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Id: 132649
Status: rejected
Priority: 0/
Queue: perl5

Owner: Nobody
Requestors: jim.avera [at] gmail.com
Cc:
AdminCc:

Operating System: (no value)
PatchStatus: (no value)
Severity: low
Type: unknown
Perl Version: (no value)
Fixed In: (no value)



Subject: Definition of % operator may be wrong for negative divisors
From: jim.avera [...] gmail.com
Date: Sun, 24 Dec 2017 00:05:05 -0800 (PST)
To: perlbug [...] perl.org
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This is a bug report for perl from jim.avera@gmail.com, generated with the help of perlbug 1.40 running under perl 5.26.0. ----------------------------------------------------------------- Hello, The documentation of the '%' operator for negative divisors seems incorrect, because if taken literally it produces an impossible result: "If $n is negative, then "$m % $n" is $m minus the smallest multiple of $n that is not less than $m" ... For example 91 % -10 = -9 ; instantiating $m = 91 and $n = -10 produces "91 % -10 is 91 minus the smallest multiple of -10 that is not less than 91" which is impossible, since no multiple of a negative number is >= a positive. My _guess_ is that a correct definitin is: "If $n is negative, then "$m % $n" is $m plus the largest multiple of $n that is not greater than -$m". That works for the examples I've tried, including the one abive where the largest multiple of -10 which is not greater than -91 is -100, and 91 + -100 = -9. Someone who knows this deeply should fix the definition in perlop. ----------------------------------------------------------------- --- Flags: category=docs severity=low --- Site configuration information for perl 5.26.0: Configured by Debian Project at Fri Sep 15 16:13:42 UTC 2017. Summary of my perl5 (revision 5 version 26 subversion 0) configuration: Platform: osname=linux osvers=4.9.0 archname=x86_64-linux-gnu-thread-multi uname='linux localhost 4.9.0 #1 smp debian 4.9.0 x86_64 gnulinux ' config_args='-Dusethreads -Duselargefiles -Dcc=x86_64-linux-gnu-gcc -Dcpp=x86_64-linux-gnu-cpp -Dld=x86_64-linux-gnu-gcc -Dccflags=-DDEBIAN -Wdate-time -D_FORTIFY_SOURCE=2 -g -O2 -fdebug-prefix-map=/build/perl-4JQEGJ/perl-5.26.0=. -fstack-protector-strong -Wformat -Werror=format-security -Dldflags= -Wl,-Bsymbolic-functions -Wl,-z,relro -Dlddlflags=-shared -Wl,-Bsymbolic-functions -Wl,-z,relro -Dcccdlflags=-fPIC -Darchname=x86_64-linux-gnu -Dprefix=/usr -Dprivlib=/usr/share/perl/5.26 -Darchlib=/usr/lib/x86_64-linux-gnu/perl/5.26 -Dvendorprefix=/usr -Dvendorlib=/usr/share/perl5 -Dvendorarch=/usr/lib/x86_64-linux-gnu/perl5/5.26 -Dsiteprefix=/usr/local -Dsitelib=/usr/local/share/perl/5.26.0 -Dsitearch=/usr/local/lib/x86_64-linux-gnu/perl/5.26.0 -Dman1dir=/usr/share/man/man1 -Dman3dir=/usr/share/man/man3 -Dsiteman1dir=/usr/local/man/man1 -Dsiteman3dir=/usr/local/man/man3 -Duse64bitint -Dman1ext=1 -Dman3ext=3perl -Dpager=/usr/bin/sensible-pager -Uafs -Ud_csh -Ud_ualarm -Uusesfio -Uusenm -Ui_libutil -Uversiononly -DDEBUGGING=-g -Doptimize=-O2 -dEs -Duseshrplib -Dlibperl=libperl.so.5.26.0' hint=recommended useposix=true d_sigaction=define useithreads=define usemultiplicity=define use64bitint=define use64bitall=define uselongdouble=undef usemymalloc=n default_inc_excludes_dot=define bincompat5005=undef Compiler: cc='x86_64-linux-gnu-gcc' ccflags ='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fwrapv -fno-strict-aliasing -pipe -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64' optimize='-O2 -g' cppflags='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fwrapv -fno-strict-aliasing -pipe -I/usr/local/include' ccversion='' gccversion='7.2.0' gccosandvers='' intsize=4 longsize=8 ptrsize=8 doublesize=8 byteorder=12345678 doublekind=3 d_longlong=define longlongsize=8 d_longdbl=define longdblsize=16 longdblkind=3 ivtype='long' ivsize=8 nvtype='double' nvsize=8 Off_t='off_t' lseeksize=8 alignbytes=8 prototype=define Linker and Libraries: ld='x86_64-linux-gnu-gcc' ldflags =' -fstack-protector-strong -L/usr/local/lib' libpth=/usr/local/lib /usr/lib/gcc/x86_64-linux-gnu/7/include-fixed /usr/include/x86_64-linux-gnu /usr/lib /lib/x86_64-linux-gnu /lib/../lib /usr/lib/x86_64-linux-gnu /usr/lib/../lib /lib libs=-lgdbm -lgdbm_compat -ldb -ldl -lm -lpthread -lc -lcrypt perllibs=-ldl -lm -lpthread -lc -lcrypt libc=libc-2.26.so so=so useshrplib=true libperl=libperl.so.5.26 gnulibc_version='2.26' Dynamic Linking: dlsrc=dl_dlopen.xs dlext=so d_dlsymun=undef ccdlflags='-Wl,-E' cccdlflags='-fPIC' lddlflags='-shared -L/usr/local/lib -fstack-protector-strong' Locally applied patches: DEBPKG:debian/cpan_definstalldirs - Provide a sensible INSTALLDIRS default for modules installed from CPAN. DEBPKG:debian/db_file_ver - https://bugs.debian.org/340047 Remove overly restrictive DB_File version check. DEBPKG:debian/doc_info - Replace generic man(1) instructions with Debian-specific information. DEBPKG:debian/enc2xs_inc - https://bugs.debian.org/290336 Tweak enc2xs to follow symlinks and ignore missing @INC directories. DEBPKG:debian/errno_ver - https://bugs.debian.org/343351 Remove Errno version check due to upgrade problems with long-running processes. DEBPKG:debian/libperl_embed_doc - https://bugs.debian.org/186778 Note that libperl-dev package is required for embedded linking DEBPKG:fixes/respect_umask - Respect umask during installation DEBPKG:debian/writable_site_dirs - Set umask approproately for site install directories DEBPKG:debian/extutils_set_libperl_path - EU:MM: set location of libperl.a under /usr/lib DEBPKG:debian/no_packlist_perllocal - Don't install .packlist or perllocal.pod for perl or vendor DEBPKG:debian/fakeroot - Postpone LD_LIBRARY_PATH evaluation to the binary targets. DEBPKG:debian/instmodsh_doc - Debian policy doesn't install .packlist files for core or vendor. DEBPKG:debian/ld_run_path - Remove standard libs from LD_RUN_PATH as per Debian policy. DEBPKG:debian/libnet_config_path - Set location of libnet.cfg to /etc/perl/Net as /usr may not be writable. DEBPKG:debian/mod_paths - Tweak @INC ordering for Debian DEBPKG:debian/prune_libs - https://bugs.debian.org/128355 Prune the list of libraries wanted to what we actually need. DEBPKG:debian/perlivp - https://bugs.debian.org/510895 Make perlivp skip include directories in /usr/local DEBPKG:debian/deprecate-with-apt - https://bugs.debian.org/747628 Point users to Debian packages of deprecated core modules DEBPKG:debian/squelch-locale-warnings - https://bugs.debian.org/508764 Squelch locale warnings in Debian package maintainer scripts DEBPKG:debian/skip-upstream-git-tests - Skip tests specific to the upstream Git repository DEBPKG:debian/patchlevel - https://bugs.debian.org/567489 List packaged patches for 5.26.0-8ubuntu1 in patchlevel.h DEBPKG:fixes/document_makemaker_ccflags - https://bugs.debian.org/628522 [rt.cpan.org #68613] Document that CCFLAGS should include $Config{ccflags} DEBPKG:debian/find_html2text - https://bugs.debian.org/640479 Configure CPAN::Distribution with correct name of html2text DEBPKG:debian/perl5db-x-terminal-emulator.patch - https://bugs.debian.org/668490 Invoke x-terminal-emulator rather than xterm in perl5db.pl DEBPKG:debian/cpan-missing-site-dirs - https://bugs.debian.org/688842 Fix CPAN::FirstTime defaults with nonexisting site dirs if a parent is writable DEBPKG:fixes/memoize_storable_nstore - [rt.cpan.org #77790] https://bugs.debian.org/587650 Memoize::Storable: respect 'nstore' option not respected DEBPKG:debian/regen-skip - Skip a regeneration check in unrelated git repositories DEBPKG:debian/makemaker-pasthru - https://bugs.debian.org/758471 Pass LD settings through to subdirectories DEBPKG:debian/makemaker-manext - https://bugs.debian.org/247370 Make EU::MakeMaker honour MANnEXT settings in generated manpage headers DEBPKG:debian/kfreebsd-softupdates - https://bugs.debian.org/796798 Work around Debian Bug#796798 DEBPKG:fixes/autodie-scope - https://bugs.debian.org/798096 Fix a scoping issue with "no autodie" and the "system" sub DEBPKG:fixes/memoize-pod - [rt.cpan.org #89441] Fix POD errors in Memoize DEBPKG:debian/hurd-softupdates - https://bugs.debian.org/822735 Fix t/op/stat.t failures on hurd DEBPKG:fixes/math_complex_doc_great_circle - https://bugs.debian.org/697567 [rt.cpan.org #114104] Math::Trig: clarify definition of great_circle_midpoint DEBPKG:fixes/math_complex_doc_see_also - https://bugs.debian.org/697568 [rt.cpan.org #114105] Math::Trig: add missing SEE ALSO DEBPKG:fixes/math_complex_doc_angle_units - https://bugs.debian.org/731505 [rt.cpan.org #114106] Math::Trig: document angle units DEBPKG:fixes/cpan_web_link - https://bugs.debian.org/367291 CPAN: Add link to main CPAN web site DEBPKG:fixes/time_piece_doc - https://bugs.debian.org/817925 Time::Piece: Improve documentation for add_months and add_years DEBPKG:fixes/extutils_makemaker_reproducible - https://bugs.debian.org/835815 https://bugs.debian.org/834190 Make perllocal.pod files reproducible DEBPKG:fixes/file_path_hurd_errno - File-Path: Fix test failure in Hurd due to hard-coded ENOENT DEBPKG:debian/hppa_op_optimize_workaround - https://bugs.debian.org/838613 Temporarily lower the optimization of op.c on hppa due to gcc-6 problems DEBPKG:debian/installman-utf8 - https://bugs.debian.org/840211 Generate man pages with UTF-8 characters DEBPKG:fixes/file_path_chmod_race - https://bugs.debian.org/863870 [rt.cpan.org #121951] Prevent directory chmod race attack. DEBPKG:fixes/extutils_file_path_compat - Correct the order of tests of chmod(). (#294) DEBPKG:fixes/getopt-long-2 - [rt.cpan.org #120300] Withdraw part of commit 5d9947fb445327c7299d8beb009d609bc70066c0, which tries to implement more GNU getopt_long campatibility. GNU DEBPKG:fixes/getopt-long-3 - provide a default value for optional arguments DEBPKG:fixes/getopt-long-4 - https://bugs.debian.org/864544 [rt.cpan.org #122068] Fix issue #122068. DEBPKG:fixes/fbm-instr-crash - [bb152a4] [perl #131575] don't call Perl_fbm_instr() with negative length DEBPKG:fixes/test-builder-reset - https://bugs.debian.org/865894 Reset inside subtest maintains parent DEBPKG:debian/CVE-2016-1238/base-pm-amends-pt2 - [a77da41] Limit dotless-INC effect on base.pm with guard: DEBPKG:debian/hppa_opmini_optimize_workaround - https://bugs.debian.org/869122 Lower the optimization level of opmini.c on hppa DEBPKG:debian/sh4_op_optimize_workaround - https://bugs.debian.org/869373 Also lower the optimization level of op.c and opmini.c on sh4 DEBPKG:fixes/json-pp-example - [rt.cpan.org #92793] https://bugs.debian.org/871837 fix RT-92793: bug in SYNOPSIS DEBPKG:debian/customized - Update customized.dat for files patched in Debian DEBPKG:fixes/CVE-2017-12837 - https://bugs.debian.org/875596 [perl #131582] [66288bb] regcomp [perl #131582] DEBPKG:fixes/CVE-2017-12883 - https://bugs.debian.org/875597 [perl #131598] [2692dda] PATCH: [perl #131598] --- @INC for perl 5.26.0: /home/jima/lib/perl /home/jima/perl5/lib/perl5/x86_64-linux-gnu-thread-multi /home/jima/perl5/lib/perl5/5.26.0/x86_64-linux-gnu-thread-multi /home/jima/perl5/lib/perl5/5.26.0 /home/jima/perl5/lib/perl5/x86_64-linux-gnu-thread-multi /home/jima/perl5/lib/perl5 /etc/perl /usr/local/lib/x86_64-linux-gnu/perl/5.26.0 /usr/local/share/perl/5.26.0 /usr/lib/x86_64-linux-gnu/perl5/5.26 /usr/share/perl5 /usr/lib/x86_64-linux-gnu/perl/5.26 /usr/share/perl/5.26 /usr/local/lib/site_perl /usr/lib/x86_64-linux-gnu/perl-base --- Environment for perl 5.26.0: HOME=/home/jima LANG=en_US.UTF-8 LANGUAGE (unset) LC_COLLATE=C LD_LIBRARY_PATH (unset) LOGDIR (unset) PATH=/home/jima/perl5/bin:/home/jima/bin:/home/jima/jima_tools/x86_64/bin:/home/jima/jima_tools/bin:/usr/bin:/bin:/usr/sbin:/sbin:/usr/bin/X11:/usr/local/bin:/usr/local/sbin:/usr/games:/usr/local/games:/usr/lib/jvm/java-8-oracle/bin:/usr/lib/jvm/java-8-oracle/db/bin:/usr/lib/jvm/java-8-oracle/jre/bin:. PERL5LIB=/home/jima/lib/perl:/home/jima/perl5/lib/perl5/x86_64-linux-gnu-thread-multi:/home/jima/perl5/lib/perl5 PERL_BADLANG (unset) PERL_LOCAL_LIB_ROOT=/home/jima/perl5 PERL_MB_OPT=--install_base /home/jima/perl5 PERL_MM_OPT=INSTALL_BASE=/home/jima/perl5 SHELL=/bin/bash
Date: Sun, 24 Dec 2017 08:25:23 +0000
To: perl5-porters [...] perl.org
Subject: Re: [perl #132649] Definition of % operator may be wrong for negative divisors
From: Zefram <zefram [...] fysh.org>
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via RT wrote: Show quoted text
>which is impossible, since no multiple of a negative number is >= a positive.
Multiples that arise from multiplying by a negative integer count. 100 is a multiple of -10, obtained by multiplying it by -10. Show quoted text
> "If $n is negative, then "$m % $n" is $m plus the largest multiple > of $n that is not greater than -$m".
That would be less clear. "Largest" isn't a synonym for "greatest", but often refers to magnitude: one would say that -110 is larger than -100, even though -100 is greater than -110. The documentation is fine as it is. -zefram


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