X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperlfaq4.pod;h=e816fb3893d976fce01cd847d5ef3791f49c381d;hb=7207e29d564e32c8ec2cb43eabae72ff68c02442;hp=1342634a2350a56f83bf461d1ab23f17fa9b60bc;hpb=89435c9650c398f35575550895182cd549b3b9fd;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perlfaq4.pod b/pod/perlfaq4.pod index 1342634..e816fb3 100644 --- a/pod/perlfaq4.pod +++ b/pod/perlfaq4.pod @@ -1,6 +1,6 @@ =head1 NAME -perlfaq4 - Data Manipulation ($Revision: 1.49 $, $Date: 1999/05/23 20:37:49 $) +perlfaq4 - Data Manipulation ($Revision: 1.9 $, $Date: 2001/12/07 21:17:58 $) =head1 DESCRIPTION @@ -39,6 +39,11 @@ arbitrary-precision decimal numbers with the Math::BigFloat module (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) to get the required precision. See L. @@ -46,19 +51,36 @@ See L. =head2 Why isn't my octal data interpreted correctly? Perl only understands octal and hex numbers as such when they occur -as literals in your program. If they are read in from somewhere and -assigned, no automatic conversion takes place. You must explicitly -use oct() or hex() if you want the values converted. oct() interprets +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 +conversion takes place. You must explicitly use oct() or hex() if you +want the values converted to decimal. oct() interprets both hex ("0x350") numbers and octal ones ("0350" or even without the leading "0", like "377"), 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. To get from decimal to hex try either +the "%x" or the "%X" formats to sprintf(). This problem shows up most often when people try using chmod(), mkdir(), -umask(), or sysopen(), which all want permissions in octal. +umask(), or sysopen(), which by widespread tradition typically take +permissions in octal. - chmod(644, $file); # WRONG -- perl -w catches this + 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 C - 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. + =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 @@ -101,24 +123,130 @@ 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. -=head2 How do I convert bits into ints? +=head2 How do I convert between numeric representations: + +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. + +=item B + +Using perl's built in conversion of 0x notation: + + $int = 0xDEADBEEF; + $dec = sprintf("%d", $int); + +Using the hex function: + + $int = hex("DEADBEEF"); + $dec = sprintf("%d", $int); + +Using pack: + + $int = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8))); + $dec = sprintf("%d", $int); + +Using the CPAN module Bit::Vector: + + use Bit::Vector; + $vec = Bit::Vector->new_Hex(32, "DEADBEEF"); + $dec = $vec->to_Dec(); + +=item B + +Using sprint: + + $hex = sprintf("%X", 3735928559); + +Using unpack + + $hex = unpack("H*", pack("N", 3735928559)); + +Using Bit::Vector + + use Bit::Vector; + $vec = Bit::Vector->new_Dec(32, -559038737); + $hex = $vec->to_Hex(); + +And Bit::Vector supports odd bit counts: + + use Bit::Vector; + $vec = Bit::Vector->new_Dec(33, 3735928559); + $vec->Resize(32); # suppress leading 0 if unwanted + $hex = $vec->to_Hex(); + +=item B + +Using Perl's built in conversion of numbers with leading zeros: + + $int = 033653337357; # note the leading 0! + $dec = sprintf("%d", $int); + +Using the oct function: + + $int = oct("33653337357"); + $dec = sprintf("%d", $int); + +Using Bit::Vector: + + use Bit::Vector; + $vec = Bit::Vector->new(32); + $vec->Chunk_List_Store(3, split(//, reverse "33653337357")); + $dec = $vec->to_Dec(); + +=item B + +Using sprintf: + + $oct = sprintf("%o", 3735928559); -To turn a string of 1s and 0s like C<10110110> into a scalar containing -its binary value, use the pack() and unpack() functions (documented in -L and L): +Using Bit::Vector - $decimal = unpack('c', pack('B8', '10110110')); + use Bit::Vector; + $vec = Bit::Vector->new_Dec(32, -559038737); + $oct = reverse join('', $vec->Chunk_List_Read(3)); -This packs the string C<10110110> into an eight bit binary structure. -This is then unpacked as a character, which returns its ordinal value. +=item B -This does the same thing: +Using pack and ord $decimal = ord(pack('B8', '10110110')); -Here's an example of going the other way: +Using pack and unpack for larger strings + + $int = unpack("N", pack("B32", + substr("0" x 32 . "11110101011011011111011101111", -32))); + $dec = sprintf("%d", $int); + + # substr() is used to left pad a 32 character string with zeros. + +Using Bit::Vector: + + $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111"); + $dec = $vec->to_Dec(); + +=item B + +Using unpack; + + $bin = unpack("B*", pack("N", 3735928559)); + +Using Bit::Vector: + + use Bit::Vector; + $vec = Bit::Vector->new_Dec(32, -559038737); + $bin = $vec->to_Bin(); + +The remaining transformations (e.g. hex -> oct, bin -> hex, etc.) +are left as an exercise to the inclined reader. - $binary_string = unpack('B*', "\x29"); =head2 Why doesn't & work the way I want it to? @@ -194,7 +322,7 @@ will not create a list of 500,000 integers. =head2 How can I output Roman numerals? -Get the http://www.perl.com/CPAN/modules/by-module/Roman module. +Get the http://www.cpan.org/modules/by-module/Roman module. =head2 Why aren't my random numbers random? @@ -206,7 +334,7 @@ than more. 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 +http://www.cpan.org/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.'' @@ -227,23 +355,6 @@ L): $day_of_year = (localtime(time()))[7]; -or more legibly (in 5.7.1 or higher): - - use Time::Piece; - $day_of_year = localtime->day_of_year(); - -You can find the week of the year by using Time::Piece's strftime(): - - $week_of_year = localtime->strftime("%U"); - $iso_week = localtime->strftime("%V"); - -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? Use the following simple functions: @@ -273,10 +384,6 @@ 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. -Also note that the core module Time::Piece overloads the addition and -subtraction operators to provide date calculation options. See -L. - =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, @@ -286,19 +393,15 @@ and Date::Manip modules from CPAN. =head2 How can I find the Julian Day? -Use Time::Piece as follows: - - use Time::Piece; - my $julian_day = localtime->julian_day; - my $mjd = localtime->mjd; # modified 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 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. +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 @@ -321,14 +424,6 @@ epoch. Take twenty-four hours off that: Then you can pass this to C 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: - - 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. @@ -464,7 +559,8 @@ 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 are the CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced; -and the byacc program. +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: @@ -517,6 +613,15 @@ Use Text::Wrap (part of the standard Perl distribution): The paragraphs you give to Text::Wrap should not contain embedded newlines. Text::Wrap doesn't justify the lines (flush-right). +Or use the CPAN module Text::Autoformat. Formatting files can be easily +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 +capabilities. + =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 @@ -593,7 +698,7 @@ To make the first letter of each word upper case: This has the strange effect of turning "C" into "C". Sometimes you might want this. Other times you might need a -more thorough solution (Suggested by brian d. foy): +more thorough solution (Suggested by brian d foy): $string =~ s/ ( (^\w) #at the beginning of the line @@ -1012,7 +1117,7 @@ Like (d), but @in contains only small positive integers: 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 Idication that you probably should have used a hash, not a list or array, to store your data. Hashes are @@ -1208,20 +1313,40 @@ lists, or you could just do something like this with an array: =head2 How do I shuffle an array randomly? -Use this: +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: - # fisher_yates_shuffle( \@array ) : - # generate a random permutation of @array in place + use List::Util 'shuffle'; + + @shuffled = shuffle(@list); + +If not, you can use this: + + # fisher_yates_shuffle + # generate a random permutation of an array in place + # As in shuffling a deck of cards + # sub fisher_yates_shuffle { - my $array = shift; - my $i; - for ($i = @$array; --$i; ) { + my $deck = shift; # $deck is a reference to an array + my $i = @$deck; + while (--$i) { my $j = int rand ($i+1); - @$array[$i,$j] = @$array[$j,$i]; + @$deck[$i,$j] = @$deck[$j,$i]; } } - fisher_yates_shuffle( \@array ); # permutes @array in place +And here is an example of using it: + + # + # shuffle my mpeg collection + # + my @mpeg =