perlfaq4 - Data Manipulation ($Revision: 1.56 $, $Date: 2004/11/03
22:47:56 $)
DESCRIPTION
This section of the FAQ answers questions related to manipulating num-
bers, dates, strings, arrays, hashes, and miscellaneous data issues.
DDaattaa:: NNuummbbeerrss
WWhhyy aamm II ggeettttiinngg lloonngg ddeecciimmaallss ((eegg,, 1199..99449999999999999999999999)) iinnsstteeaadd ooff tthhee
nnuummbbeerrss II sshhoouulldd bbee ggeettttiinngg ((eegg,, 1199..9955))??
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;
WWhhyy iiss int() bbrrookkeenn??
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.
WWhhyy iissnn''tt mmyy ooccttaall ddaattaa iinntteerrpprreetteedd ccoorrrreeccttllyy??
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.
DDooeess PPeerrll hhaavvee aa round() ffuunnccttiioonn?? WWhhaatt aabboouutt ceil() aanndd floor()??
TTrriigg ffuunnccttiioonnss??
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
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.
HHooww ddoo II ccoonnvveerrtt bbeettwweeeenn nnuummeerriicc rreepprreesseennttaattiioonnss//bbaasseess//rraaddiixxeess??
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->newHex(32, "DEADBEEF");
$dec = $vec->toDec();
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->newDec(32, -559038737);
$hex = $vec->toHex();
And Bit::Vector supports odd bit counts:
use Bit::Vector;
$vec = Bit::Vector->newDec(33, 3735928559);
$vec->Resize(32); # suppress leading 0 if unwanted
$hex = $vec->toHex();
How do I convert from octal to decimal
Using Perl's built in conversion of numbers with leading zeros:
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->newDec(32, -559038737);
$bin = $vec->toBin();
The remaining transformations (e.g. hex -> oct, bin -> hex, etc.)
are left as an exercise to the inclined reader.
WWhhyy ddooeessnn''tt && wwoorrkk tthhee wwaayy II wwaanntt iitt ttoo??
The behavior of binary arithmetic operators depends on whether they're
used on numbers or strings. The operators treat a string as a series
of bits and work with that (the string "3" is the bit pattern
00110011). The operators work with the binary form of a number (the
number 3 is treated as the bit pattern 00000011).
So, saying "11 & 3" performs the "and" operation on numbers (yielding
3). Saying "11" & "3" performs the "and" operation on strings (yield-
ing "1").
Most problems with "&" and "|" arise because the programmer thinks they
have a number but really it's a string. The rest arise because the
programmer says:
if ("\020\020" & "\101\101") {
# ...
}
but a string consisting of two null bytes (the result of ""\020\020" &
"\101\101"") is not a false value in Perl. You need:
if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
# ...
}
HHooww ddoo II mmuullttiippllyy mmaattrriicceess??
Use the Math::Matrix or Math::MatrixReal modules (available from CPAN)
or the PDL extension (also available from CPAN).
HHooww ddoo II ppeerrffoorrmm aann ooppeerraattiioonn oonn aa sseerriieess ooff iinntteeggeerrss??
To call a function on each element in an array, and collect the
results, use:
@results = map { myfunc($) } @array;
For example:
@triple = map { 3 * $ } @single;
To call a function on each element of an array, but ignore the results:
foreach $iterator (@array) {
somefunc($iterator);
}
To call a function on each integer in a (small) range, you ccaann use:
@results = map { somefunc($) } (5 .. 25);
but you should be aware that the ".." operator creates an array of all
integers in the range. This can take a lot of memory for large ranges.
Instead use:
@results = ();
for ($i=5; $i < 500005; $i++) {
push(@results, somefunc($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 $i (5 .. 500005) {
push(@results, somefunc($i));
}
will not create a list of 500,000 integers.
HHooww ccaann II oouuttppuutt RRoommaann nnuummeerraallss??
Get the http://www.cpan.org/modules/by-module/Roman module.
WWhhyy aarreenn''tt mmyy rraannddoomm nnuummbbeerrss rraannddoomm??
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 $] < 5.004 }
5.004 and later automatically call "srand" at the beginning. Don't
call "srand" more than once--you make your numbers less random, rather
than more.
Computers are good at being predictable and bad at being random
(despite appearances caused by bugs in your programs :-). see the ran-
dom article in the "Far More Than You Ever Wanted To Know" collection
in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz , courtesy of Tom
Phoenix, talks more about this. John von Neumann said, ``Anyone who
attempts to generate random numbers by deterministic means is, of
course, living in a state of sin.''
If you want numbers that are more random than "rand" with "srand" pro-
vides, you should also check out the Math::TrulyRandom module from
CPAN. It uses the imperfections in your system's timer to generate
random numbers, but this takes quite a while. If you want a better
pseudorandom generator than comes with your operating system, look at
``Numerical Recipes in C'' at http://www.nr.com/ .
HHooww ddoo II ggeett aa rraannddoomm nnuummbbeerr bbeettwweeeenn XX aanndd YY??
"rand($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: "randomintin(50,120)".
sub randomintin ($$) {
my($min, $max) = @;
# Assumes that the two arguments are integers themselves!
The localtime function returns the day of the week. Without an argu-
ment localtime uses the current time.
$dayofyear = (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 $dayofyear = strftime "%j", localtime;
my $weekofyear = 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 $weekofyear = strftime "%W",
localtime( timelocal( 0, 0, 0, 18, 11, 1987 ) );
The Date::Calc module provides two functions for to calculate these.
use Date::Calc;
my $dayofyear = DayofYear( 1987, 12, 18 );
my $weekofyear = WeekofYear( 1987, 12, 18 );
HHooww ddoo II ffiinndd tthhee ccuurrrreenntt cceennttuurryy oorr mmiilllleennnniiuumm??
Use the following simple functions:
sub getcentury {
return int((((localtime(shift || time))[5] + 1999))/100);
}
sub getmillennium {
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.
HHooww ccaann II ccoommppaarree ttwwoo ddaatteess aanndd ffiinndd tthhee ddiiffffeerreennccee??
If you're storing your dates as epoch seconds then simply subtract one
from the other. If you've got a structured date (distinct year, day,
month, hour, minute, seconds values), then for reasons of accessibil-
ity, simplicity, and efficiency, merely use either timelocal or timegm
(from the Time::Local module in the standard distribution) to reduce
structured dates to epoch seconds. However, if you don't know the pre-
cise format of your dates, then you should probably use either of the
Date::Manip and Date::Calc modules from CPAN before you go hacking up
your own parsing routine to handle arbitrary date formats.
HHooww ccaann II ttaakkee aa ssttrriinngg aanndd ttuurrnn iitt iinnttoo eeppoocchh sseeccoonnddss??
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.
HHooww ccaann II ffiinndd tthhee JJuulliiaann DDaayy??
Use the Time::JulianDay module (part of the Time-modules bundle avail-
able from CPAN.)
Before you immerse yourself too deeply in this, be sure to verify that
it is the Julian Day you really want. Are you interested in a way of
getting serial days so that you just can tell how many days they are
apart or so that you can do also other date arithmetic? If you are
interested in performing date arithmetic, this can be done using mod-
ules Date::Manip or Date::Calc.
There is too many details and much confusion on this issue to cover in
this FAQ, but the term is applied (correctly) to a calendar now sup-
planted by the Gregorian Calendar, with the Julian Calendar failing to
adjust properly for leap years on centennial years (among other annoy-
ances). The term is also used (incorrectly) to mean: [1] days in the
Gregorian Calendar; and [2] days since a particular starting time or
`epoch', usually 1970 in the Unix world and 1980 in the MS-DOS/Windows
world. If you find that it is not the first meaning that you really
want, then check out the Date::Manip and Date::Calc modules. (Thanks
to David Cassell for most of this text.)
HHooww ddoo II ffiinndd yyeesstteerrddaayy''ss ddaattee??
If you only need to find the date (and not the same time), you can use
the Date::Calc module.
use Date::Calc qw(Today AddDeltaDays);
my @date = AddDeltaDays( Today(), -1 );
print "@date\n";
Most people try to use the time rather than the calendar to figure out
dates, but that assumes that your 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. Russ Allbery offers this solu-
tion.
sub yesterday {
my $now = defined $[0] ? $[0] : time;
my $then = $now - 60 * 60 * 24;
my $ndst = (localtime $now)[8] > 0;
my $tdst = (localtime $then)[8] > 0;
$then - ($tdst - $ndst) * 60 * 60;
}
Should give you "this time yesterday" in seconds since epoch relative
to the first argument or the current time if no argument is given and
suitable for passing to localtime or whatever else you need to do with
it. $ndst is whether we're currently in daylight savings time; $tdst
is whether the point 24 hours ago was in daylight savings time. If
$tdst and $ndst are the same, a boundary wasn't crossed, and the cor-
rection will subtract 0. If $tdst is 1 and $ndst is 0, subtract an
hour more from yesterday's time since we gained an extra hour while
going off daylight savings time. If $tdst is 0 and $ndst is 1, sub-
tract a negative hour (add an hour) to yesterday's time since we lost
an hour.
All of this is because during those days when one switches off or onto
DST, a "day" isn't 24 hours long; it's either 23 or 25.
The explicit settings of $ndst and $tdst are necessary because local-
time only says it returns the system tm struct, and the system tm
struct at least on Solaris doesn't guarantee any particular positive
value (like, say, 1) for isdst, just a positive value. And that value
can potentially be negative, if DST information isn't available (this
sub just treats those cases like no DST).
Note that between 2am and 3am on the day after the time zone switches
off daylight savings time, the exact hour of "yesterday" corresponding
to the current hour is not clearly defined. Note also that if used
between 2am and 3am the day after the change to daylight savings time,
the result will be between 3am and 4am of the previous day; it's
arguable whether this is correct.
This sub does not attempt to deal with leap seconds (most things
don't).
DDooeess PPeerrll hhaavvee aa YYeeaarr 22000000 pprroobblleemm?? IIss PPeerrll YY22KK ccoommpplliiaanntt??
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.
DDaattaa:: SSttrriinnggss
HHooww ddoo II vvaalliiddaattee iinnppuutt??
The answer to this question is usually a regular expression, perhaps
with auxiliary logic. See the more specific questions (numbers, mail
addresses, etc.) for details.
HHooww ddoo II uunneessccaappee aa ssttrriinngg??
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.
HHooww ddoo II rreemmoovvee ccoonnsseeccuuttiivvee ppaaiirrss ooff cchhaarraacctteerrss??
To turn "abbcccd" into "abccd":
s/(.)\1/$1/g; # add /s to include newlines
Here's a solution that turns "abbcccd" to "abcd":
y///cs; # y == tr, but shorter :-)
HHooww ddoo II eexxppaanndd ffuunnccttiioonn ccaallllss iinn aa ssttrriinngg??
This is documented in perlref. In general, this is fraught with quot-
ing and readability problems, but it is possible. To interpolate a
subroutine call (in list context) into a string:
print "My sub returned @{[mysub(1,2,3)]} that time.\n";
See also ``How can I expand variables in text strings?'' in this sec-
tion of the FAQ.
HHooww ddoo II ffiinndd mmaattcchhiinngg//nneessttiinngg aannyytthhiinngg??
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
Use reverse() in scalar context, as documented in "reverse" in perl-
func.
$reversed = reverse $string;
HHooww ddoo II eexxppaanndd ttaabbss iinn aa ssttrriinngg??
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;
@expandedlines = expand(@lineswithtabs);
HHooww ddoo II rreeffoorrmmaatt aa ppaarraaggrraapphh??
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.
HHooww ccaann II aacccceessss oorr cchhaannggee NN cchhaarraacctteerrss ooff aa ssttrriinngg??
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";
$firstchar = 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";
HHooww ddoo II cchhaannggee tthhee NNtthh ooccccuurrrreennccee ooff ssoommeetthhiinngg??
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;
HHooww ccaann II ccoouunntt tthhee nnuummbbeerr ooff ooccccuurrrreenncceess ooff aa ssuubbssttrriinngg wwiitthhiinn aa
ssttrriinngg??
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;
HHooww ddoo II ccaappiittaalliizzee aallll tthhee wwoorrddss oonn oonnee lliinnee??
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";
}
HHooww ccaann II sspplliitt aa [[cchhaarraacctteerr]] ddeelliimmiitteedd ssttrriinngg eexxcceepptt wwhheenn iinnssiiddee
[[cchhaarraacctteerr]]??
Several modules can handle this sort of pasing--Text::Balanced,
Text::CVS, Text::CVSXS, 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.
HHooww ddoo II ssttrriipp bbllaannkk ssppaaccee ffrroomm tthhee bbeeggiinnnniinngg//eenndd ooff aa ssttrriinngg??
Although the simplest approach would seem to be
$string =~ s/^\s*(.*?)\s*$/$1/;
not only is this unnecessarily slow and destructive, it also fails with
embedded newlines. It is much faster to do this operation in two
steps:
$string =~ s/^\s+//;
$string =~ s/\s+$//;
Or more nicely written as:
for ($string) {
s/^\s+//;
s/\s+$//;
}
This idiom takes advantage of the "foreach" loop's aliasing behavior to
factor out common code. You can do this on several strings at once, or
arrays, or even the values of a hash if you use a slice:
# trim whitespace in the scalar, the array,
# and all the values in the hash
foreach ($scalar, @array, @hash{keys %hash}) {
s/^\s+//;
s/\s+$//;
}
HHooww ddoo II ppaadd aa ssttrriinngg wwiitthh bbllaannkkss oorr ppaadd aa nnuummbbeerr wwiitthh zzeerrooeess??
In the following examples, $padlen is the length to which you wish to
pad the string, $text or $num contains the string to be padded, and
$padchar contains the padding character. You can use a single charac-
ter string constant instead of the $padchar variable if you know what
it is in advance. And in the same way you can use an integer in place
of $padlen 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
$padlen.
# Left padding a string with blanks (no truncation):
$padded = sprintf("%${padlen}s", $text);
$padded = sprintf("%*s", $padlen, $text); # same thing
# Right padding a string with blanks (no truncation):
$padded = sprintf("%-${padlen}s", $text);
$padded = sprintf("%-*s", $padlen, $text); # same thing
# Left padding a number with 0 (no truncation):
$padded = sprintf("%0${padlen}d", $num);
$padded = sprintf("%0*d", $padlen, $num); # same thing
# Right padding a string with blanks using pack (will truncate):
$padded = pack("A$padlen",$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:
HHooww ddoo II eexxttrraacctt sseelleecctteedd ccoolluummnnss ffrroomm aa ssttrriinngg??
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
}
HHooww ddoo II ffiinndd tthhee ssoouunnddeexx vvaalluuee ooff aa ssttrriinngg??
Use the standard Text::Soundex module distributed with Perl. Before
you do so, you may want to determine whether `soundex' is in fact what
you think it is. Knuth's soundex algorithm compresses words into a
small space, and so it does not necessarily distinguish between two
words which you might want to appear separately. For example, the last
names `Knuth' and `Kant' are both mapped to the soundex code K530. If
Text::Soundex does not do what you are looking for, you might want to
consider the String::Approx module available at CPAN.
HHooww ccaann II eexxppaanndd vvaarriiaabblleess iinn tteexxtt ssttrriinnggss??
Let's assume that you have a string like:
$text = 'this has a $foo in it and a $bar';
If those were both global variables, then this would suffice:
$text =~ s/\$(\w+)/${$1}/g; # no /e needed
But since they are probably lexicals, or at least, they could be, you'd
have to do this:
$text =~ s/(\$\w+)/$1/eeg;
die if $@; # needed /ee, not /e
It's probably better in the general case to treat those variables as
entries in some special hash. For example:
%userdefs = (
foo => 23,
bar => 19,
);
$text =~ s/\$(\w+)/$userdefs{$1}/g;
See also ``How do I expand function calls in a string?'' in this sec-
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
WWhhyy ddoonn''tt mmyy <<< # all in one
($VAR = < your text
goes here
HERETARGET
But the HERETARGET must still be flush against the margin. If you
want that indented also, you'll have to quote in the indentation.
($quote = <<' FINIS') =~ s/^\s+//gm;
...we will have peace, when you and all your works have
perished-and the works of your dark master to whom you
would deliver us. You are a liar, Saruman, and a corrupter
of men's hearts. -Theoden in /usr/src/perl/taint.c
FINIS
$quote =~ s/\s+-/\n-/;
A nice general-purpose fixer-upper function for indented here documents
follows. It expects to be called with a here document as its argument.
It looks to see whether each line begins with a common substring, and
if so, strips that substring off. Otherwise, it takes the amount of
leading whitespace found on the first line and removes that much off
each subsequent line.
sub fix {
local $ = shift;
my ($white, $leader); # common whitespace and common leading string
if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
($white, $leader) = ($2, quotemeta($1));
} else {
($white, $leader) = (/^(\s+)/, '');
}
s/^\s*?$leader(?:$white)?//gm;
return $;
}
This works with leading special strings, dynamically determined:
$rememberthemain = fix<<' MAININTERPRETERLOOP';
@@@ int
@@@ runops() {
@@@ SAVEI32(runlevel);
@@@ runlevel++;
Or with a fixed amount of leading whitespace, with remaining indenta-
tion correctly preserved:
$poem = fix< Now far ahead the Road has gone,
And I must follow, if I can,
Pursuing it with eager feet,
Until it joins some larger way
Where many paths and errands meet.
And whither then? I cannot say.
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.
WWhhaatt iiss tthhee ddiiffffeerreennccee bbeettwweeeenn $$aarrrraayy[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 -ww flag will warn you about these
matters.
HHooww ccaann II rreemmoovvee dduupplliiccaattee eelleemmeennttss ffrroomm aa lliisstt oorr aarrrraayy??
There are several possible ways, depending on whether the array is
ordered and whether you wish to preserve the ordering.
a) If @in is sorted, and you want @out to be sorted: (this assumes all
true values in the array)
$prev = "not equal to $in[0]";
@out = grep($ ne $prev && ($prev = $, 1), @in);
This is nice in that it doesn't use much extra memory, simulating
uniq(1)'s behavior of removing only adjacent duplicates. The ", 1"
guarantees that the expression is true (so that grep picks it up)
even if the $ is 0, "", or undef.
b) If you don't know whether @in is sorted:
undef %saw;
@out = grep(!$saw{$}++, @in);
c) Like (b), but @in contains only small integers:
@out = grep(!$saw[$]++, @in);
d) A way to do (b) without any loops or greps:
undef %saw;
@saw{@in} = ();
@out = sort keys %saw; # remove sort if undesired
e) Like (d), but @in contains only small positive integers:
undef @ary;
@ary[@in] = @in;
@out = grep {defined} @ary;
But perhaps you should have been using a hash all along, eh?
HHooww ccaann II tteellll wwhheetthheerr aa cceerrttaaiinn eelleemmeenntt iiss ccoonnttaaiinneedd iinn aa lliisstt oorr
aarrrraayy??
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.
Now you can check whether $isblue{$somecolor}. 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);
@istinyprime = ();
for (@primes) { $istinyprime[$] = 1 }
# or simply @istinyprime[@primes] = (1) x @primes;
Now you check whether $istinyprime[$somenumber].
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 $n.
Please do not use
($isthere) = grep $ eq $whatever, @array;
or worse yet
($isthere) = grep /$whatever/, @array;
These are slow (checks every element even if the first matches), inef-
ficient (same reason), and potentially buggy (what if there are regex
characters in $whatever?). If you're only testing once, then use:
$isthere = 0;
foreach $elt (@array) {
if ($elt eq $elttofind) {
$isthere = 1;
last;
}
}
if ($isthere) { ... }
HHooww ddoo II ccoommppuuttee tthhee ddiiffffeerreennccee ooff ttwwoo aarrrraayyss?? HHooww ddoo II ccoommppuuttee tthhee
iinntteerrsseeccttiioonn ooff ttwwoo aarrrraayyss??
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 = ();
}
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.
HHooww ddoo II tteesstt wwhheetthheerr ttwwoo aarrrraayyss oorr hhaasshheess aarree eeqquuaall??
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.
$areequal = comparearrays(\@frogs, \@toads);
sub comparearrays {
my ($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);
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.
HHooww ddoo II ffiinndd tthhee ffiirrsstt aarrrraayy eelleemmeenntt ffoorr wwhhiicchh aa ccoonnddiittiioonn iiss ttrruuee??
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 my $element ( @array )
{
if( /Perl/ ) { $found = $element; 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;
}
}
HHooww ddoo II hhaannddllee lliinnkkeedd lliissttss??
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.
HHooww ddoo II hhaannddllee cciirrccuullaarr lliissttss??
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
HHooww ddoo II sshhuuffffllee aann aarrrraayy rraannddoommllyy??
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 fisheryatesshuffle {
my $deck = shift; # $deck is a reference to an array