=head1 NAME
-perlfaq4 - Data Manipulation ($Revision: 1.44 $, $Date: 2003/07/28 17:35:21 $)
+perlfaq4 - Data Manipulation ($Revision: 1.61 $, $Date: 2005/03/11 16:27:53 $)
=head1 DESCRIPTION
machines) will work pretty much like mathematical integers. Other numbers
are not guaranteed.
-=head2 How do I convert between numeric representations?
+=head2 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
Using perl's built in conversion of 0x notation:
- $int = 0xDEADBEEF;
- $dec = sprintf("%d", $int);
+ $dec = 0xDEADBEEF;
Using the hex function:
- $int = hex("DEADBEEF");
- $dec = sprintf("%d", $int);
+ $dec = hex("DEADBEEF");
Using pack:
- $int = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8)));
- $dec = sprintf("%d", $int);
+ $dec = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8)));
Using the CPAN module Bit::Vector:
Using sprintf:
- $hex = sprintf("%X", 3735928559);
+ $hex = sprintf("%X", 3735928559); # upper case A-F
+ $hex = sprintf("%x", 3735928559); # lower case a-f
-Using unpack
+Using unpack:
$hex = unpack("H*", pack("N", 3735928559));
-Using Bit::Vector
+Using Bit::Vector:
use Bit::Vector;
$vec = Bit::Vector->new_Dec(32, -559038737);
Using Perl's built in conversion of numbers with leading zeros:
- $int = 033653337357; # note the leading 0!
- $dec = sprintf("%d", $int);
+ $dec = 033653337357; # note the leading 0!
Using the oct function:
- $int = oct("33653337357");
- $dec = sprintf("%d", $int);
+ $dec = oct("33653337357");
Using Bit::Vector:
$oct = sprintf("%o", 3735928559);
-Using Bit::Vector
+Using Bit::Vector:
use Bit::Vector;
$vec = Bit::Vector->new_Dec(32, -559038737);
Perl 5.6 lets you write binary numbers directly with
the 0b notation:
- $number = 0b10110110;
+ $number = 0b10110110;
+
+Using oct:
-Using pack and ord
+ my $input = "10110110";
+ $decimal = oct( "0b$input" );
+
+Using pack and ord:
$decimal = ord(pack('B8', '10110110'));
-Using pack and unpack for larger strings
+Using pack and unpack for larger strings:
$int = unpack("N", pack("B32",
substr("0" x 32 . "11110101011011011111011101111", -32)));
=item How do I convert from decimal to binary
-Using unpack;
+Using sprintf (perl 5.6+):
+
+ $bin = sprintf("%b", 3735928559);
+
+Using unpack:
$bin = unpack("B*", pack("N", 3735928559));
=head2 How do I get a random number between X and Y?
-Use the following simple function. It selects a random integer between
-(and possibly including!) the two given integers, e.g.,
-C<random_int_in(50,120)>
+C<rand($x)> returns a number such that
+C<< 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 I<X> and I<Y>.
+
+That is, to get a number between 10 and 15, inclusive, you
+want a random 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: C<random_int_in(50,120)>.
sub random_int_in ($$) {
my($min, $max) = @_;
=head2 How do I find the day or week of the year?
-The localtime function returns the day of the week. Without an
+The localtime function returns the day of the year. Without an
argument localtime uses the current time.
$day_of_year = (localtime)[7];
sub get_century {
return int((((localtime(shift || time))[5] + 1999))/100);
}
+
sub get_millennium {
return 1+int((((localtime(shift || time))[5] + 1899))/1000);
}
-You can also use the POSIX strftime() function which may be a bit
-slower but is easier to read and maintain.
-
- use POSIX qw/strftime/;
-
- my $week_of_the_year = strftime "%W", localtime;
- my $day_of_the_year = strftime "%j", localtime;
-
On some systems, the POSIX module's strftime() function has
been extended in a non-standard way to use a C<%C> format,
which they sometimes claim is the "century". It isn't,
=head2 How can I find the Julian Day?
-Use the Time::JulianDay module (part of the Time-modules bundle
-available from CPAN.)
-
-Before you immerse yourself too deeply in this, be sure to verify that
-it is the I<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
-modules 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
-supplanted by the Gregorian Calendar, with the Julian Calendar failing
-to adjust properly for leap years on centennial years (among other
-annoyances). 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.)
+(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
+different ideas about Julian days. See
+http://www.hermetic.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
=head2 How do I find yesterday's date?
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://language.perl.com/news/y2k.html for
+break Y2K, people do.'' See http://www.perl.org/about/y2k.html for
a longer exposition.
=head1 Data: Strings
print "My sub returned @{[mysub(1,2,3)]} that time.\n";
-See also ``How can I expand variables in text strings?'' in this
-section of the FAQ.
-
=head2 How do I find matching/nesting anything?
This isn't something that can be done in one regular expression, no
=head2 How can I split a [character] delimited string except when inside [character]?
Several modules can handle this sort of pasing---Text::Balanced,
-Text::CVS, Text::CVS_XS, and Text::ParseWords, among others.
+Text::CSV, Text::CSV_XS, and Text::ParseWords, among others.
Take the example case of trying to split a string that is
comma-separated into its different fields. You can't use C<split(/,/)>
=head2 How do I find the soundex value of a string?
-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.
+(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.
=head2 How can I expand variables in text strings?
-Let's assume that you have a string like:
+Let's assume that you have a string that contains placeholder
+variables.
$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
+You can use a substitution with a double evaluation. The
+first /e turns C<$1> into C<$foo>, and the second /e turns
+C<$foo> into its value. You may want to wrap this in an
+C<eval>: if you try to get the value of an undeclared variable
+while running under C<use strict>, you get a fatal error.
-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
+ 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:
);
$text =~ s/\$(\w+)/$user_defs{$1}/g;
-See also ``How do I expand function calls in a string?'' in this section
-of the FAQ.
-
=head2 What's wrong with always quoting "$vars"?
The problem is that those double-quotes force stringification--
Use the rand() function (see L<perlfunc/rand>):
- # at the top of the program:
- srand; # not needed for 5.004 and later
-
- # then later on
$index = rand @array;
$element = $array[$index];
-Make sure you I<only call srand once per program, if then>.
-If you are calling it more than once (such as before each
-call to rand), you're almost certainly doing something wrong.
+Or, simply:
+ my $element = $array[ rand @array ];
=head2 How do I permute N elements of a list?
This can be conveniently combined with precalculation of keys as given
above.
-See the F<sort> artitcle article in the "Far More Than You Ever Wanted
+See the F<sort> article in the "Far More Than You Ever Wanted
To Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz for
more about this approach.
=head2 What happens if I add or remove keys from a hash while iterating over it?
-Don't do that. :-)
+(contributed by brian d foy)
-[lwall] In Perl 4, you were not allowed to modify a hash at all while
-iterating over it. In Perl 5 you can delete from it, but you still
-can't add to it, because that might cause a doubling of the hash table,
-in which half the entries get copied up to the new top half of the
-table, at which point you've totally bamboozled the iterator code.
-Even if the table doesn't double, there's no telling whether your new
-entry will be inserted before or after the current iterator position.
+The easy answer is "Don't do that!"
-Either treasure up your changes and make them after the iterator finishes
-or use keys to fetch all the old keys at once, and iterate over the list
-of keys.
+If you iterate through the hash with each(), you can delete the key
+most recently returned without worrying about it. If you delete or add
+other keys, the iterator may skip or double up on them since perl
+may rearrange the hash table. See the
+entry for C<each()> in L<perlfunc>.
=head2 How do I look up a hash element by value?
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.
-All rights reserved.
+Copyright (c) 1997-2005 Tom Christiansen, Nathan Torkington, and
+other authors as noted. All rights reserved.
This documentation is free; you can redistribute it and/or modify it
under the same terms as Perl itself.
projects / p5sagit/p5-mst-13.2.git / blobdiff