bobf++, well done.

There is another logical operator; I like to regard the sequence operator, scalar comma, as the unconditional logical operator. The right hand argument is evaluated regardless of the truth of the already-evaluated left, and the last evaluated value is returned. Naturally, this has more use in shoehorning several evaluations into a single expression than in pure logic. It's often useful that way in the body of modifier statements like s/foo/bar/g, print for <>;.

Used where logicals are often seen, sequence expressions can be a sort of front-loaded continue block - without introducing a scope.

my ($i, $j) = (1, 10);
while ($i++, $j--, $i < $j) {
    print "$i\t$j\n";
}
__END__
2       9
3       8
4       7
5       6
[download]

I think that that view of the sequence operator is not the usual one, but I find it useful. If there were an ultra-low precedence version like and and or, I'd call it then.

It's worth mentioning this tidbit I picked up from HOP: '0 but true' is a special-case that will not issue a warning when used in a numeric context.

$ perl -we 'print "0 but true" + 2'
2

$ perl -we 'print "0 but actually true" + 2'
Argument "0 but actually true" isn't numeric in addition (+) at -e lin
+e 1.
2
[download]

Interesting. I wasn't aware of "0 but true" being special cased. But you did realize that strings matching /^0E\d+$/ have this property too right?

D:\>perl -lwe "my @b=(0,1); for (qw(0E0 1E0)) { print; print qq(Bool:)
+,$b[!!$_]; print qq(0+:),0+$_; print $/; }"
0E0
Bool:1
0+:0


1E0
Bool:1
0+:1
[download]

---
$world=~s/war/peace/g

Yes, though that's a little more logical, since perldata specifies that numbers can be written in scientific notation in a form like 12.34E-10. So 0E0 is just a number, and "0E0" is a string that has length (is true) but evaluates as 0E0, which is zero (false). If you want to think about it the other way, '0' is a special case of a string that has length but is treated as false.

What I find interesting about "0 but true" is that it more clearly highlights to a reader that something unusual is going on than something like "0.0" or "0E0" -- and to such an extent that it was excepted from the usual warnings so as to make it easier to use that way. Anyone know the history of this? Or what version of Perl it first appeared in? (timtoady, are you out there?)

-xdg

Code written by xdg and posted on PerlMonks is public domain. It is provided as is with no warranties, express or implied, of any kind. Posted code may not have been tested. Use of posted code is at your own risk.

A quicker reference ;)

0/FALSE: 0, "0", "", undef
1/TRUE:  everything else 

 AND    OR     XOR   NOT
0&0=0  0|0=0  0^0=0  !0=1
0&1=0  0|1=1  0^1=1  !1=0
1&0=0  1|0=1  1^0=1
1&1=1  1|1=1  1^1=0
&      |      ^      ~     <-- bitwise
&&     ||            !     <-- logical, high precedence
and    or     xor    not   <-- logical, low precedence
[download]

Perl 6:

0/FALSE: 0, "0", "", undef ("0" is true if the type is Str)
1/TRUE:  everything else

 AND    OR     XOR   NOT
+&     +|     +^     +^    <-- bitwise, numeric
~&     ~|     ~^     ~^    <-- bitwise, string
?&     ?|     ?^     ?^    <-- bitwise, boolean
&&     ||     ^^     !     <-- logical, high precedence
and    or     xor    not   <-- logical, low precedence
[download]

• The difference between logical and bitwise boolean is that logical AND and OR return an operand, while bitwise AND and OR return boolean values (the original values are lost).
• ?^/^^, and ?^/! are practically the same.
• ^^ was missing in Perl 5.
• +^, ~^, ?^ are both XOR and NOT, depending on whether they are used infix (in between) or prefix (in front).

Juerd # { site => 'juerd.nl', plp_site => 'plp.juerd.nl', do_not_use => 'spamtrap' }

I know what you mean by the null string, but not every one does, specially not the people who need a reference. Perl doesn't have the concept of null, and we avoid using the term null string, because it's not always clear what is meant by it, the empty string, or an undefined value.

Having said that, you only list three things that are false in boolean context: the empty string (null string), the number 0, and the string "0". But you left out the most important one: the undefined value. But there are more, here's a (hopefully) complete list:

• The undefined value.
• The number 0.
• The empty string.
• The string "0".
• An empty array.
• An empty hash.
• Objects with appropriate overloaded boolean value.

When used as a number, undef is treated as 0; when used as a string, it is treated the empty string, ""; and when used as a reference that isn't being assigned to, it is treated as an error.

Well, there's also auto-vivification.

The while statement executes the block as long as the expression is true (does not evaluate to the null string "" or 0 or "0"). (see perlsyn)

I wonder why you mention while, but not for(;;) or until. Having said that, your statement is close, but not completely true. The block can also be exited with a last, return, goto, die or even a next LABEL_TO_OUTER_BLOCK statement. Furthermore, if the condition becomes false during execution of the block, the execution isn't stopped - checks are only done before entering the block. Finally, there's

    do {
        ...
    } while (0);
[download]

I've referenced this post a lot because it's so well organized. It's the easiest way for me to explain 'truthiness' to someone new to perl. I regret that I can only upvote this once. And on that topic:

"Reputation: 213 (+220 -7)"
7 downvotes? Who could downvote this?

> I regret that I can only upvote this once.

Actually your reply already pushed it up to 216! :)

> 7 downvotes? Who could downvote this?

False and untrue people? ;-D

Well honestly a 0.3 percent error shouldn't worry you and the html-formatting has some issues...

Cheers Rolf
_{(addicted to the Perl Programming Language and ☆☆☆☆ :)}

PS: Je suis Charlie!

I stopped by the Monastery while on a sabbatical and, thanks to your post, found that my once well-formed HTML had degraded into legacy syntax. As I am not familiar with current HTML/CSS conventions, I removed the offending tags to get rid of the errors. (I'd welcome formatting suggestions from anyone that can offer them...)

Thanks for alerting me to the issue. Problem fixed (for now).