware.org/pipermail/libc-alpha/2024-January/154350.html
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2024-01-31 15:15 [Bug manual/31322] New: Section 9.1 should document that the comparison function used in the example is not transitive vincent-srcware at vinc17 dot net
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