PERL - Miscellaneous Operations

Miscellaneous Operations

caller(EXPR)

caller

Returns the context of the current subroutine call:


	($package,$filename,$line) = caller;

With EXPR, returns some extra information that the debugger uses to print a stack trace. The value of EXPR indicates how many call frames to go back before the current one.

defined(EXPR)

defined EXPR

Returns a boolean value saying whether the lvalue EXPR has a real value or not. Many operations return the undefined value under exceptional conditions, such as end of file, uninitialized variable, system error and such. This function allows you to distinguish between an undefined null string and a defined null string with operations that might return a real null string, in particular referencing elements of an array. You may also check to see if arrays or subroutines exist. Use on predefined variables is not guaranteed to produce intuitive results. Examples:


	print if defined $switch{'D'};
	print "$val\n" while defined($val = pop(@ary));
	die "Can't readlink $sym: $!"
		unless defined($value = readlink $sym);
	eval '@foo = ()' if defined(@foo);
	die "No XYZ package defined" unless defined %_XYZ;
	sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }

See also undef.

dump LABEL

This causes an immediate core dump. Primarily this is so that you can use the undump program to turn your core dump into an executable binary after having initialized all your variables at the beginning of the program. When the new binary is executed it will begin by executing a "goto LABEL" (with all the restrictions that goto suffers). Think of it as a goto with an intervening core dump and reincarnation. If LABEL is omitted, restarts the program from the top. WARNING: any files opened at the time of the dump will NOT be open any more when the program is reincarnated, with possible resulting confusion on the part of perl. See also -u.

Example:


	#!/usr/bin/perl
	require 'getopt.pl';
	require 'stat.pl';
	%days = (
	    'Sun',1,
	    'Mon',2,
	    'Tue',3,
	    'Wed',4,
	    'Thu',5,
	    'Fri',6,
	    'Sat',7);

	dump QUICKSTART if $ARGV[0] eq '-d';

    QUICKSTART:
	do Getopt('f');

local(LIST)

Declares the listed variables to be local to the enclosing block, subroutine, eval or "do". All the listed elements must be legal lvalues. This operator works by saving the current values of those variables in LIST on a hidden stack and restoring them upon exiting the block, subroutine or eval. This means that called subroutines can also reference the local variable, but not the global one. The LIST may be assigned to if desired, which allows you to initialize your local variables. (If no initializer is given for a particular variable, it is created with an undefined value.) Commonly this is used to name the parameters to a subroutine. Examples:


	sub RANGEVAL {
		local($min, $max, $thunk) = @_;
		local($result) = '';
		local($i);

		# Presumably $thunk makes reference to $i

		for ($i = $min; $i < $max; $i++) {
			$result .= eval $thunk;
		}

		$result;
	}

	if ($sw eq '-v') {
	    # init local array with global array
	    local(@ARGV) = @ARGV;
	    unshift(@ARGV,'echo');
	    system @ARGV;
	}
	# @ARGV restored

	# temporarily add to digits associative array
	if ($base12) {
		# (NOTE: not claiming this is efficient!)
		local(%digits) = (%digits,'t',10,'e',11);
		do parse_num();
	}

Note that local() is a run-time command, and so gets executed every time through a loop, using up more stack storage each time until it's all released at once when the loop is exited.

require(EXPR)

require EXPR

require

Includes the library file specified by EXPR, or by $_ if EXPR is not supplied. Has semantics similar to the following subroutine:


	sub require {
	    local($filename) = @_;
	    return 1 if $INC{$filename};
	    local($realfilename,$result);
	    ITER: {
		foreach $prefix (@INC) {
		    $realfilename = "$prefix/$filename";
		    if (-f $realfilename) {
			$result = do $realfilename;
			last ITER;
		    }
		}
		die "Can't find $filename in \@INC";
	    }
	    die $@ if $@;
	    die "$filename did not return true value" unless $result;
	    $INC{$filename} = $realfilename;
	    $result;
	}

Note that the file will not be included twice under the same specified name. The file must return true as the last statement to indicate successful execution of any initialization code, so it's customary to end such a file with "1;" unless you're sure it'll return true otherwise.

reset(EXPR)

reset EXPR

reset

Generally used in a continue block at the end of a loop to clear variables and reset ?? searches so that they work again. The expression is interpreted as a list of single characters (hyphens allowed for ranges). All variables and arrays beginning with one of those letters are reset to their pristine state. If the expression is omitted, one-match searches (?pattern?) are reset to match again. Only resets variables or searches in the current package. Always returns 1. Examples:


    reset 'X';		# reset all X variables
    reset 'a-z';	# reset lower case variables
    reset;		# just reset ?? searches

Note: resetting "A-Z" is not recommended since you'll wipe out your ARGV and ENV arrays.

The use of reset on dbm associative arrays does not change the dbm file. (It does, however, flush any entries cached by perl, which may be useful if you are sharing the dbm file. Then again, maybe not.)

scalar(EXPR)

Forces EXPR to be interpreted in a scalar context and returns the value of EXPR.

undef(EXPR)

undef EXPR

undef

Undefines the value of EXPR, which must be an lvalue. Use only on a scalar value, an entire array, or a subroutine name (using &). (Undef will probably not do what you expect on most predefined variables or dbm array values.) Always returns the undefined value. You can omit the EXPR, in which case nothing is undefined, but you still get an undefined value that you could, for instance, return from a subroutine. Examples:


	undef $foo;
	undef $bar{'blurfl'};
	undef @ary;
	undef %assoc;
	undef &mysub;
	return (wantarray ? () : undef) if $they_blew_it;

wantarray

Returns true if the context of the currently executing subroutine is looking for an array value. Returns false if the context is looking for a scalar.


	return wantarray ? () : undef;