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Manpages PERLOPSection: Perl Programmers Reference Guide (1)Updated: 2005-03-28 Index Return to Main Contents NAMEperlop - Perl operators and precedenceSYNOPSISPerl operators have the following associativity and precedence, listed from highest precedence to lowest. Operators borrowed from C keep the same precedence relationship with each other, even where C's precedence is slightly screwy. (This makes learning Perl easier for C folks.) With very few exceptions, these all operate on scalar values only, not array values.
left terms and list operators (leftward) left -> nonassoc ++ -- right ** right ! ~ \ and unary + and - left =~ !~ left * / % x left + - . left << >> nonassoc named unary operators nonassoc < > <= >= lt gt le ge nonassoc == != <=> eq ne cmp left & left | ^ left && left || nonassoc .. ... right ?: right = += -= *= etc. left , => nonassoc list operators (rightward) right not left and left or xorIn the following sections, these operators are covered in precedence order. Many operators can be overloaded for objects. See overload. DESCRIPTIONTerms and List Operators (Leftward)A TERM has the highest precedence in Perl. They include variables, quote and quote-like operators, any expression in parentheses, and any function whose arguments are parenthesized. Actually, there aren't really functions in this sense, just list operators and unary operators behaving as functions because you put parentheses around the arguments. These are all documented in perlfunc.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. In the absence of parentheses, the precedence of list operators such as "print", "sort", or "chmod" is either very high or very low depending on whether you are looking at the left side or the right side of the operator. For example, in
@ary = (1, 3, sort 4, 2); print @ary; # prints 1324the commas on the right of the sort are evaluated before the sort, but the commas on the left are evaluated after. In other words, list operators tend to gobble up all arguments that follow, and then act like a simple TERM with regard to the preceding expression. Be careful with parentheses:
# These evaluate exit before doing the print: print($foo, exit); # Obviously not what you want. print $foo, exit; # Nor is this. # These do the print before evaluating exit: (print $foo), exit; # This is what you want. print($foo), exit; # Or this. print ($foo), exit; # Or even this.Also note that
print ($foo & 255) + 1, "\n";probably doesn't do what you expect at first glance. See ``Named Unary Operators'' for more discussion of this. Also parsed as terms are the "do {}" and "eval {}" constructs, as well as subroutine and method calls, and the anonymous constructors "[]" and "{}". See also ``Quote and Quote-like Operators'' toward the end of this section, as well as ``I/O Operators''. The Arrow Operator""->"" is an infix dereference operator, just as it is in C and C++. If the right side is either a "[...]", "{...}", or a "(...)" subscript, then the left side must be either a hard or symbolic reference to an array, a hash, or a subroutine respectively. (Or technically speaking, a location capable of holding a hard reference, if it's an array or hash reference being used for assignment.) See perlreftut and perlref.Otherwise, the right side is a method name or a simple scalar variable containing either the method name or a subroutine reference, and the left side must be either an object (a blessed reference) or a class name (that is, a package name). See perlobj. Auto-increment and Auto-decrement``++'' and ``--'' work as in C. That is, if placed before a variable, they increment or decrement the variable before returning the value, and if placed after, increment or decrement the variable after returning the value.The auto-increment operator has a little extra builtin magic to it. If you increment a variable that is numeric, or that has ever been used in a numeric context, you get a normal increment. If, however, the variable has been used in only string contexts since it was set, and has a value that is not the empty string and matches the pattern "/^[a-zA-Z]*[0-9]*\z/", the increment is done as a string, preserving each character within its range, with carry:
print ++($foo = '99'); # prints '100' print ++($foo = 'a0'); # prints 'a1' print ++($foo = 'Az'); # prints 'Ba' print ++($foo = 'zz'); # prints 'aaa'The auto-decrement operator is not magical. ExponentiationBinary ``**'' is the exponentiation operator. It binds even more tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is implemented using C's pow(3) function, which actually works on doubles internally.)Symbolic Unary OperatorsUnary ``!'' performs logical negation, i.e., ``not''. See also "not" for a lower precedence version of this.Unary ``-'' performs arithmetic negation if the operand is numeric. If the operand is an identifier, a string consisting of a minus sign concatenated with the identifier is returned. Otherwise, if the string starts with a plus or minus, a string starting with the opposite sign is returned. One effect of these rules is that "-bareword" is equivalent to "-bareword". Unary ``~'' performs bitwise negation, i.e., 1's complement. For example, "0666 & ~027" is 0640. (See also ``Integer Arithmetic'' and ``Bitwise String Operators''.) Note that the width of the result is platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64 bits wide on a 64-bit platform, so if you are expecting a certain bit width, remember use the & operator to mask off the excess bits. Unary ``+'' has no effect whatsoever, even on strings. It is useful syntactically for separating a function name from a parenthesized expression that would otherwise be interpreted as the complete list of function arguments. (See examples above under ``Terms and List Operators (Leftward)''.) Unary ``\'' creates a reference to whatever follows it. See perlreftut and perlref. Do not confuse this behavior with the behavior of backslash within a string, although both forms do convey the notion of protecting the next thing from interpolation. Binding OperatorsBinary ``=~'' binds a scalar expression to a pattern match. Certain operations search or modify the string $_ by default. This operator makes that kind of operation work on some other string. The right argument is a search pattern, substitution, or transliteration. The left argument is what is supposed to be searched, substituted, or transliterated instead of the default $_. When used in scalar context, the return value generally indicates the success of the operation. Behavior in list context depends on the particular operator. See ``Regexp Quote-Like Operators'' for details.If the right argument is an expression rather than a search pattern, substitution, or transliteration, it is interpreted as a search pattern at run time. This can be less efficient than an explicit search, because the pattern must be compiled every time the expression is evaluated. Binary ``!~'' is just like ``=~'' except the return value is negated in the logical sense. Multiplicative OperatorsBinary ``*'' multiplies two numbers.Binary ``/'' divides two numbers. Binary ``%'' computes the modulus of two numbers. Given integer operands $a and $b: If $b is positive, then "$a % $b" is $a minus the largest multiple of $b that is not greater than $a. If $b is negative, then "$a % $b" is $a minus the smallest multiple of $b that is not less than $a (i.e. the result will be less than or equal to zero). Note than when "use integer" is in scope, ``%'' gives you direct access to the modulus operator as implemented by your C compiler. This operator is not as well defined for negative operands, but it will execute faster. Binary ``x'' is the repetition operator. In scalar context or if the left operand is not enclosed in parentheses, it returns a string consisting of the left operand repeated the number of times specified by the right operand. In list context, if the left operand is enclosed in parentheses, it repeats the list.
print '-' x 80; # print row of dashes print "\t" x ($tab/8), ' ' x ($tab%8); # tab over @ones = (1) x 80; # a list of 80 1's @ones = (5) x @ones; # set all elements to 5 Additive OperatorsBinary ``+'' returns the sum of two numbers.Binary ``-'' returns the difference of two numbers. Binary ``.'' concatenates two strings. Shift OperatorsBinary ``<<'' returns the value of its left argument shifted left by the number of bits specified by the right argument. Arguments should be integers. (See also ``Integer Arithmetic''.)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 ``Integer Arithmetic''.) Named Unary OperatorsThe various named unary operators are treated as functions with one argument, with optional parentheses. These include the filetest operators, like "-f", "-M", etc. See perlfunc.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. 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) || diebut, because * is higher precedence than named operators:
chdir $foo * 20; # chdir ($foo * 20) chdir($foo) * 20; # (chdir $foo) * 20 chdir ($foo) * 20; # (chdir $foo) * 20 chdir +($foo) * 20; # chdir ($foo * 20) rand 10 * 20; # rand (10 * 20) rand(10) * 20; # (rand 10) * 20 rand (10) * 20; # (rand 10) * 20 rand +(10) * 20; # rand (10 * 20)See also ``Terms and List Operators (Leftward)''. Relational OperatorsBinary ``<'' returns true if the left argument is numerically less than the right argument.Binary ``>'' returns true if the left argument is numerically greater than the right argument. Binary ``<='' returns true if the left argument is numerically less than or equal to the right argument. Binary ``>='' returns true if the left argument is numerically greater than or equal to the right argument. Binary ``lt'' returns true if the left argument is stringwise less than the right argument. Binary ``gt'' returns true if the left argument is stringwise greater than the right argument. Binary ``le'' returns true if the left argument is stringwise less than or equal to the right argument. Binary ``ge'' returns true if the left argument is stringwise greater than or equal to the right argument. Equality OperatorsBinary ``=='' returns true if the left argument is numerically equal to the right argument.Binary ``!='' returns true if the left argument is numerically not equal to the right argument. Binary ``<=>'' returns -1, 0, or 1 depending on whether the left argument is numerically less than, equal to, or greater than the right argument. If your platform supports NaNs (not-a-numbers) as numeric values, using them with ``<=>'' returns undef. NaN is not ``<'', ``=='', ``>'', ``<='' or ``>='' anything (even NaN), so those 5 return false. NaN != NaN returns true, as does NaN != anything else. If your platform doesn't support NaNs then NaN is just a string with numeric value 0.
perl -le '$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. Binary ``ne'' returns true if the left argument is stringwise not equal to the right argument. Binary ``cmp'' returns -1, 0, or 1 depending on whether the left argument is stringwise less than, equal to, or greater than the right argument. ``lt'', ``le'', ``ge'', ``gt'' and ``cmp'' use the collation (sort) order specified by the current locale if "use locale" is in effect. See perllocale. Bitwise AndBinary ``&'' returns its operators ANDed together bit by bit. (See also ``Integer Arithmetic'' and ``Bitwise String Operators''.)Bitwise and Exclusive OrBinary ``|'' returns its operators ORed together bit by bit. (See also ``Integer Arithmetic'' and ``Bitwise String Operators''.)Binary ``^'' returns its operators XORed together bit by bit. (See also ``Integer Arithmetic'' and ``Bitwise String Operators''.) C-style Logical AndBinary ``&&'' performs a short-circuit logical AND operation. That is, if the left operand is false, the right operand is not even evaluated. Scalar or list context propagates down to the right operand if it is evaluated.C-style Logical OrBinary ``||'' performs a short-circuit logical OR operation. That is, if the left operand is true, the right operand is not even evaluated. Scalar or list context propagates down to the right operand if it is evaluated.The "||" and "&&" operators differ from C's in that, rather than returning 0 or 1, they return the last value evaluated. Thus, a reasonably portable way to find out the home directory (assuming it's not ``0'') might be:
$home = $ENV{'HOME'} || $ENV{'LOGDIR'} || (getpwuid($<))[7] || die "You're homeless!\n";In particular, this means that you shouldn't use this for selecting between two aggregates for assignment:
@a = @b || @c; # this is wrong @a = scalar(@b) || @c; # really meant this @a = @b ? @b : @c; # this works fine, thoughAs more readable alternatives to "&&" and "||" when used for control flow, Perl provides "and" and "or" operators (see below). The short-circuit behavior is identical. The precedence of ``and'' and ``or'' is much lower, however, so that you can safely use them after a list operator without the need for parentheses:
unlink "alpha", "beta", "gamma" or gripe(), next LINE;With the C-style operators that would have been written like this:
unlink("alpha", "beta", "gamma") || (gripe(), next LINE);Using ``or'' for assignment is unlikely to do what you want; see below. Range OperatorsBinary ``..'' is the range operator, which is really two different operators depending on the context. In list context, it returns an array of values counting (up by ones) from the left value to the right value. If the left value is greater than the right value then it returns the empty array. The range operator is useful for writing "foreach (1..10)" loops and for doing slice operations on arrays. In the current implementation, no temporary array is created when the range operator is used as the expression in "foreach" loops, but older versions of Perl might burn a lot of memory when you write something like this:
for (1 .. 1_000_000) { # code }In scalar context, ``..'' returns a boolean value. The operator is bistable, like a flip-flop, and emulates the line-range (comma) operator of sed, awk, and various editors. Each ``..'' operator maintains its own boolean state. It is false as long as its left operand is false. Once the left operand is true, the range operator stays true until the right operand is true, AFTER which the range operator becomes false again. It doesn't become false till the next time the range operator is evaluated. It can test the right operand and become false on the same evaluation it became true (as in awk), but it still returns true once. If you don't want it to test the right operand till the next evaluation, as in sed, just use three dots (``...'') instead of two. In all other regards, ``...'' behaves just like ``..'' does. The right operand is not evaluated while the operator is in the ``false'' state, and the left operand is not evaluated while the operator is in the ``true'' state. The precedence is a little lower than || and &&. The value returned is either the empty string for false, or a sequence number (beginning with 1) for true. The sequence number is reset for each range encountered. The final sequence number in a range has the string ``E0'' appended to it, which doesn't affect its numeric value, but gives you something to search for if you want to exclude the endpoint. You can exclude the beginning point by waiting for the sequence number to be greater than 1. If either operand of scalar ``..'' is a constant expression, that operand is implicitly compared to the $. variable, the current line number. Examples: As a scalar operator:
if (101 .. 200) { print; } # print 2nd hundred lines next line if (1 .. /^$/); # skip header lines s/^/> / if (/^$/ .. eof()); # quote body # parse mail messages while (<>) { $in_header = 1 .. /^$/; $in_body = /^$/ .. eof(); # do something based on those } continue { close ARGV if eof; # reset $. each file }As a list operator:
for (101 .. 200) { print; } # print $_ 100 times @foo = @foo[0 .. $#foo]; # an expensive no-op @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 itemsThe range operator (in list context) makes use of the magical auto-increment algorithm if the operands are strings. You can say
@alphabet = ('A' .. 'Z');to get all normal letters of the alphabet, or
$hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];to get a hexadecimal digit, or
@z2 = ('01' .. '31'); print $z2[$mday];to get dates with leading zeros. If the final value specified is not in the sequence that the magical increment would produce, the sequence goes until the next value would be longer than the final value specified. Conditional OperatorTernary ``?:'' is the conditional operator, just as in C. It works much like an if-then-else. If the argument before the ? is true, the argument before the : is returned, otherwise the argument after the : is returned. For example:
printf "I have %d dog%s.\n", $n, ($n == 1) ? '' : "s";Scalar or list context propagates downward into the 2nd or 3rd argument, whichever is selected.
$a = $ok ? $b : $c; # get a scalar @a = $ok ? @b : @c; # get an array $a = $ok ? @b : @c; # oops, that's just a count!The operator may be assigned to if both the 2nd and 3rd arguments are legal lvalues (meaning that you can assign to them):
($a_or_b ? $a : $b) = $c;Because this operator produces an assignable result, using assignments without parentheses will get you in trouble. For example, this:
$a % 2 ? $a += 10 : $a += 2Really means this:
(($a % 2) ? ($a += 10) : $a) += 2Rather than this:
($a % 2) ? ($a += 10) : ($a += 2)That should probably be written more simply as:
$a += ($a % 2) ? 10 : 2; Assignment Operators``='' is the ordinary assignment operator.Assignment operators work as in C. That is,
$a += 2;is equivalent to
$a = $a + 2;although without duplicating any side effects that dereferencing the lvalue might trigger, such as from tie(). Other assignment operators work similarly. The following are recognized:
**= += *= &= <<= &&= -= /= |= >>= ||= .= %= ^= x=Although these are grouped by family, they all have the precedence of assignment. Unlike in C, the scalar assignment operator produces a valid lvalue. Modifying an assignment is equivalent to doing the assignment and then modifying the variable that was assigned to. This is useful for modifying a copy of something, like this:
($tmp = $global) =~ tr [A-Z] [a-z];Likewise,
($a += 2) *= 3;is equivalent to
$a += 2; $a *= 3;Similarly, a list assignment in list context produces the list of lvalues assigned to, and a list assignment in scalar context returns the number of elements produced by the expression on the right hand side of the assignment. Comma OperatorBinary ``,'' is the comma operator. In scalar context it evaluates its left argument, throws that value away, then evaluates its right argument and returns that value. This is just like C's comma operator.In list context, it's just the list argument separator, and inserts both its arguments into the list. The => digraph is mostly just a synonym for the comma operator. It's useful for documenting arguments that come in pairs. As of release 5.001, it also forces any word to the left of it to be interpreted as a string. List Operators (Rightward)On the right side of a list operator, it has very low precedence, such that it controls all comma-separated expressions found there. The only operators with lower precedence are the logical operators ``and'', ``or'', and ``not'', which may be used to evaluate calls to list operators without the need for extra parentheses:
open HANDLE, "filename" or die "Can't open: $!\n";See also discussion of list operators in ``Terms and List Operators (Leftward)''. Logical NotUnary ``not'' returns the logical negation of the expression to its right. It's the equivalent of ``!'' except for the very low precedence.Logical AndBinary ``and'' returns the logical conjunction of the two surrounding expressions. It's equivalent to && except for the very low precedence. This means that it short-circuits: i.e., the right expression is evaluated only if the left expression is true.Logical or and Exclusive OrBinary ``or'' returns the logical disjunction of the two surrounding expressions. It's equivalent to || except for the very low precedence. This makes it useful for control flow
print FH $data or die "Can't write to FH: $!";This means that it short-circuits: i.e., the right expression is evaluated only if the left expression is false. Due to its precedence, you should probably avoid using this for assignment, only for control flow.
$a = $b or $c; # bug: this is wrong ($a = $b) or $c; # really means this $a = $b || $c; # better written this wayHowever, when it's a list-context assignment and you're trying to use ``||'' for control flow, you probably need ``or'' so that the assignment takes higher precedence.
@info = stat($file) || die; # oops, scalar sense of stat! @info = stat($file) or die; # better, now @info gets its dueThen again, you could always use parentheses. Binary ``xor'' returns the exclusive-OR of the two surrounding expressions. It cannot short circuit, of course. C Operators Missing From PerlHere is what C has that Perl doesn't:
Quote and Quote-like OperatorsWhile we usually think of quotes as literal values, in Perl they function as operators, providing various kinds of interpolating and pattern matching capabilities. Perl provides customary quote characters for these behaviors, but also provides a way for you to choose your quote character for any of them. In the following table, a "{}" represents any pair of delimiters you choose.
Customary Generic Meaning Interpolates '' q{} Literal no "" qq{} Literal yes `` qx{} Command yes (unless '' is delimiter) qw{} Word list no // m{} Pattern match yes (unless '' is delimiter) qr{} Pattern yes (unless '' is delimiter) s{}{} Substitution yes (unless '' is delimiter) tr{}{} Transliteration no (but see below)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
q{foo{bar}baz}is the same as
'foo{bar}baz'Note, however, that this does not always work for quoting Perl code:
$s = q{ if($a eq "}") ... }; # WRONGis a syntax error. The "Text::Balanced" module on CPAN is able to do this properly. There can be whitespace between the operator and the quoting characters, except when "#" is being used as the quoting character. "q#foo#" is parsed as the string "foo", while "q #foo#" is the operator "q" followed by a comment. Its argument will be taken from the next line. This allows you to write:
s {foo} # Replace foo {bar} # with bar.For constructs that do interpolate, variables beginning with ""$"`` or ''"@"" are interpolated, as are the following escape sequences. Within a transliteration, the first eleven of these sequences may be used.
\t tab (HT, TAB) \n newline (NL) \r return (CR) \f form feed (FF) \b backspace (BS) \a alarm (bell) (BEL) \e escape (ESC) \033 octal char (ESC) \x1b hex char (ESC) \x{263a} wide hex char (SMILEY) \c[ control char (ESC) \N{name} named char \l lowercase next char \u uppercase next char \L lowercase till \E \U uppercase till \E \E end case modification \Q quote non-word characters till \EIf "use locale" is in effect, the case map used by "\l", "\L", "\u" and "\U" is taken from the current locale. See perllocale. For documentation of "\N{name}", see charnames. All systems use the virtual "\n" to represent a line terminator, called a ``newline''. There is no such thing as an unvarying, physical newline character. It is only an illusion that the operating system, device drivers, C libraries, and Perl all conspire to preserve. Not all systems read "\r" as ASCII CR and "\n" as ASCII LF. For example, on a Mac, these are reversed, and on systems without line terminator, printing "\n" may emit no actual data. In general, use "\n" when you mean a ``newline'' for your system, but use the literal ASCII when you need an exact character. For example, most networking protocols expect and prefer a CR+LF ("\015\012" or "\cM\cJ") for line terminators, and although they often accept just "\012", they seldom tolerate just "\015". If you get in the habit of using "\n" for networking, you may be burned some day. You cannot include a literal "$" or "@" within a "\Q" sequence. An unescaped "$" or "@" interpolates the corresponding variable, while escaping will cause the literal string "\$" to be inserted. You'll need to write something like "m/\Quser\E\@\Qhost/". Patterns are subject to an additional level of interpretation as a regular expression. This is done as a second pass, after variables are interpolated, so that regular expressions may be incorporated into the pattern from the variables. If this is not what you want, use "\Q" to interpolate a variable literally. Apart from the behavior described above, Perl does not expand multiple levels of interpolation. In particular, contrary to the expectations of shell programmers, back-quotes do NOT interpolate within double quotes, nor do single quotes impede evaluation of variables when used within double quotes. Regexp Quote-Like OperatorsHere are the quote-like operators that apply to pattern matching and related activities.
Gory details of parsing quoted constructsWhen presented with something that might have several different interpretations, Perl uses the DWIM (that's ``Do What I Mean'') principle to pick the most probable interpretation. This strategy is so successful that Perl programmers often do not suspect the ambivalence of what they write. But from time to time, Perl's notions differ substantially from what the author honestly meant.This section hopes to clarify how Perl handles quoted constructs. Although the most common reason to learn this is to unravel labyrinthine regular expressions, because the initial steps of parsing are the same for all quoting operators, they are all discussed together. The most important Perl parsing rule is the first one discussed below: when processing a quoted construct, Perl first finds the end of that construct, then interprets its contents. If you understand this rule, you may skip the rest of this section on the first reading. The other rules are likely to contradict the user's expectations much less frequently than this first one. Some passes discussed below are performed concurrently, but because 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.
I/O OperatorsThere 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 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 $/ 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 $? (see perlvar for the interpretation of $?). Unlike in csh, 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 "qx//". (Because backticks always undergo shell expansion as well, see perlsec 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 "undef" at end-of-file or on error. When $/ is set to "undef" (sometimes known as file-slurp mode) and the file is empty, it returns '' the first time, followed by "undef" subsequently. Ordinarily you must assign the returned value to a variable, but there is one situation where an automatic assignment happens. If and only if the input symbol is the only thing inside the conditional of a "while" statement (even if disguised as a "for(;;)" 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 $_ variable is not implicitly localized. You'll have to put a "local $_;" before the loop if you want that to happen. The following lines are equivalent:
while (defined($_ = <STDIN>)) { print; } while ($_ = <STDIN>) { print; } while (<STDIN>) { print; } for (;<STDIN>;) { print; } print while defined($_ = <STDIN>); print while ($_ = <STDIN>); print while <STDIN>;This also behaves similarly, but avoids $_ :
while (my $line = <STDIN>) { print $line }In these loop constructs, the assigned value (whether assignment is automatic or explicit) is then tested to see whether it is defined. The defined test avoids problems where line has a string value that would be treated as false by Perl, for example a "`` or a ''0" with no trailing newline. If you really mean for such values to terminate the loop, they should be tested for explicitly:
while (($_ = <STDIN>) ne '0') { ... } while (<STDIN>) { last unless $_; ... }In other boolean contexts, "<filehandle>" without an explicit "defined" test or comparison elicit a warning if the "use warnings" pragma or the -w command-line switch (the $^W variable) is in effect. The filehandles STDIN, STDOUT, and STDERR are predefined. (The filehandles "stdin", "stdout", and "stderr" will also work except in packages, where they would be interpreted as local identifiers rather than global.) Additional filehandles may be created with the open() function, amongst others. See perlopentut and ``open'' in perlfunc for details on this. If a <FILEHANDLE> is used in a context that is looking for a list, a list comprising all input lines is returned, one line per list element. It's easy to grow to a rather large data space this way, so use with care. <FILEHANDLE> may also be spelled "readline(*FILEHANDLE)". See ``readline'' in perlfunc. The null filehandle <> is special: it can be used to emulate the behavior of sed and awk. Input from <> comes either from standard input, or from each file listed on the command line. Here's how it works: the first time <> is evaluated, the @ARGV array is checked, and if it is empty, $ARGV[0] is set to ``-'', which when opened gives you standard input. The @ARGV array is then processed as a list of filenames. The loop
while (<>) { ... # code for each line }is equivalent to the following Perl-like pseudo code:
unshift(@ARGV, '-') unless @ARGV; while ($ARGV = shift) { open(ARGV, $ARGV); while (<ARGV>) { ... # code for each line } }except that it isn't so cumbersome to say, and will actually work. It really does shift the @ARGV array and put the current filename into the $ARGV variable. It also uses filehandle ARGV internally--<> is just a synonym for <ARGV>, which is magical. (The pseudo code above doesn't work because it treats <ARGV> as non-magical.) You can modify @ARGV before the first <> as long as the array ends up containing the list of filenames you really want. Line numbers ($.) continue as though the input were one big happy file. See the example in ``eof'' in perlfunc for how to reset line numbers on each file. If you want to set @ARGV to your own list of files, go right ahead. This sets @ARGV to all plain text files if no @ARGV was given:
@ARGV = grep { -f && -T } glob('*') unless @ARGV;You can even set them to pipe commands. For example, this automatically filters compressed arguments through gzip:
@ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;If you want to pass switches into your script, you can use one of the Getopts modules or put a loop on the front like this:
while ($_ = $ARGV[0], /^-/) { shift; last if /^--$/; if (/^-D(.*)/) { $debug = $1 } if (/^-v/) { $verbose++ } # ... # other switches } while (<>) { # ... # code for each line }The <> symbol will return "undef" for end-of-file only once. If you call it again after this, it will assume you are processing another @ARGV list, and if you haven't set @ARGV, will read input from STDIN. If angle brackets contain is a simple scalar variable (e.g., <$foo>), then that variable contains the name of the filehandle to input from, or its typeglob, or a reference to the same. For example:
$fh = \*STDIN; $line = <$fh>;If what's within the angle brackets is neither a filehandle nor a simple scalar variable containing a filehandle name, typeglob, or typeglob reference, it is interpreted as a filename pattern to be globbed, and either a list of filenames or the next filename in the list is returned, depending on context. This distinction is determined on syntactic grounds alone. That means "<$x>" is always a readline() from an indirect handle, but "<$hash{key}>" is always a glob(). That's because $x is a simple scalar variable, but $hash{key} is not-it's a hash element. One level of double-quote interpretation is done first, but you can't say "<$foo>" because that's an indirect filehandle as explained in the previous paragraph. (In older versions of Perl, programmers would insert curly brackets to force interpretation as a filename glob: "<${foo}>". These days, it's considered cleaner to call the internal function directly as "glob($foo)", which is probably the right way to have done it in the first place.) For example:
while (<*.c>) { chmod 0644, $_; }is roughly equivalent to:
open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|"); while (<FOO>) { chomp; chmod 0644, $_; }except that the globbing is actually done internally using the standard "File::Glob" extension. Of course, the shortest way to do the above is:
chmod 0644, <*.c>;A (file)glob evaluates its (embedded) argument only when it is starting a new list. All values must be read before it will start over. In list context, this isn't important because you automatically get them all anyway. However, in scalar context the operator returns the next value each time it's called, or "undef" when the list has run out. As with filehandle reads, an automatic "defined" is generated when the glob occurs in the test part of a "while", because legal glob returns (e.g. a file called 0) would otherwise terminate the loop. Again, "undef" is returned only once. So if you're expecting a single value from a glob, it is much better to say
($file) = <blurch*>;than
$file = <blurch*>;because the latter will alternate between returning a filename and returning false. It you're trying to do variable interpolation, it's definitely better to use the glob() function, because the older notation can cause people to become confused with the indirect filehandle notation.
@files = glob("$dir/*.[ch]"); @files = glob($files[$i]); Constant FoldingLike C, Perl does a certain amount of expression evaluation at compile time whenever it determines that all arguments to an operator are static and have no side effects. In particular, string concatenation happens at compile time between literals that don't do variable substitution. Backslash interpolation also happens at compile time. You can say
'Now is the time for all' . "\n" . 'good men to come to.'and this all reduces to one string internally. Likewise, if you say
foreach $file (@filenames) { if (-s $file > 5 + 100 * 2**16) { } }the compiler will precompute the number which that expression represents so that the interpreter won't have to. Bitwise String OperatorsBitstrings of any size may be manipulated by the bitwise operators ("~ | & ^").If the operands to a binary bitwise op are strings of different sizes, | and ^ ops act as though the shorter operand had additional zero bits on the right, while the & op acts as though the longer operand were truncated to the length of the shorter. The granularity for such extension or truncation is one or more bytes.
# ASCII-based examples print "j p \n" ^ " a h"; # prints "JAPH\n" print "JA" | " ph\n"; # prints "japh\n" print "japh\nJunk" & '_____'; # prints "JAPH\n"; print 'p N$' ^ " E<H\n"; # prints "Perl\n";If you are intending to manipulate bitstrings, be certain that you're supplying bitstrings: If an operand is a number, that will imply a numeric bitwise operation. You may explicitly show which type of operation you intend by using "" or "0+", as in the examples below.
$foo = 150 | 105 ; # yields 255 (0x96 | 0x69 is 0xFF) $foo = '150' | 105 ; # yields 255 $foo = 150 | '105'; # yields 255 $foo = '150' | '105'; # yields string '155' (under ASCII) $baz = 0+$foo & 0+$bar; # both ops explicitly numeric $biz = "$foo" ^ "$bar"; # both ops explicitly stringySee ``vec'' in perlfunc for information on how to manipulate individual bits in a bit vector. Integer ArithmeticBy default, Perl assumes that it must do most of its arithmetic in floating point. But by saying
use integer;you may tell the compiler that it's okay to use integer operations (if it feels like it) from here to the end of the enclosing BLOCK. An inner BLOCK may countermand this by saying
no integer;which lasts until the end of that BLOCK. Note that this doesn't mean everything is only an integer, merely that Perl may use integer operations if it is so inclined. For example, even under "use integer", if you take the sqrt(2), you'll still get 1.4142135623731 or so. Used on numbers, the bitwise operators (``&'', ``|'', ``^'', ``~'', ``<<'', and ``>>'') always produce integral results. (But see also ``Bitwise String Operators''.) However, "use integer" still has meaning for them. By default, their results are interpreted as unsigned integers, but if "use integer" is in effect, their results are interpreted as signed integers. For example, "~0" usually evaluates to a large integral value. However, "use integer; ~0" is "-1" on twos-complement machines. Floating-point ArithmeticWhile "use integer" provides integer-only arithmetic, there is no analogous mechanism to provide automatic rounding or truncation to a certain number of decimal places. For rounding to a certain number of digits, sprintf() or printf() is usually the easiest route. See perlfaq4.Floating-point numbers are only approximations to what a mathematician would call real numbers. There are infinitely more reals than floats, so some corners must be cut. For example:
printf "%.20g\n", 123456789123456789; # produces 123456789123456784Testing for exact equality of floating-point equality or inequality is not a good idea. Here's a (relatively expensive) work-around to compare whether two floating-point numbers are equal to a particular number of decimal places. See Knuth, volume II, for a more robust treatment of this topic.
sub fp_equal { my ($X, $Y, $POINTS) = @_; my ($tX, $tY); $tX = sprintf("%.${POINTS}g", $X); $tY = sprintf("%.${POINTS}g", $Y); return $tX eq $tY; }The POSIX module (part of the standard perl distribution) implements ceil(), floor(), and other mathematical and trigonometric functions. The Math::Complex module (part of the standard perl distribution) defines mathematical functions that work on both the reals and the imaginary numbers. Math::Complex not as efficient as POSIX, but POSIX can't work with complex numbers. Rounding in financial applications can have serious implications, and the rounding method used should be specified precisely. In these cases, it probably pays not to trust whichever system rounding is being used by Perl, but to instead implement the rounding function you need yourself. Bigger NumbersThe 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 considerable speed, they avoid the normal pitfalls associated with limited-precision representations.
use Math::BigInt; $x = Math::BigInt->new('123456789123456789'); print $x * $x; # prints +15241578780673678515622620750190521There 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 libraryChoose wisely.
Index
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