X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperlfaq4.pod;h=e816fb3893d976fce01cd847d5ef3791f49c381d;hb=7207e29d564e32c8ec2cb43eabae72ff68c02442;hp=ecbd65243e5ff28a3966b50bc16c180cb0e5be43;hpb=bf4acbe410c9fcc2bff9bfa63411be8c6c46902a;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perlfaq4.pod b/pod/perlfaq4.pod index ecbd652..e816fb3 100644 --- a/pod/perlfaq4.pod +++ b/pod/perlfaq4.pod @@ -1,10 +1,10 @@ =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 $) =head1 DESCRIPTION -The section of the FAQ answers question related to the manipulation +The section of the FAQ answers questions related to the manipulation of data as numbers, dates, strings, arrays, hashes, and miscellaneous data issues. @@ -13,13 +13,13 @@ data issues. =head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)? The infinite set that a mathematician thinks of as the real numbers can -only be approximate on a computer, since the computer only has a finite +only be approximated on a computer, since the computer only has a finite number of bits to store an infinite number of, um, numbers. Internally, your computer represents floating-point numbers in binary. Floating-point numbers read in from a file or appearing as literals in your program are converted from their decimal floating-point -representation (eg, 19.95) to the internal binary representation. +representation (eg, 19.95) to an internal binary representation. However, 19.95 can't be precisely represented as a binary floating-point number, just like 1/3 can't be exactly represented as a @@ -29,7 +29,7 @@ of 19.95, therefore, isn't exactly 19.95. When a floating-point number gets printed, the binary floating-point representation is converted back to decimal. These decimal numbers are displayed in either the format you specify with printf(), or the -current output format for numbers (see L if you use +current output format for numbers. (See L if you use print. C<$#> has a different default value in Perl5 than it did in Perl4. Changing C<$#> yourself is deprecated.) @@ -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 @@ -75,7 +97,7 @@ functions. $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 @@ -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); -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 unpack - $decimal = unpack('c', pack('B8', '10110110')); + $hex = unpack("H*", pack("N", 3735928559)); -This packs the string C<10110110> into an eight bit binary structure. -This is then unpacked as a character, which returns its ordinal value. +Using Bit::Vector -This does the same thing: + 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); + +Using Bit::Vector + + use Bit::Vector; + $vec = Bit::Vector->new_Dec(32, -559038737); + $oct = reverse join('', $vec->Chunk_List_Read(3)); + +=item B + +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? @@ -206,8 +334,8 @@ 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 -Phoenix, talks more about this.. John von Neumann said, ``Anyone who +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 5.004 or higher): - - use Time::localtime; - $day_of_year = localtime(time())->yday; - -You can find the week of the year by dividing this by 7: - - $week_of_year = int($day_of_year / 7); - -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. - =head2 How do I find the current century or millennium? Use the following simple functions: @@ -285,21 +396,23 @@ and Date::Manip modules from CPAN. 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 they really just interested in -a way of getting serial days so that they can do date arithmetic? If you +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 -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.) +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? @@ -370,7 +483,7 @@ you can. Is that the pencil's fault? Of course it isn't. 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 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. @@ -398,7 +511,7 @@ addresses, etc.) for details. It depends just what you mean by ``escape''. URL escapes are dealt with in L. Shell escapes with the backslash (C<\>) -character are removed with: +character are removed with s/\\(.)/$1/g; @@ -446,7 +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: @@ -499,6 +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}' $*" + +See the documentation for Text::Autoformat to appreciate its many +capabilities. + =head2 How can I access/change the first N letters of a string? There are many ways. If you just want to grab a copy, use @@ -512,7 +635,7 @@ use substr() as an lvalue: substr($a, 0, 3) = "Tom"; Although those with a pattern matching kind of thought process will -likely prefer: +likely prefer $a =~ s/^.../Tom/; @@ -549,7 +672,7 @@ repetition count and repeated pattern like this: =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 function like so: @@ -574,8 +697,8 @@ To make the first letter of each word upper case: $line =~ s/\b(\w)/\U$1/g; This has the strange effect of turning "C" into "C". 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 @@ -637,15 +760,15 @@ distribution) lets you say: 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: $string =~ s/^\s+//; @@ -740,7 +863,7 @@ you can use this kind of thing: =head2 How do I find the soundex value of a string? Use the standard Text::Soundex module distributed with Perl. -But before you do so, you may want to determine whether `soundex' is in +Before you do so, you may want to determine whether `soundex' is in fact what you think it is. Knuth's soundex algorithm compresses words into a small space, and so it does not necessarily distinguish between two words which you might want to appear separately. For example, the @@ -779,9 +902,9 @@ of the FAQ. =head2 What's wrong with always quoting "$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? @@ -857,13 +980,13 @@ in the indentation. 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 { @@ -886,7 +1009,7 @@ This works with leading special strings, dynamically determined: @@@ } 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< 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. @@ -924,7 +1047,7 @@ last value to be returned: 9. =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). @@ -948,33 +1071,43 @@ ordered and whether you wish to preserve the ordering. =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[$_]++, @in); -=item d) A way to do (b) without any loops or greps: +=item d) + +A way to do (b) without any loops or greps: undef %saw; @saw{@in} = (); @out = sort keys %saw; # remove sort if undesired -=item e) Like (d), but @in contains only small positive integers: +=item e) + +Like (d), but @in contains only small positive integers: undef @ary; @ary[@in] = @in; @@ -984,7 +1117,7 @@ nice in that it won't work with false values like undef, 0, or ""; But perhaps you should have been using a hash all along, eh? -=head2 How can I tell whether a list or array contains a certain element? +=head2 How can I tell whether a certain element is contained in a list or array? Hearing the word "in" is an Idication that you probably should have used a hash, not a list or array, to store your data. Hashes are @@ -1023,11 +1156,11 @@ Now check whether C is true for some C<$n>. 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 @@ -1057,7 +1190,7 @@ each element is unique in a given array: } Note that this is the I, 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? @@ -1148,7 +1281,7 @@ You could walk the list this way: } 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 @@ -1180,24 +1313,43 @@ 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); - next if $i == $j; - @$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 =