a2p
accept
access
acct
addftinfo
addr2line
adjtime
afmtodit
after
aio_cancel
aio_error
aio_read
aio_return
aio_suspend
aio_waitcomplete
aio_write
alias
aliases
alloc
anvil
append
apply
apropos
ar
array
as
asa
asn1parse
at
atq
atrm
attemptckalloc
attemptckrealloc
authlib
authtest
autopoint
awk
b64decode
b64encode
basename
batch
bc
bdes
bell
bg
bgerror
biff
big5
binary
bind
bindkey
bindtags
bindtextdomain
bio
bitmap
blowfish
bn
bootparams
bootptab
bounce
brandelf
break
breaksw
brk
bsdiff
bsdtar
bsnmpd
bspatch
bthost
btsockstat
buffer
builtin
builtins
bunzip2
button
byacc
bzcat
bzegrep
bzfgrep
bzgrep
bzip2
c2ph
c89
c99
ca
cal
calendar
canvas
cap_mkdb
case
cat
catch
catman
cc
cd
cdcontrol
chdir
checkbutton
checknr
chflags
chfn
chgrp
chio
chkey
chmod
chown
chpass
chroot
chsh
ci
ciphers
ckalloc
ckdist
ckfree
ckrealloc
cksum
cleanup
clear
clipboard
clock
clock_getres
clock_gettime
clock_settime
close
cmp
co
col
colcrt
colldef
colors
colrm
column
comm
command
compile_et
complete
compress
concat
config
connect
console
continue
core
courierlogger
couriertcpd
cp
cpan
cpio
cpp
creat
crl
crontab
crunchgen
crunchide
crypt
crypto
csh
csplit
ctags
ctm
ctm_dequeue
ctm_rmail
ctm_smail
cu
cursor
cursors
cut
cvs
date
dbiprof
dbiproxy
dc
dcgettext
dcngettext
dd
dde
default
defer
deliverquota
des
destroy
devfs
df
dgettext
dgst
dh
dhparam
dialog
diff
diff3
dig
dir
dirent
dirname
dirs
discard
disktab
dngettext
do
domainname
done
dprofpp
dsa
dsaparam
dtmfdecode
du
dup
dup2
eaccess
ec
ecdsa
echo
echotc
ecparam
ed
edit
editrc
ee
egrep
elf
elfdump
elif
else
enc
enc2xs
encoding
end
endif
endsw
engine
enigma
entry
env
envsubst
eof
eqn
err
errno
error
errstr
esac
ethers
euc
eui64
eval
event
evp
ex
exec
execve
exit
expand
export
exports
expr
extattr
extattr_delete_fd
extattr_delete_file
extattr_get_fd
extattr_get_file
extattr_set_fd
extattr_set_file
f77
false
famm
famx
fblocked
fbtab
fc
fchdir
fchflags
fchmod
fchown
fcntl
fconfigure
fcopy
fdescfs
fdformat
fdread
fdwrite
fetch
fg
fgrep
fhopen
fhstat
fhstatfs
fi
file
file2c
fileevent
filename
filetest
find
find2perl
finger
flex
flock
flush
fmt
focus
fold
font
fontedit
for
foreach
fork
format
forward
fpathconf
frame
from
fs
fstab
fstat
fstatfs
fsync
ftp
ftpchroot
ftpusers
ftruncate
futimes
g711conv
gb2312
gb18030
gbk
gcc
gcore
gcov
gdb
gencat
gendsa
genrsa
gensnmptree
getconf
getdents
getdirentries
getdtablesize
getegid
geteuid
getfacl
getfh
getfsstat
getgid
getgroups
getitimer
getlogin
getopt
getopts
getpeername
getpgid
getpgrp
getpid
getppid
getpriority
getresgid
getresuid
getrlimit
getrusage
gets
getsid
getsockname
getsockopt
gettext
gettextize
gettimeofday
gettytab
getuid
glob
global
gmake
goto
gperf
gprof
grab
grep
grid
grn
grodvi
groff
groff_font
groff_out
groff_tmac
grog
grolbp
grolj4
grops
grotty
group
groups
gunzip
gzcat
gzexe
gzip
h2ph
h2xs
hash
hashstat
hd
head
help2man
hesinfo
hexdump
history
host
hostname
hosts
hosts_access
hosts_options
hpftodit
http
hup
i386_get_ioperm
i386_get_ldt
i386_set_ioperm
i386_set_ldt
i386_vm86
iconv
id
ident
idprio
if
ifnames253
ifnames259
image
imapd
incr
indent
indxbib
info
infokey
inode
install
instmodsh
interp
intro
introduction
ioctl
ipcrm
ipcs
ipf
ipftest
ipnat
ippool
ipresend
issetugid
jail
jail_attach
jobid
jobs
join
jot
kbdcontrol
kbdmap
kcon
kdestroy
kdump
kenv
kevent
keycap
keylogin
keylogout
keymap
keysyms
kgdb
kill
killall
killpg
kinit
kldfind
kldfirstmod
kldload
kldnext
kldstat
kldsym
kldunload
klist
kpasswd
kqueue
kse
kse_create
kse_exit
kse_release
kse_switchin
kse_thr_interrupt
kse_wakeup
ktrace
label
labelframe
lam
lappend
last
lastcomm
lastlog
lchflags
lchmod
lchown
ld
ldap
ldapadd
ldapcompare
ldapdelete
ldapmodify
ldapmodrdn
ldappasswd
ldapsearch
ldapwhoami
ldd
leave
less
lesskey
lex
lgetfh
lhash
libnetcfg
library
limit
limits
lindex
link
linprocfs
linsert
lint
lio_listio
list
listbox
listen
lj4_font
lkbib
llength
lmtp
ln
load
loadfont
local
locale
locate
lock
lockf
log
logger
login
logins
logname
logout
look
lookbib
lorder
lower
lp
lpq
lpr
lprm
lptest
lrange
lreplace
ls
lsearch
lseek
lset
lsort
lstat
lsvfs
lutimes
lynx
m4
madvise
magic
mail
maildiracl
maildirkw
maildirmake
mailq
mailx
make
makeinfo
makewhatis
man
manpath
master
mc
mcedit
mcview
md2
md4
md5
mdc2
memory
menu
menubar
menubutton
merge
mesg
message
mincore
minherit
minigzip
mkdep
mkdir
mkfifo
mkimapdcert
mklocale
mknod
mkpop3dcert
mkstr
mktemp
mlock
mlockall
mmap
mmroff
modfind
modfnext
modnext
modstat
moduli
more
motd
mount
mprotect
mptable
msdos
msdosfs
msgattrib
msgcat
msgcmp
msgcomm
msgconv
msgen
msgexec
msgfilter
msgfmt
msggrep
msginit
msgmerge
msgs
msgunfmt
msguniq
mskanji
msql2mysql
msync
mt
munlock
munlockall
munmap
mv
myisamchk
myisamlog
myisampack
mysql
mysqlaccess
mysqladmin
mysqlbinlog
mysqlcheck
mysqld
mysqldump
mysqld_multi
mysqld_safe
mysqlhotcopy
mysqlimport
mysqlshow
mysql_config
mysql_fix_privilege_tables
mysql_zap
namespace
nanosleep
nawk
nc
ncal
ncplist
ncplogin
ncplogout
neqn
netconfig
netgroup
netid
netstat
networks
newaliases
newgrp
nex
nfsstat
nfssvc
ngettext
nice
nl
nm
nmount
nohup
nologin
notify
nroff
nseq
nslookup
ntp_adjtime
ntp_gettime
nvi
nview
objcopy
objdump
objformat
ocsp
od
onintr
open
openssl
opieaccess
opieinfo
opiekey
opiekeys
opiepasswd
option
options
oqmgr
pack
package
packagens
pagesize
palette
pam_auth
panedwindow
parray
passwd
paste
patch
pathchk
pathconf
pawd
pax
pbm
pcre
pcreapi
pcrebuild
pcrecallout
pcrecompat
pcrecpp
pcregrep
pcrematching
pcrepartial
pcrepattern
pcreperform
pcreposix
pcreprecompile
pcresample
pcretest
perl
perl56delta
perl58delta
perl561delta
perl570delta
perl571delta
perl572delta
perl573delta
perl581delta
perl582delta
perl583delta
perl584delta
perl585delta
perl586delta
perl587delta
perl588delta
perl5004delta
perl5005delta
perlaix
perlamiga
perlapi
perlapio
perlapollo
perlartistic
perlbeos
perlbook
perlboot
perlbot
perlbs2000
perlbug
perlcall
perlcc
perlce
perlcheat
perlclib
perlcn
perlcompile
perlcygwin
perldata
perldbmfilter
perldebguts
perldebtut
perldebug
perldelta
perldgux
perldiag
perldoc
perldos
perldsc
perlebcdic
perlembed
perlepoc
perlfaq
perlfaq1
perlfaq2
perlfaq3
perlfaq4
perlfaq5
perlfaq6
perlfaq7
perlfaq8
perlfaq9
perlfilter
perlfork
perlform
perlfreebsd
perlfunc
perlglossary
perlgpl
perlguts
perlhack
perlhist
perlhpux
perlhurd
perlintern
perlintro
perliol
perlipc
perlirix
perlivp
perljp
perlko
perllexwarn
perllinux
perllocale
perllol
perlmachten
perlmacos
perlmacosx
perlmint
perlmod
perlmodinstall
perlmodlib
perlmodstyle
perlmpeix
perlnetware
perlnewmod
perlnumber
perlobj
perlop
perlopenbsd
perlopentut
perlos2
perlos390
perlos400
perlothrtut
perlpacktut
perlplan9
perlpod
perlpodspec
perlport
perlqnx
perlre
perlref
perlreftut
perlrequick
perlreref
perlretut
perlrun
perlsec
perlsolaris
perlstyle
perlsub
perlsyn
perlthrtut
perltie
perltoc
perltodo
perltooc
perltoot
perltrap
perltru64
perltw
perlunicode
perluniintro
perlutil
perluts
perlvar
perlvmesa
perlvms
perlvos
perlwin32
perlxs
perlxstut
perror
pfbtops
pftp
pgrep
phones
photo
pic
pickup
piconv
pid
pipe
pkcs7
pkcs8
pkcs12
pkg_add
pkg_check
pkg_create
pkg_delete
pkg_info
pkg_sign
pkg_version
pkill
pl2pm
place
pod2html
pod2latex
pod2man
pod2text
pod2usage
podchecker
podselect
poll
popd
popup
posix_madvise
postalias
postcat
postconf
postdrop
postfix
postkick
postlock
postlog
postmap
postqueue
postsuper
pr
pread
preadv
printcap
printenv
printf
proc
procfs
profil
protocols
prove
proxymap
ps
psed
psroff
pstruct
ptrace
publickey
pushd
puts
pwd
pwrite
pwritev
qmgr
qmqpd
quota
quotactl
radiobutton
raise
rand
ranlib
rcp
rcs
rcsclean
rcsdiff
rcsfile
rcsfreeze
rcsintro
rcsmerge
read
readelf
readlink
readonly
readv
realpath
reboot
recv
recvfrom
recvmsg
red
ree
refer
regexp
registry
regsub
rehash
remote
rename
repeat
replace
req
reset
resolver
resource
return
rev
revoke
rfcomm_sppd
rfork
rhosts
ripemd
ripemd160
rlog
rlogin
rm
rmd160
rmdir
rpc
rpcgen
rs
rsa
rsautl
rsh
rtld
rtprio
rup
ruptime
rusers
rwall
rwho
s2p
safe
sasl
sasldblistusers2
saslpasswd2
sbrk
scache
scale
scan
sched
sched_getparam
sched_getscheduler
sched_get_priority_max
sched_get_priority_min
sched_rr_get_interval
sched_setparam
sched_setscheduler
sched_yield
scon
scp
script
scrollbar
sdiff
sed
seek
select
selection
semctl
semget
semop
send
sendbug
sendfile
sendmail
sendmsg
sendto
services
sess_id
set
setegid
setenv
seteuid
setfacl
setgid
setgroups
setitimer
setlogin
setpgid
setpgrp
setpriority
setregid
setresgid
setresuid
setreuid
setrlimit
setsid
setsockopt
settc
settimeofday
setty
setuid
setvar
sftp
sh
sha
sha1
sha256
shar
shells
shift
shmat
shmctl
shmdt
shmget
showq
shutdown
sigaction
sigaltstack
sigblock
sigmask
sigpause
sigpending
sigprocmask
sigreturn
sigsetmask
sigstack
sigsuspend
sigvec
sigwait
size
slapadd
slapcat
slapd
slapdn
slapindex
slappasswd
slaptest
sleep
slogin
slurpd
smbutil
smime
smtp
smtpd
socket
socketpair
sockstat
soelim
sort
source
spawn
speed
spinbox
spkac
splain
split
squid
squid_ldap_auth
squid_ldap_group
squid_unix_group
sscop
ssh
sshd_config
ssh_config
stab
startslip
stat
statfs
stop
string
strings
strip
stty
su
subst
sum
suspend
swapoff
swapon
switch
symlink
sync
sysarch
syscall
sysconftool
sysconftoolcheck
systat
s_client
s_server
s_time
tabs
tail
talk
tar
tbl
tclsh
tcltest
tclvars
tcopy
tcpdump
tcpslice
tcsh
tee
tell
telltc
telnet
term
termcap
terminfo
test
texindex
texinfo
text
textdomain
tfmtodit
tftp
then
threads
time
tip
tk
tkerror
tkvars
tkwait
tlsmgr
tmac
top
toplevel
touch
tput
tr
trace
trafshow
trap
troff
true
truncate
truss
tset
tsort
tty
ttys
type
tzfile
ui
ul
ulimit
umask
unalias
uname
uncomplete
uncompress
undelete
unexpand
unhash
unifdef
unifdefall
uniq
units
unknown
unlimit
unlink
unmount
unset
unsetenv
until
unvis
update
uplevel
uptime
upvar
usbhidaction
usbhidctl
users
utf8
utimes
utmp
utrace
uudecode
uuencode
uuidgen
vacation
variable
verify
version
vfork
vgrind
vgrindefs
vi
vidcontrol
vidfont
view
virtual
vis
vt220keys
vwait
w
wait
wait3
wait4
waitpid
wall
wc
wget
what
whatis
where
whereis
which
while
who
whoami
whois
window
winfo
wish
wm
write
writev
wtmp
x509
xargs
xgettext
xmlwf
xstr
xsubpp
yacc
yes
ypcat
ypchfn
ypchpass
ypchsh
ypmatch
yppasswd
ypwhich
yyfix
zcat
zcmp
zdiff
zegrep
zfgrep
zforce
zgrep
zmore
znew
_exit
__syscall
 
FreeBSD/Linux/UNIX General Commands Manual
Hypertext Man Pages
perlfaq4
 
PERLFAQ4(1)	       Perl Programmers Reference Guide 	   PERLFAQ4(1)



NAME
       perlfaq4 - Data Manipulation ($Revision: 1.73 $, $Date: 2005/12/31
       00:54:37 $)

DESCRIPTION
       This section of the FAQ answers questions related to manipulating num-
       bers, dates, strings, arrays, hashes, and miscellaneous data issues.

Data: Numbers
       Why am I getting long decimals (eg, 19.9499999999999) instead of the
       numbers I should be getting (eg, 19.95)?

       Internally, your computer represents floating-point numbers in binary.
       Digital (as in powers of two) computers cannot store all numbers
       exactly.  Some real numbers lose precision in the process.  This is a
       problem with how computers store numbers and affects all computer lan-
       guages, not just Perl.

       perlnumber show the gory details of number representations and conver-
       sions.

       To limit the number of decimal places in your numbers, you can use the
       printf or sprintf function.  See the "Floating Point Arithmetic" for
       more details.

	       printf "%.2f", 10/3;

	       my $number = sprintf "%.2f", 10/3;

       Why is int() broken?

       Your int() is most probably working just fine.  It's the numbers that
       aren't quite what you think.

       First, see the above item "Why am I getting long decimals (eg,
       19.9499999999999) instead of the numbers I should be getting (eg,
       19.95)?".

       For example, this

	   print int(0.6/0.2-2), "\n";

       will in most computers print 0, not 1, because even such simple numbers
       as 0.6 and 0.2 cannot be presented exactly by floating-point numbers.
       What you think in the above as 'three' is really more like
       2.9999999999999995559.

       Why isn't my octal data interpreted correctly?

       Perl only understands octal and hex numbers as such when they occur as
       literals in your program.  Octal literals in perl must start with a
       leading "0" and hexadecimal literals must start with a leading "0x".
       If they are read in from somewhere and assigned, no automatic conver-
       sion takes place.  You must explicitly use oct() or hex() if you want
       the values converted to decimal.  oct() interprets hex ("0x350"), octal
       ("0350" or even without the leading "0", like "377") and binary
       ("0b1010") numbers, while hex() only converts hexadecimal ones, with or
       without a leading "0x", like "0x255", "3A", "ff", or "deadbeef".  The
       inverse mapping from decimal to octal can be done with either the "%o"
       or "%O" sprintf() formats.

       This problem shows up most often when people try using chmod(),
       mkdir(), umask(), or sysopen(), which by widespread tradition typically
       take permissions in octal.

	   chmod(644,  $file); # WRONG
	   chmod(0644, $file); # right

       Note the mistake in the first line was specifying the decimal literal
       644, rather than the intended octal literal 0644.  The problem can be
       seen with:

	   printf("%#o",644); # prints 01204

       Surely you had not intended "chmod(01204, $file);" - did you?  If you
       want to use numeric literals as arguments to chmod() et al. then please
       try to express them as octal constants, that is with a leading zero and
       with the following digits restricted to the set 0..7.

       Does Perl have a round() function?  What about ceil() and floor()?
       Trig functions?

       Remember that int() merely truncates toward 0.  For rounding to a cer-
       tain number of digits, sprintf() or printf() is usually the easiest
       route.

	   printf("%.3f", 3.1415926535);       # prints 3.142

       The POSIX module (part of the standard Perl distribution) implements
       ceil(), floor(), and a number of other mathematical and trigonometric
       functions.

	   use POSIX;
	   $ceil   = ceil(3.5); 		       # 4
	   $floor  = floor(3.5);		       # 3

       In 5.000 to 5.003 perls, trigonometry was done in the Math::Complex
       module.	With 5.004, the Math::Trig module (part of the standard Perl
       distribution) implements the trigonometric functions. Internally it
       uses the Math::Complex module and some functions can break out from the
       real axis into the complex plane, for example the inverse sine of 2.

       Rounding in financial applications can have serious implications, and
       the rounding method used should be specified precisely.	In these
       cases, it probably pays not to trust whichever system rounding is being
       used by Perl, but to instead implement the rounding function you need
       yourself.

       To see why, notice how you'll still have an issue on half-way-point
       alternation:

	   for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}

	   0.0 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.7 0.7
	   0.8 0.8 0.9 0.9 1.0 1.0

       Don't blame Perl.  It's the same as in C.  IEEE says we have to do
       this.  Perl numbers whose absolute values are integers under 2**31 (on
       32 bit machines) will work pretty much like mathematical integers.
       Other numbers are not guaranteed.

       How do I convert between numeric representations/bases/radixes?

       As always with Perl there is more than one way to do it.  Below are a
       few examples of approaches to making common conversions between number
       representations.  This is intended to be representational rather than
       exhaustive.

       Some of the examples below use the Bit::Vector module from CPAN.  The
       reason you might choose Bit::Vector over the perl built in functions is
       that it works with numbers of ANY size, that it is optimized for speed
       on some operations, and for at least some programmers the notation
       might be familiar.

       How do I convert hexadecimal into decimal
	   Using perl's built in conversion of 0x notation:

	       $dec = 0xDEADBEEF;

	   Using the hex function:

	       $dec = hex("DEADBEEF");

	   Using pack:

	       $dec = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8)));

	   Using the CPAN module Bit::Vector:

	       use Bit::Vector;
	       $vec = Bit::Vector->new_Hex(32, "DEADBEEF");
	       $dec = $vec->to_Dec();

       How do I convert from decimal to hexadecimal
	   Using sprintf:

	       $hex = sprintf("%X", 3735928559); # upper case A-F
	       $hex = sprintf("%x", 3735928559); # lower case a-f

	   Using unpack:

	       $hex = unpack("H*", pack("N", 3735928559));

	   Using Bit::Vector:

	       use Bit::Vector;
	       $vec = Bit::Vector->new_Dec(32, -559038737);
	       $hex = $vec->to_Hex();

	   And Bit::Vector supports odd bit counts:

	       use Bit::Vector;
	       $vec = Bit::Vector->new_Dec(33, 3735928559);
	       $vec->Resize(32); # suppress leading 0 if unwanted
	       $hex = $vec->to_Hex();

       How do I convert from octal to decimal
	   Using Perl's built in conversion of numbers with leading zeros:

	       $dec = 033653337357; # note the leading 0!

	   Using the oct function:

	       $dec = oct("33653337357");

	   Using Bit::Vector:

	       use Bit::Vector;
	       $vec = Bit::Vector->new(32);
	       $vec->Chunk_List_Store(3, split(//, reverse "33653337357"));
	       $dec = $vec->to_Dec();

       How do I convert from decimal to octal
	   Using sprintf:

	       $oct = sprintf("%o", 3735928559);

	   Using Bit::Vector:

	       use Bit::Vector;
	       $vec = Bit::Vector->new_Dec(32, -559038737);
	       $oct = reverse join('', $vec->Chunk_List_Read(3));

       How do I convert from binary to decimal
	   Perl 5.6 lets you write binary numbers directly with the 0b nota-
	   tion:

	       $number = 0b10110110;

	   Using oct:

	       my $input = "10110110";
	       $decimal = oct( "0b$input" );

	   Using pack and ord:

	       $decimal = ord(pack('B8', '10110110'));

	   Using pack and unpack for larger strings:

	       $int = unpack("N", pack("B32",
		   substr("0" x 32 . "11110101011011011111011101111", -32)));
	       $dec = sprintf("%d", $int);

	       # substr() is used to left pad a 32 character string with zeros.

	   Using Bit::Vector:

	       $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111");
	       $dec = $vec->to_Dec();

       How do I convert from decimal to binary
	   Using sprintf (perl 5.6+):

	       $bin = sprintf("%b", 3735928559);

	   Using unpack:

	       $bin = unpack("B*", pack("N", 3735928559));

	   Using Bit::Vector:

	       use Bit::Vector;
	       $vec = Bit::Vector->new_Dec(32, -559038737);
	       $bin = $vec->to_Bin();

	   The remaining transformations (e.g. hex -> oct, bin -> hex, etc.)
	   are left as an exercise to the inclined reader.

       Why doesn't & work the way I want it to?

       The behavior of binary arithmetic operators depends on whether they're
       used on numbers or strings.  The operators treat a string as a series
       of bits and work with that (the string "3" is the bit pattern
       00110011).  The operators work with the binary form of a number (the
       number 3 is treated as the bit pattern 00000011).

       So, saying "11 & 3" performs the "and" operation on numbers (yielding
       3).  Saying "11" & "3" performs the "and" operation on strings (yield-
       ing "1").

       Most problems with "&" and "|" arise because the programmer thinks they
       have a number but really it's a string.	The rest arise because the
       programmer says:

	   if ("\020\020" & "\101\101") {
	       # ...
	   }

       but a string consisting of two null bytes (the result of ""\020\020" &
       "\101\101"") is not a false value in Perl.  You need:

	   if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
	       # ...
	   }

       How do I multiply matrices?

       Use the Math::Matrix or Math::MatrixReal modules (available from CPAN)
       or the PDL extension (also available from CPAN).

       How do I perform an operation on a series of integers?

       To call a function on each element in an array, and collect the
       results, use:

	   @results = map { my_func($_) } @array;

       For example:

	   @triple = map { 3 * $_ } @single;

       To call a function on each element of an array, but ignore the results:

	   foreach $iterator (@array) {
	       some_func($iterator);
	   }

       To call a function on each integer in a (small) range, you can use:

	   @results = map { some_func($_) } (5 .. 25);

       but you should be aware that the ".." operator creates an array of all
       integers in the range.  This can take a lot of memory for large ranges.
       Instead use:

	   @results = ();
	   for ($i=5; $i < 500_005; $i++) {
	       push(@results, some_func($i));
	   }

       This situation has been fixed in Perl5.005. Use of ".." in a "for" loop
       will iterate over the range, without creating the entire range.

	   for my $i (5 .. 500_005) {
	       push(@results, some_func($i));
	   }

       will not create a list of 500,000 integers.

       How can I output Roman numerals?

       Get the http://www.cpan.org/modules/by-module/Roman module.

       Why aren't my random numbers random?

       If you're using a version of Perl before 5.004, you must call "srand"
       once at the start of your program to seed the random number generator.

		BEGIN { srand() if $] < 5.004 }

       5.004 and later automatically call "srand" at the beginning.  Don't
       call "srand" more than once---you make your numbers less random, rather
       than more.

       Computers are good at being predictable and bad at being random
       (despite appearances caused by bugs in your programs :-).  see the ran-
       dom article in the "Far More Than You Ever Wanted To Know" collection
       in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz , courtesy of Tom
       Phoenix, talks more about this.	John von Neumann said, "Anyone who
       attempts to generate random numbers by deterministic means is, of
       course, living in a state of sin."

       If you want numbers that are more random than "rand" with "srand" pro-
       vides, you should also check out the Math::TrulyRandom module from
       CPAN.  It uses the imperfections in your system's timer to generate
       random numbers, but this takes quite a while.  If you want a better
       pseudorandom generator than comes with your operating system, look at
       "Numerical Recipes in C" at http://www.nr.com/ .

       How do I get a random number between X and Y?

       "rand($x)" returns a number such that "0 <= rand($x) < $x". Thus what
       you want to have perl figure out is a random number in the range from 0
       to the difference between your X and Y.

       That is, to get a number between 10 and 15, inclusive, you want a ran-
       dom number between 0 and 5 that you can then add to 10.

	   my $number = 10 + int rand( 15-10+1 );

       Hence you derive the following simple function to abstract that. It
       selects a random integer between the two given integers (inclusive),
       For example: "random_int_in(50,120)".

	  sub random_int_in ($$) {
	    my($min, $max) = @_;
	     # Assumes that the two arguments are integers themselves!
	    return $min if $min == $max;
	    ($min, $max) = ($max, $min)  if  $min > $max;
	    return $min + int rand(1 + $max - $min);
	  }

Data: Dates
       How do I find the day or week of the year?

       The localtime function returns the day of the year.  Without an argu-
       ment localtime uses the current time.

	       $day_of_year = (localtime)[7];

       The POSIX module can also format a date as the day of the year or week
       of the year.

	       use POSIX qw/strftime/;
	       my $day_of_year	= strftime "%j", localtime;
	       my $week_of_year = strftime "%W", localtime;

       To get the day of year for any date, use the Time::Local module to get
       a time in epoch seconds for the argument to localtime.

	       use POSIX qw/strftime/;
	       use Time::Local;
	       my $week_of_year = strftime "%W",
		       localtime( timelocal( 0, 0, 0, 18, 11, 1987 ) );

       The Date::Calc module provides two functions to calculate these.

	       use Date::Calc;
	       my $day_of_year	= Day_of_Year(	1987, 12, 18 );
	       my $week_of_year = Week_of_Year( 1987, 12, 18 );

       How do I find the current century or millennium?

       Use the following simple functions:

	   sub get_century    {
	       return int((((localtime(shift || time))[5] + 1999))/100);
	   }

	   sub get_millennium {
	       return 1+int((((localtime(shift || time))[5] + 1899))/1000);
	   }

       On some systems, the POSIX module's strftime() function has been
       extended in a non-standard way to use a %C format, which they sometimes
       claim is the "century".	It isn't, because on most such systems, this
       is only the first two digits of the four-digit year, and thus cannot be
       used to reliably determine the current century or millennium.

       How can I compare two dates and find the difference?

       (contributed by brian d foy)

       You could just store all your dates as a number and then subtract. Life
       isn't always that simple though. If you want to work with formatted
       dates, the Date::Manip, Date::Calc, or DateTime modules can help you.

       How can I take a string and turn it into epoch seconds?

       If it's a regular enough string that it always has the same format, you
       can split it up and pass the parts to "timelocal" in the standard
       Time::Local module.  Otherwise, you should look into the Date::Calc and
       Date::Manip modules from CPAN.

       How can I find the Julian Day?

       (contributed by brian d foy and Dave Cross)

       You can use the Time::JulianDay module available on CPAN.  Ensure that
       you really want to find a Julian day, though, as many people have dif-
       ferent ideas about Julian days.	See http://www.her-
       metic.ch/cal_stud/jdn.htm for instance.

       You can also try the DateTime module, which can convert a date/time to
       a Julian Day.

	 $ perl -MDateTime -le'print DateTime->today->jd'
	 2453401.5

       Or the modified Julian Day

	 $ perl -MDateTime -le'print DateTime->today->mjd'
	 53401

       Or even the day of the year (which is what some people think of as a
       Julian day)

	 $ perl -MDateTime -le'print DateTime->today->doy'
	 31

       How do I find yesterday's date?

       (contributed by brian d foy)

       Use one of the Date modules. The "DateTime" module makes it simple, and
       give you the same time of day, only the day before.

	       use DateTime;

	       my $yesterday = DateTime->now->subtract( days => 1 );

	       print "Yesterday was $yesterday\n";

       You can also use the "Date::Calc" module using its Today_and_Now func-
       tion.

	       use Date::Calc qw( Today_and_Now Add_Delta_DHMS );

	       my @date_time = Add_Delta_DHMS( Today_and_Now(), -1, 0, 0, 0 );

	       print "@date\n";

       Most people try to use the time rather than the calendar to figure out
       dates, but that assumes that days are twenty-four hours each.  For most
       people, there are two days a year when they aren't: the switch to and
       from summer time throws this off. Let the modules do the work.

       Does Perl have a Year 2000 problem?  Is Perl Y2K compliant?

       Short answer: No, Perl does not have a Year 2000 problem.  Yes, Perl is
       Y2K compliant (whatever that means).  The programmers you've hired to
       use it, however, probably are not.

       Long answer: The question belies a true understanding of the issue.
       Perl is just as Y2K compliant as your pencil--no more, and no less.
       Can you use your pencil to write a non-Y2K-compliant memo?  Of course
       you can.  Is that the pencil's fault?  Of course it isn't.

       The date and time functions supplied with Perl (gmtime and localtime)
       supply adequate information to determine the year well beyond 2000
       (2038 is when trouble strikes for 32-bit machines).  The year returned
       by these functions when used in a list context is the year minus 1900.
       For years between 1910 and 1999 this happens to be a 2-digit decimal
       number. To avoid the year 2000 problem simply do not treat the year as
       a 2-digit number.  It isn't.

       When gmtime() and localtime() are used in scalar context they return a
       timestamp string that contains a fully-expanded year.  For example,
       "$timestamp = gmtime(1005613200)" sets $timestamp to "Tue Nov 13
       01:00:00 2001".	There's no year 2000 problem here.

       That doesn't mean that Perl can't be used to create non-Y2K compliant
       programs.  It can.  But so can your pencil.  It's the fault of the
       user, not the language.	At the risk of inflaming the NRA: "Perl
       doesn't break Y2K, people do."  See http://www.perl.org/about/y2k.html
       for a longer exposition.

Data: Strings
       How do I validate input?

       (contributed by brian d foy)

       There are many ways to ensure that values are what you expect or want
       to accept. Besides the specific examples that we cover in the perlfaq,
       you can also look at the modules with "Assert" and "Validate" in their
       names, along with other modules such as "Regexp::Common".

       Some modules have validation for particular types of input, such as
       "Business::ISBN", "Business::CreditCard", "Email::Valid", and
       "Data::Validate::IP".

       How do I unescape a string?

       It depends just what you mean by "escape".  URL escapes are dealt with
       in perlfaq9.  Shell escapes with the backslash ("\") character are
       removed with

	   s/\\(.)/$1/g;

       This won't expand "\n" or "\t" or any other special escapes.

       How do I remove consecutive pairs of characters?

       (contributed by brian d foy)

       You can use the substitution operator to find pairs of characters (or
       runs of characters) and replace them with a single instance. In this
       substitution, we find a character in "(.)". The memory parentheses
       store the matched character in the back-reference "\1" and we use that
       to require that the same thing immediately follow it. We replace that
       part of the string with the character in $1.

	   s/(.)\1/$1/g;

       We can also use the transliteration operator, "tr///". In this example,
       the search list side of our "tr///" contains nothing, but the "c"
       option complements that so it contains everything. The replacement list
       also contains nothing, so the transliteration is almost a no-op since
       it won't do any replacements (or more exactly, replace the character
       with itself). However, the "s" option squashes duplicated and consecu-
       tive characters in the string so a character does not show up next to
       itself

	       my $str = 'Haarlem';   # in the Netherlands
	   $str =~ tr///cs;	  # Now Harlem, like in New York

       How do I expand function calls in a string?

       (contributed by brian d foy)

       This is documented in perlref, and although it's not the easiest thing
       to read, it does work. In each of these examples, we call the function
       inside the braces used to dereference a reference. If we have a more
       than one return value, we can construct and dereference an anonymous
       array. In this case, we call the function in list context.

	       print "The time values are @{ [localtime] }.\n";

       If we want to call the function in scalar context, we have to do a bit
       more work. We can really have any code we like inside the braces, so we
       simply have to end with the scalar reference, although how you do that
       is up to you, and you can use code inside the braces.

	       print "The time is ${\(scalar localtime)}.\n"

	       print "The time is ${ my $x = localtime; \$x }.\n";

       If your function already returns a reference, you don't need to create
       the reference yourself.

	       sub timestamp { my $t = localtime; \$t }

	       print "The time is ${ timestamp() }.\n";

       The "Interpolation" module can also do a lot of magic for you. You can
       specify a variable name, in this case "E", to set up a tied hash that
       does the interpolation for you. It has several other methods to do this
       as well.

	       use Interpolation E => 'eval';
	       print "The time values are $E{localtime()}.\n";

       In most cases, it is probably easier to simply use string concatena-
       tion, which also forces scalar context.

	       print "The time is " . localtime . ".\n";

       How do I find matching/nesting anything?

       This isn't something that can be done in one regular expression, no
       matter how complicated.	To find something between two single charac-
       ters, a pattern like "/x([^x]*)x/" will get the intervening bits in $1.
       For multiple ones, then something more like "/alpha(.*?)omega/" would
       be needed.  But none of these deals with nested patterns.  For balanced
       expressions using "(", "{", "[" or "<" as delimiters, use the CPAN mod-
       ule Regexp::Common, or see "(??{ code })" in perlre.  For other cases,
       you'll have to write a parser.

       If you are serious about writing a parser, there are a number of mod-
       ules or oddities that will make your life a lot easier.	There are the
       CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced; and
       the byacc program.   Starting from perl 5.8 the Text::Balanced is part
       of the standard distribution.

       One simple destructive, inside-out approach that you might try is to
       pull out the smallest nesting parts one at a time:

	   while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
	       # do something with $1
	   }

       A more complicated and sneaky approach is to make Perl's regular
       expression engine do it for you.  This is courtesy Dean Inada, and
       rather has the nature of an Obfuscated Perl Contest entry, but it
       really does work:

	   # $_ contains the string to parse
	   # BEGIN and END are the opening and closing markers for the
	   # nested text.

	   @( = ('(','');
	   @) = (')','');
	   ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
	   @$ = (eval{/$re/},$@!~/unmatched/i);
	   print join("\n",@$[0..$#$]) if( $$[-1] );

       How do I reverse a string?

       Use reverse() in scalar context, as documented in "reverse" in perl-
       func.

	   $reversed = reverse $string;

       How do I expand tabs in a string?

       You can do it yourself:

	   1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;

       Or you can just use the Text::Tabs module (part of the standard Perl
       distribution).

	   use Text::Tabs;
	   @expanded_lines = expand(@lines_with_tabs);

       How do I reformat a paragraph?

       Use Text::Wrap (part of the standard Perl distribution):

	   use Text::Wrap;
	   print wrap("\t", '  ', @paragraphs);

       The paragraphs you give to Text::Wrap should not contain embedded new-
       lines.  Text::Wrap doesn't justify the lines (flush-right).

       Or use the CPAN module Text::Autoformat.  Formatting files can be eas-
       ily done by making a shell alias, like so:

	   alias fmt="perl -i -MText::Autoformat -n0777 \
	       -e 'print autoformat $_, {all=>1}' $*"

       See the documentation for Text::Autoformat to appreciate its many capa-
       bilities.

       How can I access or change N characters of a string?

       You can access the first characters of a string with substr().  To get
       the first character, for example, start at position 0 and grab the
       string of length 1.

	       $string = "Just another Perl Hacker";
	   $first_char = substr( $string, 0, 1 );  #  'J'

       To change part of a string, you can use the optional fourth argument
       which is the replacement string.

	   substr( $string, 13, 4, "Perl 5.8.0" );

       You can also use substr() as an lvalue.

	   substr( $string, 13, 4 ) =  "Perl 5.8.0";

       How do I change the Nth occurrence of something?

       You have to keep track of N yourself.  For example, let's say you want
       to change the fifth occurrence of "whoever" or "whomever" into "whoso-
       ever" or "whomsoever", case insensitively.  These all assume that $_
       contains the string to be altered.

	   $count = 0;
	   s{((whom?)ever)}{
	       ++$count == 5	       # is it the 5th?
		   ? "${2}soever"      # yes, swap
		   : $1 	       # renege and leave it there
	   }ige;

       In the more general case, you can use the "/g" modifier in a "while"
       loop, keeping count of matches.

	   $WANT = 3;
	   $count = 0;
	   $_ = "One fish two fish red fish blue fish";
	   while (/(\w+)\s+fish\b/gi) {
	       if (++$count == $WANT) {
		   print "The third fish is a $1 one.\n";
	       }
	   }

       That prints out: "The third fish is a red one."	You can also use a
       repetition count and repeated pattern like this:

	   /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;

       How can I count the number of occurrences of a substring within a
       string?

       There are a number of ways, with varying efficiency.  If you want a
       count of a certain single character (X) within a string, you can use
       the "tr///" function like so:

	   $string = "ThisXlineXhasXsomeXx'sXinXit";
	   $count = ($string =~ tr/X//);
	   print "There are $count X characters in the string";

       This is fine if you are just looking for a single character.  However,
       if you are trying to count multiple character substrings within a
       larger string, "tr///" won't work.  What you can do is wrap a while()
       loop around a global pattern match.  For example, let's count negative
       integers:

	   $string = "-9 55 48 -2 23 -76 4 14 -44";
	   while ($string =~ /-\d+/g) { $count++ }
	   print "There are $count negative numbers in the string";

       Another version uses a global match in list context, then assigns the
       result to a scalar, producing a count of the number of matches.

	       $count = () = $string =~ /-\d+/g;

       How do I capitalize all the words on one line?

       To make the first letter of each word upper case:

	       $line =~ s/\b(\w)/\U$1/g;

       This has the strange effect of turning ""don't do it"" into ""Don'T Do
       It"".  Sometimes you might want this.  Other times you might need a
       more thorough solution (Suggested by brian d foy):

	   $string =~ s/ (
			(^\w)	 #at the beginning of the line
			  |	 # or
			(\s\w)	 #preceded by whitespace
			  )
		       /\U$1/xg;
	   $string =~ /([\w']+)/\u\L$1/g;

       To make the whole line upper case:

	       $line = uc($line);

       To force each word to be lower case, with the first letter upper case:

	       $line =~ s/(\w+)/\u\L$1/g;

       You can (and probably should) enable locale awareness of those charac-
       ters by placing a "use locale" pragma in your program.  See perllocale
       for endless details on locales.

       This is sometimes referred to as putting something into "title case",
       but that's not quite accurate.  Consider the proper capitalization of
       the movie Dr. Strangelove or: How I Learned to Stop Worrying and Love
       the Bomb, for example.

       Damian Conway's Text::Autoformat module provides some smart case trans-
       formations:

	   use Text::Autoformat;
	   my $x = "Dr. Strangelove or: How I Learned to Stop ".
	     "Worrying and Love the Bomb";

	   print $x, "\n";
	   for my $style (qw( sentence title highlight ))
	   {
	       print autoformat($x, { case => $style }), "\n";
	   }

       How can I split a [character] delimited string except when inside
       [character]?

       Several modules can handle this sort of pasing---Text::Balanced,
       Text::CSV, Text::CSV_XS, and Text::ParseWords, among others.

       Take the example case of trying to split a string that is comma-sepa-
       rated into its different fields. You can't use "split(/,/)" because you
       shouldn't split if the comma is inside quotes.  For example, take a
       data line like this:

	   SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"

       Due to the restriction of the quotes, this is a fairly complex problem.
       Thankfully, we have Jeffrey Friedl, author of Mastering Regular Expres-
       sions, to handle these for us.  He suggests (assuming your string is
       contained in $text):

	    @new = ();
	    push(@new, $+) while $text =~ m{
		"([^\"\\]*(?:\\.[^\"\\]*)*)",?	# groups the phrase inside the quotes
	      | ([^,]+),?
	      | ,
	    }gx;
	    push(@new, undef) if substr($text,-1,1) eq ',';

       If you want to represent quotation marks inside a quotation-mark-delim-
       ited field, escape them with backslashes (eg, "like \"this\"".

       Alternatively, the Text::ParseWords module (part of the standard Perl
       distribution) lets you say:

	   use Text::ParseWords;
	   @new = quotewords(",", 0, $text);

       There's also a Text::CSV (Comma-Separated Values) module on CPAN.

       How do I strip blank space from the beginning/end of a string?

       (contributed by brian d foy)

       A substitution can do this for you. For a single line, you want to
       replace all the leading or trailing whitespace with nothing. You can do
       that with a pair of substitutions.

	       s/^\s+//;
	       s/\s+$//;

       You can also write that as a single substitution, although it turns out
       the combined statement is slower than the separate ones. That might not
       matter to you, though.

	       s/^\s+|\s+$//g;

       In this regular expression, the alternation matches either at the
       beginning or the end of the string since the anchors have a lower
       precedence than the alternation. With the "/g" flag, the substitution
       makes all possible matches, so it gets both. Remember, the trailing
       newline matches the "\s+", and  the "$" anchor can match to the physi-
       cal end of the string, so the newline disappears too. Just add the new-
       line to the output, which has the added benefit of preserving "blank"
       (consisting entirely of whitespace) lines which the "^\s+" would remove
       all by itself.

	       while( <> )
		       {
		       s/^\s+|\s+$//g;
		       print "$_\n";
		       }

       For a multi-line string, you can apply the regular expression to each
       logical line in the string by adding the "/m" flag (for "multi-line").
       With the "/m" flag, the "$" matches before an embedded newline, so it
       doesn't remove it. It still removes the newline at the end of the
       string.

	   $string =~ s/^\s+|\s+$//gm;

       Remember that lines consisting entirely of whitespace will disappear,
       since the first part of the alternation can match the entire string and
       replace it with nothing. If need to keep embedded blank lines, you have
       to do a little more work. Instead of matching any whitespace (since
       that includes a newline), just match the other whitespace.

	       $string =~ s/^[\t\f ]+|[\t\f ]+$//mg;

       How do I pad a string with blanks or pad a number with zeroes?

       In the following examples, $pad_len is the length to which you wish to
       pad the string, $text or $num contains the string to be padded, and
       $pad_char contains the padding character. You can use a single charac-
       ter string constant instead of the $pad_char variable if you know what
       it is in advance. And in the same way you can use an integer in place
       of $pad_len if you know the pad length in advance.

       The simplest method uses the "sprintf" function. It can pad on the left
       or right with blanks and on the left with zeroes and it will not trun-
       cate the result. The "pack" function can only pad strings on the right
       with blanks and it will truncate the result to a maximum length of
       $pad_len.

	   # Left padding a string with blanks (no truncation):
	       $padded = sprintf("%${pad_len}s", $text);
	       $padded = sprintf("%*s", $pad_len, $text);  # same thing

	   # Right padding a string with blanks (no truncation):
	       $padded = sprintf("%-${pad_len}s", $text);
	       $padded = sprintf("%-*s", $pad_len, $text); # same thing

	   # Left padding a number with 0 (no truncation):
	       $padded = sprintf("%0${pad_len}d", $num);
	       $padded = sprintf("%0*d", $pad_len, $num); # same thing

	   # Right padding a string with blanks using pack (will truncate):
	   $padded = pack("A$pad_len",$text);

       If you need to pad with a character other than blank or zero you can
       use one of the following methods.  They all generate a pad string with
       the "x" operator and combine that with $text. These methods do not
       truncate $text.

       Left and right padding with any character, creating a new string:

	   $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
	   $padded = $text . $pad_char x ( $pad_len - length( $text ) );

       Left and right padding with any character, modifying $text directly:

	   substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
	   $text .= $pad_char x ( $pad_len - length( $text ) );

       How do I extract selected columns from a string?

       Use substr() or unpack(), both documented in perlfunc.  If you prefer
       thinking in terms of columns instead of widths, you can use this kind
       of thing:

	   # determine the unpack format needed to split Linux ps output
	   # arguments are cut columns
	   my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);

	   sub cut2fmt {
	       my(@positions) = @_;
	       my $template  = '';
	       my $lastpos   = 1;
	       for my $place (@positions) {
		   $template .= "A" . ($place - $lastpos) . " ";
		   $lastpos   = $place;
	       }
	       $template .= "A*";
	       return $template;
	   }

       How do I find the soundex value of a string?

       (contributed by brian d foy)

       You can use the Text::Soundex module. If you want to do fuzzy or close
       matching, you might also try the String::Approx, and Text::Metaphone,
       and Text::DoubleMetaphone modules.

       How can I expand variables in text strings?

       Let's assume that you have a string that contains placeholder vari-
       ables.

	   $text = 'this has a $foo in it and a $bar';

       You can use a substitution with a double evaluation.  The first /e
       turns $1 into $foo, and the second /e turns $foo into its value.  You
       may want to wrap this in an "eval": if you try to get the value of an
       undeclared variable while running under "use strict", you get a fatal
       error.

	   eval { $text =~ s/(\$\w+)/$1/eeg };
	   die if $@;

       It's probably better in the general case to treat those variables as
       entries in some special hash.  For example:

	   %user_defs = (
	       foo  => 23,
	       bar  => 19,
	   );
	   $text =~ s/\$(\w+)/$user_defs{$1}/g;

       What's wrong with always quoting "$vars"?

       The problem is that those double-quotes force stringification-- coerc-
       ing numbers and references into strings--even when you don't want them
       to be strings.  Think of it this way: double-quote expansion is used to
       produce new strings.  If you already have a string, why do you need
       more?

       If you get used to writing odd things like these:

	   print "$var";       # BAD
	   $new = "$old";      # BAD
	   somefunc("$var");   # BAD

       You'll be in trouble.  Those should (in 99.8% of the cases) be the sim-
       pler and more direct:

	   print $var;
	   $new = $old;
	   somefunc($var);

       Otherwise, besides slowing you down, you're going to break code when
       the thing in the scalar is actually neither a string nor a number, but
       a reference:

	   func(\@array);
	   sub func {
	       my $aref = shift;
	       my $oref = "$aref";  # WRONG
	   }

       You can also get into subtle problems on those few operations in Perl
       that actually do care about the difference between a string and a num-
       ber, such as the magical "++" autoincrement operator or the syscall()
       function.

       Stringification also destroys arrays.

	   @lines = `command`;
	   print "@lines";	       # WRONG - extra blanks
	   print @lines;	       # right

       Why don't my < documents work?

       Check for these three things:

       There must be no space after the << part.
       There (probably) should be a semicolon at the end.
       You can't (easily) have any space in front of the tag.

       If you want to indent the text in the here document, you can do this:

	   # all in one
	   ($VAR = <op_ppaddr)() );
	       @@@     TAINT_NOT;
	       @@@     return 0;
	       @@@ }
	   MAIN_INTERPRETER_LOOP

       Or with a fixed amount of leading whitespace, with remaining indenta-
       tion correctly preserved:

	   $poem = fix<Data: Arrays
       What is the difference between a list and an array?

       An array has a changeable length.  A list does not.  An array is some-
       thing you can push or pop, while a list is a set of values.  Some peo-
       ple make the distinction that a list is a value while an array is a
       variable.  Subroutines are passed and return lists, you put things into
       list context, you initialize arrays with lists, and you foreach()
       across a list.  "@" variables are arrays, anonymous arrays are arrays,
       arrays in scalar context behave like the number of elements in them,
       subroutines access their arguments through the array @_, and
       push/pop/shift only work on arrays.

       As a side note, there's no such thing as a list in scalar context.
       When you say

	   $scalar = (2, 5, 7, 9);

       you're using the comma operator in scalar context, so it uses the
       scalar comma operator.  There never was a list there at all!  This
       causes the last value to be returned: 9.

       What is the difference between $array[1] and @array[1]?

       The former is a scalar value; the latter an array slice, making it a
       list with one (scalar) value.  You should use $ when you want a scalar
       value (most of the time) and @ when you want a list with one scalar
       value in it (very, very rarely; nearly never, in fact).

       Sometimes it doesn't make a difference, but sometimes it does.  For
       example, compare:

	   $good[0] = `some program that outputs several lines`;

       with

	   @bad[0]  = `same program that outputs several lines`;

       The "use warnings" pragma and the -w flag will warn you about these
       matters.

       How can I remove duplicate elements from a list or array?

       (contributed by brian d foy)

       Use a hash. When you think the words "unique" or "duplicated", think
       "hash keys".

       If you don't care about the order of the elements, you could just cre-
       ate the hash then extract the keys. It's not important how you create
       that hash: just that you use "keys" to get the unique elements.

	  my %hash   = map { $_, 1 } @array;
	  # or a hash slice: @hash{ @array } = ();
	  # or a foreach: $hash{$_} = 1 foreach ( @array );

	  my @unique = keys %hash;

       You can also go through each element and skip the ones you've seen
       before. Use a hash to keep track. The first time the loop sees an ele-
       ment, that element has no key in %Seen. The "next" statement creates
       the key and immediately uses its value, which is "undef", so the loop
       continues to the "push" and increments the value for that key. The next
       time the loop sees that same element, its key exists in the hash and
       the value for that key is true (since it's not 0 or undef), so the next
       skips that iteration and the loop goes to the next element.

	       my @unique = ();
	       my %seen   = ();

	       foreach my $elem ( @array )
		       {
		       next if $seen{ $elem }++;
		       push @unique, $elem;
		       }

       You can write this more briefly using a grep, which does the same
       thing.

	  my %seen = ();
	  my @unique = grep { ! $seen{ $_ }++ } @array;

       How can I tell whether a certain element is contained in a list or
       array?

       (portions of this answer contributed by Anno Siegel)

       Hearing the word "in" is an indication that you probably should have
       used a hash, not a list or array, to store your data.  Hashes are
       designed to answer this question quickly and efficiently.  Arrays
       aren't.

       That being said, there are several ways to approach this.  If you are
       going to make this query many times over arbitrary string values, the
       fastest way is probably to invert the original array and maintain a
       hash whose keys are the first array's values.

	   @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
	   %is_blue = ();
	   for (@blues) { $is_blue{$_} = 1 }

       Now you can check whether $is_blue{$some_color}.  It might have been a
       good idea to keep the blues all in a hash in the first place.

       If the values are all small integers, you could use a simple indexed
       array.  This kind of an array will take up less space:

	   @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
	   @is_tiny_prime = ();
	   for (@primes) { $is_tiny_prime[$_] = 1 }
	   # or simply	@istiny_prime[@primes] = (1) x @primes;

       Now you check whether $is_tiny_prime[$some_number].

       If the values in question are integers instead of strings, you can save
       quite a lot of space by using bit strings instead:

	   @articles = ( 1..10, 150..2000, 2017 );
	   undef $read;
	   for (@articles) { vec($read,$_,1) = 1 }

       Now check whether "vec($read,$n,1)" is true for some $n.

       These methods guarantee fast individual tests but require a re-organi-
       zation of the original list or array.  They only pay off if you have to
       test multiple values against the same array.

       If you are testing only once, the standard module List::Util exports
       the function "first" for this purpose.  It works by stopping once it
       finds the element. It's written in C for speed, and its Perl equivalant
       looks like this subroutine:

	       sub first (&@) {
		       my $code = shift;
		       foreach (@_) {
			       return $_ if &{$code}();
		       }
		       undef;
	       }

       If speed is of little concern, the common idiom uses grep in scalar
       context (which returns the number of items that passed its condition)
       to traverse the entire list. This does have the benefit of telling you
       how many matches it found, though.

	       my $is_there = grep $_ eq $whatever, @array;

       If you want to actually extract the matching elements, simply use grep
       in list context.

	       my @matches = grep $_ eq $whatever, @array;

       How do I compute the difference of two arrays?  How do I compute the
       intersection of two arrays?

       Use a hash.  Here's code to do both and more.  It assumes that each
       element is unique in a given array:

	   @union = @intersection = @difference = ();
	   %count = ();
	   foreach $element (@array1, @array2) { $count{$element}++ }
	   foreach $element (keys %count) {
	       push @union, $element;
	       push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
	   }

       Note that this is the symmetric difference, that is, all elements in
       either A or in B but not in both.  Think of it as an xor operation.

       How do I test whether two arrays or hashes are equal?

       The following code works for single-level arrays.  It uses a stringwise
       comparison, and does not distinguish defined versus undefined empty
       strings.  Modify if you have other needs.

	   $are_equal = compare_arrays(\@frogs, \@toads);

	   sub compare_arrays {
	       my ($first, $second) = @_;
	       no warnings;  # silence spurious -w undef complaints
	       return 0 unless @$first == @$second;
	       for (my $i = 0; $i < @$first; $i++) {
		   return 0 if $first->[$i] ne $second->[$i];
	       }
	       return 1;
	   }

       For multilevel structures, you may wish to use an approach more like
       this one.  It uses the CPAN module FreezeThaw:

	   use FreezeThaw qw(cmpStr);
	   @a = @b = ( "this", "that", [ "more", "stuff" ] );

	   printf "a and b contain %s arrays\n",
	       cmpStr(\@a, \@b) == 0
		   ? "the same"
		   : "different";

       This approach also works for comparing hashes.  Here we'll demonstrate
       two different answers:

	   use FreezeThaw qw(cmpStr cmpStrHard);

	   %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
	   $a{EXTRA} = \%b;
	   $b{EXTRA} = \%a;

	   printf "a and b contain %s hashes\n",
	       cmpStr(\%a, \%b) == 0 ? "the same" : "different";

	   printf "a and b contain %s hashes\n",
	       cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";

       The first reports that both those the hashes contain the same data,
       while the second reports that they do not.  Which you prefer is left as
       an exercise to the reader.

       How do I find the first array element for which a condition is true?

       To find the first array element which satisfies a condition, you can
       use the first() function in the List::Util module, which comes with
       Perl 5.8.  This example finds the first element that contains "Perl".

	       use List::Util qw(first);

	       my $element = first { /Perl/ } @array;

       If you cannot use List::Util, you can make your own loop to do the same
       thing.  Once you find the element, you stop the loop with last.

	       my $found;
	       foreach ( @array )
		       {
		       if( /Perl/ ) { $found = $_; last }
		       }

       If you want the array index, you can iterate through the indices and
       check the array element at each index until you find one that satisfies
       the condition.

	       my( $found, $index ) = ( undef, -1 );
	       for( $i = 0; $i < @array; $i++ )
		       {
		       if( $array[$i] =~ /Perl/ )
			       {
			       $found = $array[$i];
			       $index = $i;
			       last;
			       }
		       }

       How do I handle linked lists?

       In general, you usually don't need a linked list in Perl, since with
       regular arrays, you can push and pop or shift and unshift at either
       end, or you can use splice to add and/or remove arbitrary number of
       elements at arbitrary points.  Both pop and shift are both O(1) opera-
       tions on Perl's dynamic arrays.	In the absence of shifts and pops,
       push in general needs to reallocate on the order every log(N) times,
       and unshift will need to copy pointers each time.

       If you really, really wanted, you could use structures as described in
       perldsc or perltoot and do just what the algorithm book tells you to
       do.  For example, imagine a list node like this:

	   $node = {
	       VALUE => 42,
	       LINK  => undef,
	   };

       You could walk the list this way:

	   print "List: ";
	   for ($node = $head;	$node; $node = $node->{LINK}) {
	       print $node->{VALUE}, " ";
	   }
	   print "\n";

       You could add to the list this way:

	   my ($head, $tail);
	   $tail = append($head, 1);	   # grow a new head
	   for $value ( 2 .. 10 ) {
	       $tail = append($tail, $value);
	   }

	   sub append {
	       my($list, $value) = @_;
	       my $node = { VALUE => $value };
	       if ($list) {
		   $node->{LINK} = $list->{LINK};
		   $list->{LINK} = $node;
	       } else {
		   $_[0] = $node;      # replace caller's version
	       }
	       return $node;
	   }

       But again, Perl's built-in are virtually always good enough.

       How do I handle circular lists?

       Circular lists could be handled in the traditional fashion with linked
       lists, or you could just do something like this with an array:

	   unshift(@array, pop(@array));  # the last shall be first
	   push(@array, shift(@array));   # and vice versa

       How do I shuffle an array randomly?

       If you either have Perl 5.8.0 or later installed, or if you have
       Scalar-List-Utils 1.03 or later installed, you can say:

	   use List::Util 'shuffle';

	       @shuffled = shuffle(@list);

       If not, you can use a Fisher-Yates shuffle.

	   sub fisher_yates_shuffle {
	       my $deck = shift;  # $deck is a reference to an array
	       my $i = @$deck;
	       while (--$i) {
		   my $j = int rand ($i+1);
		   @$deck[$i,$j] = @$deck[$j,$i];
	       }
	   }

	   # shuffle my mpeg collection
	   #
	   my @mpeg = 
=28515
+132
(46)