September 10, 2013 06:15 Hans Ginzel wrote:
> Is there a shorter way to write $a = ! $a, please?
> Something analogous to ++ and -- operators like $a !! or !! $a would
> negate the variable $a and return its previous or new value
> respectively.
I don't believe Perl has boolean pre-invert or post-invert unary
operators. You might be able to build something with objects and
operator overloading, but that would mean overloading existing
operators. Adding entirely new operators could require hacking the Perl
source code (?).
On 09/10/13 04:14, Dr.Ruud wrote:
$a^=1
The bitwise xor-equals binary operator is an interesting suggestion.
Yes, it inverts the boolean sense of variables containing canonical
boolean values (undef, empty string, numerical zero, and numerical one).
But, thinking in boolean and applying this operator to other kinds of
values may be confusing (see script and output, below).
Assuming canonical boolean values, post-invert semantics (save the new
value into another variable) can be can be obtained with assignment:
$new = ($var ^= 1)
It appears that xor-equals has higher precedence than assignment, so the
parentheses are not required:
$new = $var ^= 1
Pre-invert semantics (save the old value into another variable) can be
obtained with multiple assignment, protected within parentheses, so that
xor-equals operates on the correct lvalue:
($var = $old = $var) ^= 1
Note that the following will make a copy and then invert the copy.
leaving the original untouched:
($varn = $var) ^= 1
HTH,
David
2013-09-10 14:27:36 dpchrist@desktop ~/sandbox/perl
$ cat xor-equal.pl
#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;
my ( $t, $f, $u, $e, $z, $o, $i, $n, $s);
my ($newt, $newf, $newu, $newe, $newz, $newo, $newi, $newn, $news);
my ($oldt, $oldf, $oldu, $olde, $oldz, $oldo, $oldi, $oldn, $olds);
sub dump_new {
Data::Dumper->Dump(
[$newt, $newf, $newu, $newe, $newz, $newo, $newi, $newn, $news],
[qw(newt newf newu newe newz newo newi newn news)]);
}
sub dump_old {
Data::Dumper->Dump(
[$oldt, $oldf, $oldu, $olde, $oldz, $oldo, $oldi, $oldn, $olds],
[qw(oldt oldf oldu olde oldz oldo oldi oldn olds)]);
}
sub dump_var {
Data::Dumper->Dump([$t, $f, $u, $e, $z, $o, $i, $n, $s],
[qw(t f u e z o i n s)]);
}
sub reset_var {
$t = (1 == 1); # true
$f = (1 != 1); # false
$u = undef; # undefined value
$e = ''; # empty string
$z = 0; # zero
$o = 1; # one
$i = 255; # integer
$n = 3.141592653589793238462; # double
$s = "Just another Perl hacker"; # string
}
sub save_new {
$newt = $t ^= 1;
$newf = $f ^= 1;
$newu = $u ^= 1;
$newe = $e ^= 1;
$newz = $z ^= 1;
$newo = $o ^= 1;
$newi = $i ^= 1;
$newn = $n ^= 1;
$news = $s ^= 1;
}
sub save_old {
($t = $oldt = $t) ^= 1;
($f = $oldf = $f) ^= 1;
($u = $oldu = $u) ^= 1;
($e = $olde = $e) ^= 1;
($z = $oldz = $z) ^= 1;
($o = $oldo = $o) ^= 1;
($i = $oldi = $i) ^= 1;
($n = $oldn = $n) ^= 1;
($s = $olds = $s) ^= 1;
}
reset_var();
print "### reset values ###\n", dump_var();
save_new();
print "### xor-equals, save new values ###\n", dump_var(), dump_new();
save_new();
print "### again ###\n", dump_var(), dump_new();
reset_var();
print "### reset values ###\n", dump_var();
save_old();
print "### xor-equals, save old values ###\n", dump_var(), dump_old();
save_old();
print "### again ###\n", dump_var(), dump_old();
2013-09-10 14:27:38 dpchrist@desktop ~/sandbox/perl
$ perl xor-equal.pl
### reset values ###
$t = 1;
$f = '';
$u = undef;
$e = '';
$z = 0;
$o = 1;
$i = 255;
$n = '3.14159265358979';
$s = 'Just another Perl hacker';
Argument "" isn't numeric in bitwise xor (^) at xor-equal.pl line 43.
Argument "Just another Perl hacker" isn't numeric in bitwise xor (^) at
xor-equal.pl line 48.
### xor-equals, save new values ###
$t = 0;
$f = 1;
$u = 1;
$e = 1;
$z = 1;
$o = 0;
$i = 254;
$n = 2;
$s = 1;
$newt = 0;
$newf = 1;
$newu = 1;
$newe = 1;
$newz = 1;
$newo = 0;
$newi = 254;
$newn = 2;
$news = 1;
### again ###
$t = 1;
$f = 0;
$u = 0;
$e = 0;
$z = 0;
$o = 1;
$i = 255;
$n = 3;
$s = 0;
$newt = 1;
$newf = 0;
$newu = 0;
$newe = 0;
$newz = 0;
$newo = 1;
$newi = 255;
$newn = 3;
$news = 0;
### reset values ###
$t = 1;
$f = '';
$u = undef;
$e = '';
$z = 0;
$o = 1;
$i = 255;
$n = '3.14159265358979';
$s = 'Just another Perl hacker';
Argument "" isn't numeric in bitwise xor (^) at xor-equal.pl line 55.
Argument "Just another Perl hacker" isn't numeric in bitwise xor (^) at
xor-equal.pl line 60.
### xor-equals, save old values ###
$t = 0;
$f = 1;
$u = 1;
$e = 1;
$z = 1;
$o = 0;
$i = 254;
$n = 2;
$s = 1;
$oldt = 1;
$oldf = '';
$oldu = undef;
$olde = '';
$oldz = 0;
$oldo = 1;
$oldi = 255;
$oldn = '3.14159265358979';
$olds = 'Just another Perl hacker';
### again ###
$t = 1;
$f = 0;
$u = 0;
$e = 0;
$z = 0;
$o = 1;
$i = 255;
$n = 3;
$s = 0;
$oldt = 0;
$oldf = 1;
$oldu = 1;
$olde = 1;
$oldz = 1;
$oldo = 0;
$oldi = 254;
$oldn = 2;
$olds = 1;
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