X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperlop.pod;h=9ae391821a97f31934ee69e63cda0ce7b19eb329;hb=5269aecde866056a77e32c937c7c3182bb599487;hp=e97a25bc9b9aea3b879557c41a12159918611c1e;hpb=22d4bb9ccb8701e68f9243547d7e3a3c55f70908;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perlop.pod b/pod/perlop.pod index e97a25b..9ae3918 100644 --- a/pod/perlop.pod +++ b/pod/perlop.pod @@ -233,6 +233,18 @@ Binary ">>" returns the value of its left argument shifted right by the number of bits specified by the right argument. Arguments should be integers. (See also L.) +Note that both "<<" and ">>" in Perl are implemented directly using +"<<" and ">>" in C. If C (see L) is +in force then signed C integers are used, else unsigned C integers are +used. Either way, the implementation isn't going to generate results +larger than the size of the integer type Perl was built with (32 bits +or 64 bits). + +The result of overflowing the range of the integers is undefined +because it is undefined also in C. In other words, using 32-bit +integers, C<< 1 << 32 >> is undefined. Shifting by a negative number +of bits is also undefined. + =head2 Named Unary Operators The various named unary operators are treated as functions with one @@ -242,14 +254,15 @@ operators, like C<-f>, C<-M>, etc. See L. If any list operator (print(), etc.) or any unary operator (chdir(), etc.) is followed by a left parenthesis as the next token, the operator and arguments within parentheses are taken to be of highest precedence, -just like a normal function call. Examples: +just like a normal function call. For example, +because named unary operators are higher precedence than ||: chdir $foo || die; # (chdir $foo) || die chdir($foo) || die; # (chdir $foo) || die chdir ($foo) || die; # (chdir $foo) || die chdir +($foo) || die; # (chdir $foo) || die -but, because * is higher precedence than ||: +but, because * is higher precedence than named operators: chdir $foo * 20; # chdir ($foo * 20) chdir($foo) * 20; # (chdir $foo) * 20 @@ -300,8 +313,13 @@ to the right argument. Binary "<=>" returns -1, 0, or 1 depending on whether the left argument is numerically less than, equal to, or greater than the right argument. If your platform supports NaNs (not-a-numbers) as numeric -values, using them with "<=>" (or any other numeric comparison) -returns undef. +values, using them with "<=>" returns undef. NaN is not "<", "==", ">", +"<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN +returns true, as does NaN != anything else. If your platform doesn't +support NaNs then NaN is just a string with numeric value 0. + + perl -le '$a = NaN; print "No NaN support here" if $a == $a' + perl -le '$a = NaN; print "NaN support here" if $a != $a' Binary "eq" returns true if the left argument is stringwise equal to the right argument. @@ -640,13 +658,15 @@ any pair of delimiters you choose. Customary Generic Meaning Interpolates '' q{} Literal no "" qq{} Literal yes - `` qx{} Command yes (unless '' is delimiter) + `` qx{} Command yes* qw{} Word list no - // m{} Pattern match yes (unless '' is delimiter) - qr{} Pattern yes (unless '' is delimiter) - s{}{} Substitution yes (unless '' is delimiter) + // m{} Pattern match yes* + qr{} Pattern yes* + s{}{} Substitution yes* tr{}{} Transliteration no (but see below) + * unless the delimiter is ''. + Non-bracketing delimiters use the same character fore and aft, but the four sorts of brackets (round, angle, square, curly) will all nest, which means that @@ -661,8 +681,9 @@ Note, however, that this does not always work for quoting Perl code: $s = q{ if($a eq "}") ... }; # WRONG -is a syntax error. The C module on CPAN is able to do this -properly. +is a syntax error. The C module (from CPAN, and +starting from Perl 5.8 part of the standard distribution) is able +to do this properly. There can be whitespace between the operator and the quoting characters, except when C<#> is being used as the quoting character. @@ -673,9 +694,8 @@ from the next line. This allows you to write: s {foo} # Replace foo {bar} # with bar. -For constructs that do interpolate, variables beginning with "C<$>" -or "C<@>" are interpolated, as are the following escape sequences. Within -a transliteration, the first eleven of these sequences may be used. +The following escape sequences are available in constructs that interpolate +and in transliterations. \t tab (HT, TAB) \n newline (NL) @@ -690,6 +710,9 @@ a transliteration, the first eleven of these sequences may be used. \c[ control char (ESC) \N{name} named char +The following escape sequences are available in constructs that interpolate +but not in transliterations. + \l lowercase next char \u uppercase next char \L lowercase till \E @@ -715,6 +738,16 @@ and although they often accept just C<"\012">, they seldom tolerate just C<"\015">. If you get in the habit of using C<"\n"> for networking, you may be burned some day. +For constructs that do interpolate, variables beginning with "C<$>" +or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or +C<$href->{key}[0]> are also interpolated, as are array and hash slices. +But method calls such as C<$obj->meth> are not. + +Interpolating an array or slice interpolates the elements in order, +separated by the value of C<$">, so is equivalent to interpolating +C. "Punctuation" arrays such as C<@+> are only +interpolated if the name is enclosed in braces C<@{+}>. + You cannot include a literal C<$> or C<@> within a C<\Q> sequence. An unescaped C<$> or C<@> interpolates the corresponding variable, while escaping will cause the literal string C<\$> to be inserted. @@ -755,7 +788,7 @@ patterns local to the current package are reset. reset if eof; # clear ?? status for next file } -This usage is vaguely depreciated, which means it just might possibly +This usage is vaguely deprecated, which means it just might possibly be removed in some distant future version of Perl, perhaps somewhere around the year 2168. @@ -798,7 +831,7 @@ the trailing delimiter. This avoids expensive run-time recompilations, and is useful when the value you are interpolating won't change over the life of the script. However, mentioning C constitutes a promise that you won't change the variables in the pattern. If you change them, -Perl won't even notice. See also L<"qr//">. +Perl won't even notice. See also L<"qr/STRING/imosx">. If the PATTERN evaluates to the empty string, the last I matched regular expression is used instead. @@ -951,7 +984,7 @@ A double-quoted, interpolated string. =item qr/STRING/imosx -This operators quotes--and compiles--its I as a regular +This operator quotes (and possibly compiles) its I as a regular expression. I is interpolated the same way as I in C. If "'" is used as the delimiter, no interpolation is done. Returns a Perl value which may be used instead of the @@ -1338,7 +1371,7 @@ their results are the same, we consider them individually. For different quoting constructs, Perl performs different numbers of passes, from one to five, but these passes are always performed in the same order. -=over +=over 4 =item Finding the end @@ -1392,7 +1425,7 @@ used in parsing. The next step is interpolation in the text obtained, which is now delimiter-independent. There are four different cases. -=over +=over 4 =item C<<<'EOF'>, C, C, C, C @@ -1557,19 +1590,19 @@ There are several I/O operators you should know about. A string enclosed by backticks (grave accents) first undergoes double-quote interpolation. It is then interpreted as an external command, and the output of that command is the value of the -pseudo-literal, j -string consisting of all output is returned. In list context, a -list of values is returned, one per line of output. (You can set -C<$/> to use a different line terminator.) The command is executed -each time the pseudo-literal is evaluated. The status value of the -command is returned in C<$?> (see L for the interpretation -of C<$?>). Unlike in B, no translation is done on the return -data--newlines remain newlines. Unlike in any of the shells, single -quotes do not hide variable names in the command from interpretation. -To pass a literal dollar-sign through to the shell you need to hide -it with a backslash. The generalized form of backticks is C. -(Because backticks always undergo shell expansion as well, see -L for security concerns.) +backtick string, like in a shell. In scalar context, a single string +consisting of all output is returned. In list context, a list of +values is returned, one per line of output. (You can set C<$/> to use +a different line terminator.) The command is executed each time the +pseudo-literal is evaluated. The status value of the command is +returned in C<$?> (see L for the interpretation of C<$?>). +Unlike in B, no translation is done on the return data--newlines +remain newlines. Unlike in any of the shells, single quotes do not +hide variable names in the command from interpretation. To pass a +literal dollar-sign through to the shell you need to hide it with a +backslash. The generalized form of backticks is C. (Because +backticks always undergo shell expansion as well, see L for +security concerns.) In scalar context, evaluating a filehandle in angle brackets yields the next line from that file (the newline, if any, included), or @@ -1584,7 +1617,7 @@ of a C statement (even if disguised as a C loop), the value is automatically assigned to the global variable $_, destroying whatever was there previously. (This may seem like an odd thing to you, but you'll use the construct in almost every Perl -script you write.) The $_ variables is not implicitly localized. +script you write.) The $_ variable is not implicitly localized. You'll have to put a C before the loop if you want that to happen. @@ -1695,7 +1728,7 @@ The <> symbol will return C for end-of-file only once. If you call it again after this, it will assume you are processing another @ARGV list, and if you haven't set @ARGV, will read input from STDIN. -If angle brackets contain is a simple scalar variable (e.g., +If what the angle brackets contain is a simple scalar variable (e.g., <$foo>), then that variable contains the name of the filehandle to input from, or its typeglob, or a reference to the same. For example: @@ -1729,7 +1762,7 @@ is roughly equivalent to: open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|"); while () { - chop; + chomp; chmod 0644, $_; } @@ -1759,7 +1792,7 @@ than because the latter will alternate between returning a filename and returning false. -It you're trying to do variable interpolation, it's definitely better +If you're trying to do variable interpolation, it's definitely better to use the glob() function, because the older notation can cause people to become confused with the indirect filehandle notation. @@ -1842,8 +1875,8 @@ integer>, if you take the C, you'll still get C<1.4142135623731> or so. Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<", -and ">>") always produce integral results. (But see also L.) However, C still has meaning for +and ">>") always produce integral results. (But see also +L.) However, C still has meaning for them. By default, their results are interpreted as unsigned integers, but if C is in effect, their results are interpreted as signed integers. For example, C<~0> usually evaluates to a large @@ -1896,7 +1929,7 @@ need yourself. The standard Math::BigInt and Math::BigFloat modules provide variable-precision arithmetic and overloaded operators, although -they're currently pretty slow. At the cost of some space and +they're currently pretty slow. At the cost of some space and considerable speed, they avoid the normal pitfalls associated with limited-precision representations. @@ -1906,8 +1939,25 @@ limited-precision representations. # prints +15241578780673678515622620750190521 -The non-standard modules SSLeay::BN and Math::Pari provide -equivalent functionality (and much more) with a substantial -performance savings. +There are several modules that let you calculate with (bound only by +memory and cpu-time) unlimited or fixed precision. There are also +some non-standard modules that provide faster implementations via +external C libraries. + +Here is a short, but incomplete summary: + + Math::Fraction big, unlimited fractions like 9973 / 12967 + Math::String treat string sequences like numbers + Math::FixedPrecision calculate with a fixed precision + Math::Currency for currency calculations + Bit::Vector manipulate bit vectors fast (uses C) + Math::BigIntFast Bit::Vector wrapper for big numbers + Math::Pari provides access to the Pari C library + Math::BigInteger uses an external C library + Math::Cephes uses external Cephes C library (no big numbers) + Math::Cephes::Fraction fractions via the Cephes library + Math::GMP another one using an external C library + +Choose wisely. =cut