Re: Evaluate arbitrary boolean expressions
by Corion (Pope) on Mar 13, 2018 at 10:26 UTC

Do you want to do algebraic symbolic reduction of the terms or do you just want to evaluate it?
One approach would be to evaluate every atomic expression lefttoright until you have no more operators, and then using De Morgan's laws to resolve parentheses.
 [reply] 

Thanks for the quick response.
In answer to your question: I am only interested in whether the expression I generate is true or false. I do not need to do any algebraic symbolic reduction of the terms.
 [reply] 

#!perl w
use strict;
my @terms = (
'( ( T  F ) && T )',
'( ( T  F ) && ( F && F ))'
);
sub simplify {
my( $term ) = @_;
$term =~ s!\s+!!g; # eliminate all whitespace
my $changed;
do {
$changed = 0;
$changed = ($term =~ s!F\\!!g);
#print "$term ($changed)\n";
$changed = ($term =~ s!\\F!!g);
#print "$term ($changed)\n";
$changed = ($term =~ s!T\&\&!!g);
#print "$term ($changed)\n";
$changed = ($term =~ s!\&\&T!!g);
#print "$term ($changed)\n";
$changed = ($term =~ s!F\&\&!!g);
#print "$term ($changed)\n";
$changed = ($term =~ s!\&\&F!!g);
#print "$term ($changed)\n";
$changed = ($term =~ s!T\\!!g);
#print "$term ($changed)\n";
$changed = ($term =~ s!\\T!!g);
#print "$term ($changed)\n";
$changed = ($term =~ s!\(([FT])\)!$1!g);
#print "$term ($changed)\n";
} while $changed;
return $term
}
for my $t (@terms) {
print "$t => " . simplify( $t ), "\n";
};
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Re: Evaluate arbitrary boolean expressions
by hippo (Canon) on Mar 13, 2018 at 10:53 UTC

my $bool = (foo() && !foo())  (!foo() && !foo());
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 [reply] 

if ($bool) {
print "It's true\n";
}
else {
print "It's false\n";
}
...roboticus
When your only tool is a hammer, all problems look like your thumb.  [reply] [d/l] 



Re: Evaluate arbitrary boolean expressions
by LanX (Bishop) on Mar 13, 2018 at 17:49 UTC

double negation !! will return the "normalized" boolean value in Perl
 !!0 is "the" False
 !!1 is "the" True
D:\Users\RolfLangsdorf>perl de0
Loading DB routines from perl5db.pl version 1.37
Editor support available.
Enter h or 'h h' for help, or 'perldoc perldebug' for more help.
main::(e:1): 0
DB<1> x [!!0,!!1]
0 ARRAY(0x28d1a8)
0 ''
1 1
DB<2>
if you need more syntactic sugar, than map them with use constant or use Readonly
updates
DB<1> use constant {TRUE => !!1, FALSE=> !!0}
DB<2> x [TRUE,FALSE]
0 ARRAY(0x4c6170)
0 1
1 ''
Cheers Rolf
_{(addicted to the Perl Programming Language and ☆☆☆☆ :)
Wikisyntax for the Monastery
}
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Re: Evaluate arbitrary boolean expressions
by Eily (Prior) on Mar 13, 2018 at 13:44 UTC

It looks like you want something that is true or false when used in boolean context (meaning, && and  work as expected) and is the string "true" or "false" when used in string context. Do note that perl doesn't have a specific boolean type, but uses either strings or numbers instead. You can obtain what (I think) you want with an overloaded object. I got lazy so what I did is use the code of boolean and replaced the "overload" call with:
use overload
'0+' => sub { ${$_[0]} },
'""' => sub { ${$_[0]} ? "true" : "false"},
'!' => sub { ${$_[0]} ? $false : $true },
fallback => 1;
Then the return value of true and false can be used a string. There are some possible issues though, the string "false" is not empty, so it is actually true, and the string "true" turns into the number 0 so it will be different from the number you may get from true() (which is 1):
use boolean;
$\ = $/;
my $false = false;
print "\$false is $false" unless $false;
my $copy = $false;
print '$copy is also false' if not $copy;
my $str_false = "$false";
print '$str_false is true?' if $str_false;
print 'But equal to false' if $str_false eq $false;
my $true = true;
my $str_true = "$true";
print '$str_true is true but != from true' if $str_true and ($str_true
+ != true);
print "The value of this test is: ", boolean(truefalse);
$false is false
$copy is also false
$str_false is true?
But equal to false
$str_true is true but != from true
The value of this test is: true
It kind of works as long as you do not try to convert the values to a string or a number, and then use them again as boolean. The best solution IMHO would rather be to translate the value to the boolean string whenever you want to print it, and keep the number otherwise, with sub to_bool_str { $_[0] ? "true" : "false" }; print "1 is ", to_bool_str (1);  [reply] [d/l] [select] 
Re: Evaluate arbitrary boolean expressions
by tybalt89 (Priest) on Mar 13, 2018 at 15:23 UTC

#!/usr/bin/perl
# http://perlmonks.org/?node_id=1210791
use strict;
use warnings;
sub error { die s/\G/ *** Expected @_ >/r, "\n" }
sub want { /\G\s+/gc; /\G\Q$_[1]\E/gc ? shift : error "'$_[1]'" }
sub expr
{
my $p = shift;
/\G\s+/gc;
my $value =
/\G(TF)/gc ? $1 :
/\G\(/gc ? want expr(0), ')' :
error "Operand";
$value =
/\G\s+/gc ? $value :
$p <= 1 && /\G\&\&/gc ? $value eq 'T' ? expr(2) : (expr(2), 'F') :
$p <= 0 && /\G\\/gc ? $value eq 'T' ? (expr(1), 'T') : expr(1) :
return $value while 1;
}
while(<DATA>)
{
my $answer = expr(0);
/\G\s*\z/ or error "Complete Parse";
print tr/\n//dr, " = $answer\n";
}
__DATA__
(T && F)  (F && F)
(T && T)  (F && F)
(T && F)  T
Outputs:
(T && F)  (F && F) = F
(T && T)  (F && F) = T
(T && F)  T = T
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Re: Evaluate arbitrary boolean expressions
by choroba (Bishop) on Mar 13, 2018 at 17:44 UTC

If you don't want to use eval for security purposes, write a parser. Marpa::R2 is a module that can help you with that:
#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };
use Marpa::R2;
my $dsl = << '__DSL__';
:default ::= action => ::first
lexeme default = latm => 1
Exp ::= Term
 Exp (disj) Term action => disj
Term ::= Factor
 Term (conj) Factor action => conj
Factor ::= Atom
 Negation
 Bracketed
Bracketed ::= (lbra) Exp (rbra)
Negation ::= (excl) Factor action => neg
Atom ::= true action => true
 false action => false
true ~ 'T'
false ~ 'F'
excl ~ '!'
conj ~ '&&'
disj ~ ''
lbra ~ '('
rbra ~ ')'
ws ~ [\s]+
:discard ~ ws
__DSL__
sub true { 1 }
sub false { 0 }
sub neg { ! $_[1] }
sub conj { $_[1] && $_[2] }
sub disj { $_[1]  $_[2] }
my $grammar = 'Marpa::R2::Scanless::G'>new({source => \$dsl});
my $input = '((T)  (!(T))) && !(F && F && F && F && ! (T))';
my $value = ${ $grammar>parse(\$input, 'main') };
say $value ? 'TRUE' : 'FALSE';
($q=q:Sq=~/;[c](.)(.)/;chr(5+lengthSq)`"SoS2"`map{chr +ord
}map{substrSq`S_+`}3E`7**23:)=~y+S`+$1,++print+eval$q,q,a,
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Re: Evaluate arbitrary boolean expressions
by Marshall (Abbot) on Mar 13, 2018 at 16:30 UTC

It's been maybe 25 years since I wrote a prefix(algebraic) to postfix (RPN) routine to parse an algebraic expression. I would have to work on this for awhile to get it working.
Example using stack:
(2+3)*5 results in: push 2, push 3, add, push 5, multiply
This requires an op precedence table.
I think you are better off with some form of Perl eval if the actual equation is unknown until runtime.
As another technique, for 8 variables, you can make an array of say 256 entries, use one bit for each variable, precalculate the results for the 8 variables and use a simple lookup table for the result.
This is also possible with a Perl hash table. Just calculate values where the 8 or whatever variables describe a "true" result.
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Re: Evaluate arbitrary boolean expressions
by Anonymous Monk on Mar 13, 2018 at 12:42 UTC

If you need to evaluate an arbitrary and unforeseen expression, eval an assignmentstatement. (And, arrange to catch any runtime errors that may result.)  [reply] 