=head1 NAME
-perlfaq4 - Data Manipulation ($Revision: 1.24 $, $Date: 1998/07/05 15:07:20 $)
+perlfaq4 - Data Manipulation ($Revision: 1.26 $, $Date: 1998/08/05 12:04:00 $)
=head1 DESCRIPTION
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
+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.
However, 19.95 can't be precisely represented as a binary
=head2 Does perl have a round function? What about ceil() and floor()? Trig functions?
-Remember that int() merely truncates toward 0. For rounding to a certain
-number of digits, sprintf() or printf() is usually the easiest route.
+Remember that int() merely truncates toward 0. For rounding to a
+certain 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
=head2 How do I convert bits into ints?
-To turn a string of 1s and 0s like '10110110' into a scalar containing
+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">):
=head2 Why aren't my random numbers random?
-John von Neumann said, ``Anyone who attempts to generate random numbers by
-deterministic means is, of course, living in a state of sin.''
-
The short explanation is that you're getting pseudorandom numbers, not
-random ones, because that's how these things work. A longer explanation
-is available on http://www.perl.com/CPAN/doc/FMTEYEWTK/random, courtesy
-of Tom Phoenix.
-
-You should also check out the Math::TrulyRandom module from CPAN.
+random ones, because computers are good at being predictable and bad
+at being random (despite appearances caused by bugs in your programs
+:-). A longer explanation is available on
+http://www.perl.com/CPAN/doc/FMTEYEWTK/random, courtesy of Tom
+Phoenix. John von Neumann said, ``Anyone who attempts to generate
+random numbers by deterministic means is, of course, living in a state
+of sin.''
+
+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://nr.harvard.edu/nr/bookc.html .
=head1 Data: Dates
$week_of_year = int($day_of_year / 7);
-Of course, this believes that weeks start at zero.
+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 business consider ``week 1'' to be the same; for example,
+American business often consider the first week with a Monday
+in it to be Work Week #1, despite ISO 8601, which consider
+WW1 to be the frist week with a Thursday in it.
-=head2 How can I compare two date strings?
+=head2 How can I compare two dates and find the difference?
-Use the Date::Manip or Date::DateCalc modules from CPAN.
+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.
=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,
-you can split it up and pass the parts to timelocal in the standard
-Time::Local module. Otherwise, you should look into one of the
-Date modules from CPAN.
+you can split it up and pass the parts to C<timelocal> in the standard
+Time::Local module. Otherwise, you should look into the Date::Calc
+and Date::Manip modules from CPAN.
=head2 How can I find the Julian Day?
-Neither Date::Manip nor Date::DateCalc deal with Julian days.
-Instead, there is an example of Julian date calculation in
-http://www.perl.com/CPAN/authors/David_Muir_Sharnoff/modules/Time/JulianDay.pm.gz,
-which should help.
+Neither Date::Manip nor Date::Calc deal with Julian days. Instead,
+there is 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 Does Perl have a year 2000 problem? Is Perl Y2K compliant?
-Perl is just as Y2K compliant as your pencil--no more, and no less.
-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.
-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.
+Short answer: No, Perl does not have a Year 2000 problem. Yes,
+Perl is Y2K compliant. The programmers you've hired to use it,
+however, probably are not.
+
+Long answer: Perl is just as Y2K compliant as your pencil--no more,
+and no less. 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. 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.
When gmtime() and localtime() are used in scalar context they return
a timestamp string that contains a fully-expanded year. For example,
=head2 How do I unescape a string?
-It depends just what you mean by "escape". URL escapes are dealt with
-in L<perlfaq9>. Shell escapes with the backslash (\)
+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:
s/\\(.)/$1/g;
-Note that this won't expand \n or \t or any other special escapes.
+This won't expand C<"\n"> or C<"\t"> or any other special escapes.
=head2 How do I remove consecutive pairs of characters?
-To turn "abbcccd" into "abccd":
+To turn C<"abbcccd"> into C<"abccd">:
s/(.)\1/$1/g;
print "That yields ${\($n + 5)} widgets\n";
-See also "How can I expand variables in text strings?" in this section
-of the FAQ.
+Version 5.004 of Perl had a bug that gave list context to the
+expression in C<${...}>, but this is fixed in version 5.005.
+
+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 tackled in one regular expression, no
-matter how complicated. To find something between two single characters,
-a pattern like C</x([^x]*)x/> will get the intervening bits in $1. For
-multiple ones, then something more like C</alpha(.*?)omega/> would
-be needed. But none of these deals with nested patterns, nor can they.
-For that you'll have to write a parser.
+This isn't something that can be done in one regular expression, no
+matter how complicated. To find something between two single
+characters, a pattern like C</x([^x]*)x/> will get the intervening
+bits in $1. For multiple ones, then something more like
+C</alpha(.*?)omega/> would be needed. But none of these deals with
+nested patterns, nor can they. For that you'll have to write a
+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, and Mark-Jason Dominus's excellent I<py> tool at
+http://www.plover.com/~mjd/perl/py/ .
-One destructive, inside-out approach that you might try is to pull
-out the smallest nesting parts one at a time:
+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
use Text::Wrap;
print wrap("\t", ' ', @paragraphs);
-The paragraphs you give to Text::Wrap may not contain embedded
+The paragraphs you give to Text::Wrap should not contain embedded
newlines. Text::Wrap doesn't justify the lines (flush-right).
=head2 How can I access/change the first N letters of a string?
There are many ways. If you just want to grab a copy, use
-substr:
+substr():
$first_byte = substr($a, 0, 1);
substr($a, 0, 3) = "Tom";
-Although those with a regexp kind of thought process will likely prefer
+Although those with a pattern matching kind of thought process will
+likely prefer:
$a =~ s/^.../Tom/;
=head2 How do I change the Nth occurrence of something?
-You have to keep track. For example, let's say you want
-to change the fifth occurrence of "whoever" or "whomever"
-into "whosoever" or "whomsoever", case insensitively.
+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.
$count = 0;
s{((whom?)ever)}{
}
}
-That prints out: "The third fish is a red one." You can also use a
+That prints out: C<"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;
count of a certain single character (X) within a string, you can use the
C<tr///> function like so:
- $string = "ThisXlineXhasXsomeXx'sXinXit":
+ $string = "ThisXlineXhasXsomeXx'sXinXit";
$count = ($string =~ tr/X//);
print "There are $count X charcters in the string";
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
-Foy E<lt>comdog@computerdog.comE<gt>):
+Foy):
$string =~ s/ (
(^\w) #at the beginning of the line
You can (and probably should) enable locale awareness of those
characters by placing a C<use locale> pragma in your program.
-See L<perllocale> for endless details.
+See L<perllocale> for endless details on locales.
=head2 How can I split a [character] delimited string except when inside
[character]? (Comma-separated files)
s/\s+$//;
}
-This idiom takes advantage of the for(each) loop's aliasing
+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:
);
$text =~ s/\$(\w+)/$user_defs{$1}/g;
-See also "How do I expand function calls in a string?" in this section
+See also ``How do I expand function calls in a string?'' in this section
of the FAQ.
=head2 What's wrong with always quoting "$vars"?
projects / p5sagit/p5-mst-13.2.git / blobdiff