=head1 DESCRIPTION
-The section of the FAQ answers question related to the manipulation
+The section of the FAQ answers questions related to the manipulation
of data as numbers, dates, strings, arrays, hashes, and miscellaneous
data issues.
=head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
The infinite set that a mathematician thinks of as the real numbers can
-only be approximate on a computer, since the computer only has a finite
+only be approximated on a computer, since the computer only has a finite
number of bits to store an infinite number of, um, numbers.
Internally, your computer represents floating-point numbers in binary.
Floating-point numbers read in from a file or appearing as literals
in your program are converted from their decimal floating-point
-representation (eg, 19.95) to the internal binary representation.
+representation (eg, 19.95) to an internal binary representation.
However, 19.95 can't be precisely represented as a binary
floating-point number, just like 1/3 can't be exactly represented as a
When a floating-point number gets printed, the binary floating-point
representation is converted back to decimal. These decimal numbers
are displayed in either the format you specify with printf(), or the
-current output format for numbers (see L<perlvar/"$#"> if you use
+current output format for numbers. (See L<perlvar/"$#"> if you use
print. C<$#> has a different default value in Perl5 than it did in
Perl4. Changing C<$#> yourself is deprecated.)
(part of the standard Perl distribution), but mathematical operations
are consequently slower.
+If precision is important, such as when dealing with money, it's good
+to work with integers and then divide at the last possible moment.
+For example, work in pennies (1995) instead of dollars and cents
+(19.95) and divide by 100 at the end.
+
To get rid of the superfluous digits, just use a format (eg,
C<printf("%.2f", 19.95)>) to get the required precision.
See L<perlop/"Floating-point Arithmetic">.
$ceil = ceil(3.5); # 4
$floor = floor(3.5); # 3
-In 5.000 to 5.003 Perls, trigonometry was done in the Math::Complex
+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
Computers are good at being predictable and bad at being random
(despite appearances caused by bugs in your programs :-).
-http://www.perl.com/CPAN/doc/FMTEYEWTK/random, courtesy of Tom
-Phoenix, talks more about this.. John von Neumann said, ``Anyone who
+http://www.perl.com/CPAN/doc/FMTEYEWTK/random , 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.''
$day_of_year = (localtime(time()))[7];
-or more legibly (in 5.004 or higher):
+or more legibly (in 5.7.1 or higher):
- use Time::localtime;
- $day_of_year = localtime(time())->yday;
+ use Time::Piece;
+ $day_of_year = localtime->day_of_year();
-You can find the week of the year by dividing this by 7:
+You can find the week of the year by using Time::Piece's strftime():
- $week_of_year = int($day_of_year / 7);
+ $week_of_year = localtime->strftime("%U");
+ $iso_week = localtime->strftime("%V");
-Of course, this believes that weeks start at zero. The Date::Calc
-module from CPAN has a lot of date calculation functions, including
-day of the year, week of the year, and so on. Note that not
-all businesses consider ``week 1'' to be the same; for example,
-American businesses often consider the first week with a Monday
-in it to be Work Week #1, despite ISO 8601, which considers
-WW1 to be the first week with a Thursday in it.
+The difference between %U and %V is that %U assumes that the first day
+of week 1 is the first Sunday of the year, whereas ISO 8601:1988 uses
+the first week that has at least 4 days in the current year, and with
+Monday as the first day of the week. You can also use %W, which will
+return the week of the year with Monday as the first day of week 1. See
+your strftime(3) man page for more details.
=head2 How do I find the current century or millennium?
Date::Calc modules from CPAN before you go hacking up your own parsing
routine to handle arbitrary date formats.
+Also note that the core module Time::Piece overloads the addition and
+subtraction operators to provide date calculation options. See
+L<Time::Piece/Date Calculations>.
+
=head2 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,
=head2 How can I find the Julian Day?
-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.
+Use Time::Piece as follows:
-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.)
+ use Time::Piece;
+ my $julian_day = localtime->julian_day;
+ my $mjd = localtime->mjd; # modified julian day
-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
+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
+Time::Piece (standard module since Perl 5.8), or by 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.)
=head2 How do I find yesterday's date?
Then you can pass this to C<localtime()> and get the individual year,
month, day, hour, minute, seconds values.
+Alternatively, you can use Time::Piece to subtract a day from the value
+returned from C<localtime()>:
+
+ use Time::Piece;
+ use Time::Seconds; # imports seconds constants, like ONE_DAY
+ my $today = localtime();
+ my $yesterday = $today - ONE_DAY;
+
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.
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 an array context is the year minus 1900.
+by these functions when used in a list context is the year minus 1900.
For years between 1910 and 1999 this I<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.
It depends just what you mean by ``escape''. URL escapes are dealt
with in L<perlfaq9>. Shell escapes with the backslash (C<\>)
-character are removed with:
+character are removed with
s/\\(.)/$1/g;
parser.
If you are serious about writing a parser, there are a number of
-modules or oddities that will make your life a lot easier. There is
-the CPAN module Parse::RecDescent, the standard module Text::Balanced,
-the byacc program, the CPAN module Parse::Yapp, and Mark-Jason
-Dominus's excellent I<py> tool at http://www.plover.com/%7Emjd/perl/py/
-.
+modules 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:
# $_ contains the string to parse
# BEGIN and END are the opening and closing markers for the
# nested text.
-
+
@( = ('(','');
@) = (')','');
($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
substr($a, 0, 3) = "Tom";
Although those with a pattern matching kind of thought process will
-likely prefer:
+likely prefer
$a =~ s/^.../Tom/;
=head2 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
+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
C<tr///> function like so:
$line =~ s/\b(\w)/\U$1/g;
This has the strange effect of turning "C<don't do it>" into "C<Don'T
-Do It>". Sometimes you might want this, instead (Suggested by brian d.
-foy):
+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
use Text::ParseWords;
@new = quotewords(",", 0, $text);
-There's also a Text::CSV module on CPAN.
+There's also a Text::CSV (Comma-Separated Values) module on CPAN.
=head2 How do I strip blank space from the beginning/end of a string?
-Although the simplest approach would seem to be:
+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
+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:
$string =~ s/^\s+//;
=head2 How do I find the soundex value of a string?
Use the standard Text::Soundex module distributed with Perl.
-But before you do so, you may want to determine whether `soundex' is in
+Before you do so, you may want to determine whether `soundex' is in
fact what you think it is. Knuth's soundex algorithm compresses words
into a small space, and so it does not necessarily distinguish between
two words which you might want to appear separately. For example, the
=head2 What's wrong with always quoting "$vars"?
-The problem is that those double-quotes force stringification,
-coercing numbers and references into strings, even when you
-don't want them to be. Think of it this way: double-quote
+The problem is that those double-quotes force stringification--
+coercing 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?
print "@lines"; # WRONG - extra blanks
print @lines; # right
-=head2 Why don't my E<lt>E<lt>HERE documents work?
+=head2 Why don't my <<HERE documents work?
Check for these three things:
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 off. Otherwise, it takes the amount of leading
-white space found on the first line and removes that much off each
+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, $leader); # common white space and common leading string
+ my ($white, $leader); # common whitespace and common leading string
if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
($white, $leader) = ($2, quotemeta($1));
} else {
@@@ }
MAIN_INTERPRETER_LOOP
-Or with a fixed amount of leading white space, with remaining
+Or with a fixed amount of leading whitespace, with remaining
indentation correctly preserved:
$poem = fix<<EVER_ON_AND_ON;
context, you initialize arrays with lists, and you foreach() across
a list. C<@> 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 C<@_>, push/pop/shift only work
+access their arguments through the array C<@_>, and push/pop/shift only work
on arrays.
As a side note, there's no such thing as a list in scalar context.
=head2 What is the difference between $array[1] and @array[1]?
-The former is a scalar value, the latter an array slice, which makes
+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).
@bad[0] = `same program that outputs several lines`;
-The B<-w> flag will warn you about these matters.
+The C<use warnings> pragma and the B<-w> flag will warn you about these
+matters.
=head2 How can I remove duplicate elements from a list or array?
=over 4
-=item a) If @in is sorted, and you want @out to be sorted:
+=item a)
+
+If @in is sorted, and you want @out to be sorted:
(this assumes all true values in the array)
- $prev = 'nonesuch';
- @out = grep($_ ne $prev && ($prev = $_), @in);
+ $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. It's less
-nice in that it won't work with false values like undef, 0, or "";
-"0 but true" is OK, though.
+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 $_ is 0, "", or undef.
+
+=item b)
-=item b) If you don't know whether @in is sorted:
+If you don't know whether @in is sorted:
undef %saw;
@out = grep(!$saw{$_}++, @in);
-=item c) Like (b), but @in contains only small integers:
+=item c)
+
+Like (b), but @in contains only small integers:
@out = grep(!$saw[$_]++, @in);
-=item d) A way to do (b) without any loops or greps:
+=item d)
+
+A way to do (b) without any loops or greps:
undef %saw;
@saw{@in} = ();
@out = sort keys %saw; # remove sort if undesired
-=item e) Like (d), but @in contains only small positive integers:
+=item e)
+
+Like (d), but @in contains only small positive integers:
undef @ary;
@ary[@in] = @in;
But perhaps you should have been using a hash all along, eh?
-=head2 How can I tell whether a list or array contains a certain element?
+=head2 How can I tell whether a certain element is contained in a list or array?
Hearing the word "in" is an I<in>dication that you probably should have
used a hash, not a list or array, to store your data. Hashes are
Please do not use
- $is_there = grep $_ eq $whatever, @array;
+ ($is_there) = grep $_ eq $whatever, @array;
or worse yet
- $is_there = grep /$whatever/, @array;
+ ($is_there) = grep /$whatever/, @array;
These are slow (checks every element even if the first matches),
inefficient (same reason), and potentially buggy (what if there are
}
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.
+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?
sub compare_arrays {
my ($first, $second) = @_;
- local $^W = 0; # silence spurious -w undef complaints
+ no warnings; # silence spurious -w undef complaints
return 0 unless @$first == @$second;
for (my $i = 0; $i < @$first; $i++) {
return 0 if $first->[$i] ne $second->[$i];
}
print "\n";
-You could grow the list this way:
+You could add to the list this way:
my ($head, $tail);
$tail = append($head, 1); # grow a new head
my $i;
for ($i = @$array; --$i; ) {
my $j = int rand ($i+1);
- next if $i == $j;
@$array[$i,$j] = @$array[$j,$i];
}
}
fisher_yates_shuffle( \@array ); # permutes @array in place
You've probably seen shuffling algorithms that work using splice,
-randomly picking another element to swap the current element with:
+randomly picking another element to swap the current element with
srand;
@new = ();
@list = sort { $a <=> $b } @list;
The default sort function is cmp, string comparison, which would
-sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<E<lt>=E<gt>>, used above, is
+sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is
the numerical comparison operator.
If you have a complicated function needed to pull out the part you
}
@sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
-Which could also be written this way, using a trick
+which could also be written this way, using a trick
that's come to be known as the Schwartzian Transform:
@sorted = map { $_->[0] }
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,
+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.
$num_keys = scalar keys %hash;
-In void context, the keys() function just resets the iterator, which is
+The keys() function also resets the iterator, which in void context is
faster for tied hashes than would be iterating through the whole
hash, one key-value pair at a time.
} keys %hash; # and by value
Here we'll do a reverse numeric sort by value, and if two keys are
-identical, sort by length of key, and if that fails, by straight ASCII
-comparison of the keys (well, possibly modified by your locale -- see
+identical, sort by length of key, or if that fails, by straight ASCII
+comparison of the keys (well, possibly modified by your locale--see
L<perllocale>).
@keys = sort {
sub is_numeric { defined getnum($_[0]) }
-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.
+Or you could check out the String::Scanf module on CPAN instead. The
+POSIX module (part of the standard Perl distribution) provides the
+C<strtod> and C<strtol> for converting strings to double and longs,
+respectively.
=head2 How do I keep persistent data across program calls?
For some specific applications, you can use one of the DBM modules.
See L<AnyDBM_File>. More generically, you should consult the FreezeThaw,
-Storable, or Class::Eroot modules from CPAN. Here's one example using
+Storable, or Class::Eroot modules from CPAN. Starting from Perl 5.8
+Storable is part of the standard distribution. Here's one example using
Storable's C<store> and C<retrieve> functions:
use Storable;
projects / p5sagit/p5-mst-13.2.git / blobdiff