=head1 NAME
-perlfaq4 - Data Manipulation ($Revision: 1.40 $, $Date: 1999/01/08 04:26:39 $)
+perlfaq4 - Data Manipulation ($Revision: 1.49 $, $Date: 1999/05/23 20:37:49 $)
=head1 DESCRIPTION
=head2 How do I convert bits into ints?
To turn a string of 1s and 0s like C<10110110> into a scalar containing
-its binary value, use the pack() function (documented in
-L<perlfunc/"pack">):
+its binary value, use the pack() and unpack() functions (documented in
+L<perlfunc/"pack" L<perlfunc/"unpack">):
- $decimal = pack('B8', '10110110');
+ $decimal = unpack('c', pack('B8', '10110110'));
+
+This packs the string C<10110110> into an eight bit binary structure.
+This is then unpack as a character, which returns its ordinal value.
+
+This does the same thing:
+
+ $decimal = ord(pack('B8', '10110110'));
Here's an example of going the other way:
- $binary_string = join('', unpack('B*', "\x29"));
+ $binary_string = unpack('B*', "\x29");
=head2 Why doesn't & work the way I want it to?
in it to be Work Week #1, despite ISO 8601, which considers
WW1 to be the first week with a Thursday in it.
+=head2 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, you'll find that 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, 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.
+
=head2 How can I compare two dates and find the difference?
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 use one of the Date::Manip
-and Date::Calc modules from CPAN.
+month, hour, minute, seconds values), then for reasons of accessibility,
+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 precise 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.
=head2 How can I take a string and turn it into epoch seconds?
=head2 How can I find the Julian Day?
-Neither Date::Manip nor Date::Calc deal with Julian days. Instead,
-there is an example of Julian date calculation that should help you in
-Time::JulianDay (part of the Time-modules bundle) which can be found at
-http://www.perl.com/CPAN/modules/by-module/Time/.
+You could use Date::Calc's Delta_Days function and calculate the number
+of days from there. Assuming that's what you really want, that is.
+
+Before you immerse yourself too deeply in this, be sure to verify that it
+is the I<Julian> Day you really want. Are they really just interested in
+a way of getting serial days so that they can do date arithmetic? If you
+are interested in performing date arithmetic, this can be done using
+either Date::Manip or Date::Calc, without converting to Julian Day first.
+
+There is too 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.)
+There is also an example of Julian date calculation that should help you in
+http://www.perl.com/CPAN/authors/David_Muir_Sharnoff/modules/Time/JulianDay.pm.gz
=head2 How do I find yesterday's date?
The C<time()> function returns the current time in seconds since the
-epoch. Take one day off that:
+epoch. Take twenty-four hours off that:
$yesterday = time() - ( 24 * 60 * 60 );
Then you can pass this to C<localtime()> and get the individual year,
month, day, hour, minute, seconds values.
+Note very carefully that the code above 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.
+A solution to this issue is offered by Russ Allbery.
+
+ 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 correction
+ # 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, subtract 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 localtime
+ # only says it returns the system tm struct, and the system tm struct at
+ # least on Solaris doesn't guarantee any particuliar 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).
+ #
+ # Copyright relinquished 1999 by Russ Allbery <rra@stanford.edu>
+ # This code is in the public domain
+
=head2 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
To turn C<"abbcccd"> into C<"abccd">:
- s/(.)\1/$1/g;
+ s/(.)\1/$1/g; # add /s to include newlines
+
+Here's a solution that turns "abbcccd" to "abcd":
+
+ y///cs; # y == tr, but shorter :-)
=head2 How do I expand function calls in a string?
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) {
+ while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
# do something with $1
}
You have to keep track of N yourself. For example, let's say you want
to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
-C<"whosoever"> or C<"whomsoever">, case insensitively.
+C<"whosoever"> or C<"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
- }igex;
+ }ige;
In the more general case, you can use the C</g> modifier in a C<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";
- # Warning: don't `last' out of this loop
}
}
$string = "ThisXlineXhasXsomeXx'sXinXit";
$count = ($string =~ tr/X//);
- print "There are $count X charcters in the string";
+ 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
See L<perllocale> for endless details on locales.
This is sometimes referred to as putting something into "title
-case", but that's not quite accurate. Consdier the proper
+case", but that's not quite accurate. Consider the proper
capitalization of the movie I<Dr. Strangelove or: How I Learned to
Stop Worrying and Love the Bomb>, for example.
$string =~ s/^\s*(.*?)\s*$/$1/;
-This is unnecessarily slow, destructive, and fails with embedded newlines.
-It is much better faster to do this in two steps:
+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+//;
$string =~ s/\s+$//;
This idiom takes advantage of the C<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 slide:
+values of a hash if you use a slice:
# trim whitespace in the scalar, the array,
# and all the values in the hash
=head2 How do I pad a string with blanks or pad a number with zeroes?
-(This answer contributed by Uri Guttman)
+(This answer contributed by Uri Guttman, with kibitzing from
+Bart Lateur.)
In the following examples, C<$pad_len> is the length to which you wish
-to pad the string, C<$text> or C<$num> contains the string to be
-padded, and C<$pad_char> contains the padding character. You can use a
-single character string constant instead of the C<$pad_char> variable
-if you know what it is in advance.
+to pad the string, C<$text> or C<$num> contains the string to be padded,
+and C<$pad_char> contains the padding character. You can use a single
+character string constant instead of the C<$pad_char> variable if you
+know what it is in advance. And in the same way you can use an integer in
+place of C<$pad_len> if you know the pad length in advance.
-The simplest method use the C<sprintf> function. It can pad on the
-left or right with blanks and on the left with zeroes.
+The simplest method uses the C<sprintf> function. It can pad on the left
+or right with blanks and on the left with zeroes and it will not
+truncate the result. The C<pack> function can only pad strings on the
+right with blanks and it will truncate the result to a maximum length of
+C<$pad_len>.
- # Left padding with blank:
- $padded = sprintf( "%${pad_len}s", $text ) ;
+ # Left padding a string with blanks (no truncation):
+ $padded = sprintf("%${pad_len}s", $text);
- # Right padding with blank:
- $padded = sprintf( "%${pad_len}s", $text ) ;
+ # Right padding a string with blanks (no truncation):
+ $padded = sprintf("%-${pad_len}s", $text);
- # Left padding with 0:
- $padded = sprintf( "%0${pad_len}d", $num ) ;
+ # Left padding a number with 0 (no truncation):
+ $padded = sprintf("%0${pad_len}d", $num);
-If you need to pad with a character other than blank or zero you can use
-one of the following methods.
+ # Right padding a string with blanks using pack (will truncate):
+ $padded = pack("A$pad_len",$text);
-These methods generate a pad string with the C<x> operator and
-concatenate that with the original text.
+If you need to pad with a character other than blank or zero you can use
+one of the following methods. They all generate a pad string with the
+C<x> operator and combine that with C<$text>. These methods do
+not truncate C<$text>.
-Left and right padding with any character:
+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 ) ) ;
+ $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
+ $padded = $text . $pad_char x ( $pad_len - length( $text ) );
-Or you can left or right pad $text directly:
+Left and right padding with any character, modifying C<$text> directly:
- $text .= $pad_char x ( $pad_len - length( $text ) ) ;
- substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) ) ;
+ substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
+ $text .= $pad_char x ( $pad_len - length( $text ) );
=head2 How do I extract selected columns from a string?
=head2 How do I find the soundex value of a string?
Use the standard Text::Soundex module distributed with perl.
+But 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.
=head2 How can I expand variables in text strings?
@@@ runops() {
@@@ SAVEI32(runlevel);
@@@ runlevel++;
- @@@ while ( op = (*op->op_ppaddr)() ) ;
+ @@@ while ( op = (*op->op_ppaddr)() );
@@@ TAINT_NOT;
@@@ return 0;
@@@ }
$scalar = (2, 5, 7, 9);
-you're using the comma operator in scalar context, so it evaluates the
-left hand side, then evaluates and returns the left hand side. This
-causes the last value to be returned: 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.
=head2 What is the difference between $array[1] and @array[1]?
The B<-w> flag will warn you about these matters.
-=head2 How can I extract just the unique elements of an array?
+=head2 How can I remove duplicate elements from a list or array?
There are several possible ways, depending on whether the array is
ordered and whether you wish to preserve the ordering.
@primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
undef @is_tiny_prime;
- for (@primes) { $is_tiny_prime[$_] = 1; }
+ for (@primes) { $is_tiny_prime[$_] = 1 }
+ # or simply @istiny_prime[@primes] = (1) x @primes;
Now you check whether $is_tiny_prime[$some_number].
These are slow (checks every element even if the first matches),
inefficient (same reason), and potentially buggy (what if there are
-regexp characters in $whatever?). If you're only testing once, then
+regex characters in $whatever?). If you're only testing once, then
use:
$is_there = 0;
push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
}
+Note that this is the I<symmetric difference>, that is, all elements in
+either A or in B, but not in both. Think of it as an xor operation.
+
=head2 How do I test whether two arrays or hashes are equal?
The following code works for single-level arrays. It uses a stringwise
fisher_yates_shuffle( \@array ); # permutes @array in place
-You've probably seen shuffling algorithms that works using splice,
+You've probably seen shuffling algorithms that work using splice,
randomly picking another element to swap the current element with:
srand;
@sorted = map { $_->[0] }
sort { $a->[1] cmp $b->[1] }
- map { [ $_, uc((/\d+\s*(\S+)/ )[0] ] } @data;
+ map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
If you need to sort on several fields, the following paradigm is useful.
=head2 What happens if I add or remove keys from a hash while iterating over it?
-Don't do that.
+Don't do that. :-)
+
+[lwall] In Perl 4, you were not allowed to modify a hash at all while
+interating 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 interator 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.
+
+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.
=head2 How do I look up a hash element by value?
$by_value{$value} = $key;
}
-If your hash could have repeated values, the methods above will only
-find one of the associated keys. This may or may not worry you.
+If your hash could have repeated values, the methods above will only find
+one of the associated keys. This may or may not worry you. If it does
+worry you, you can always reverse the hash into a hash of arrays instead:
+
+ while (($key, $value) = each %by_key) {
+ push @{$key_list_by_value{$value}}, $key;
+ }
=head2 How can I know how many entries are in a hash?
$num_keys = scalar keys %hash;
-In void context it just resets the iterator, which is faster
-for tied hashes.
+In void context, the keys() function just resets the iterator, which is
+faster for tied hashes than would be iterating through the whole
+hash, one key-value pair at a time.
=head2 How do I sort a hash (optionally by value instead of key)?
=head2 How can I get the unique keys from two hashes?
-First you extract the keys from the hashes into arrays, and then solve
-the uniquifying the array problem described above. For example:
+First you extract the keys from the hashes into lists, then solve
+the "removing duplicates" problem described above. For example:
%seen = ();
for $element (keys(%foo), keys(%bar)) {
print "Your kernel is GNU-zip enabled!\n";
}
-On some legacy systems, however, you have to play tedious games with
-"text" versus "binary" files. See L<perlfunc/"binmode">, or the upcoming
-L<perlopentut> manpage.
+On less elegant (read: Byzantine) systems, however, you have
+to play tedious games with "text" versus "binary" files. See
+L<perlfunc/"binmode"> or L<perlopentut>. Most of these ancient-thinking
+systems are curses out of Microsoft, who seem to be committed to putting
+the backward into backward compatibility.
If you're concerned about 8-bit ASCII data, then see L<perllocale>.
sub is_numeric { defined &getnum }
-Or you could check out String::Scanf which can be found at
-http://www.perl.com/CPAN/modules/by-module/String/.
-The POSIX module (part of the standard Perl distribution) provides
-the C<strtol> and C<strtod> for converting strings to double
+Or you could check out
+http://www.perl.com/CPAN/modules/by-module/String/String-Scanf-1.1.tar.gz
+instead. The POSIX module (part of the standard Perl distribution)
+provides the C<strtol> and C<strtod> for converting strings to double
and longs, respectively.
=head2 How do I keep persistent data across program calls?
When included as part of the Standard Version of Perl, or as part of
its complete documentation whether printed or otherwise, this work
-may be distributed only under the terms of Perl's Artistic Licence.
+may be distributed only under the terms of Perl's Artistic License.
Any distribution of this file or derivatives thereof I<outside>
of that package require that special arrangements be made with
copyright holder.
encouraged to use this code in your own programs for fun
or for profit as you see fit. A simple comment in the code giving
credit would be courteous but is not required.
-
https://perl5.git.perl.org / perl5.git / blobdiff