question about the Truth

sH4ke has asked for the wisdom of the Perl Monks concerning the following question:

Hello, Once and a while I have problems with boolean values in my scripts. Can someone please explain why is this:

if (false) { print "I do always get printed...\n" }
if (false==true) { print "..and even do I" }
[download]

and the same in "x ? y : z" style. Only when I'm using numerical forms i.e. false=0 and true=1, 0 never is true: if (0) { print "they never print me." } What's wrong with literal values? I'm using -v 5.6.1 and ActiveState's 5.8.

Comment on question about the Truth Select or Download Code

Replies are listed 'Best First'.
Re: question about the Truth by Roger (Parson) on Jan 15, 2004 at 07:10 UTC
Did you declare 'false' and 'true' as constants? `use constant false=>0; use constant true=>1; if (false) { print "I do always get printed...\n" } if (false==true) { print "..and even do I" }` [download] Otherwise the barewords 'false' and 'true' are treated as strings. Now if you rewrite `false==true` as `false eq true`, the second print will never be executed.	[reply] [d/l] [select]
Re: Re: question about the Truth by sH4ke (Initiate) on Jan 15, 2004 at 07:35 UTC
Thank you for your reply. Yet another reason why one should always use 'perl -w'	[reply]
Re: question about the Truth by Anonymous Monk on Jan 15, 2004 at 07:12 UTC
Perl has never had the values "true" and "false" assigned to literals. You could do `use constant true => 1; use constant false => 0;`	[reply] [d/l]
Re: question about the Truth by bart (Canon) on Jan 16, 2004 at 13:05 UTC
I have to warn you, that comparing to `true` or `false` is a very unperlish thing to do. 1 is considered as true, yes, but so is 2, 3.14, and "aardvark". In short: don't ever do `if($flag==true) { ...` [download] because you'll get a `false` response for any case where `$flag` is a true value, but not the same as the value for your constant `true`. In Perl, you should just be using `if($flag) { ...` [download] if you want to test if it's true, and any of `if(!$flag) { ... if(not $flag) { ... unless($flag) { ...` [download] to test for false. p.s. An additional warning: All of `undef`, `0`, `""` and `"0"` are considered false in Perl. Perhaps that's too forgiving for you, and you explicitely want to distinguish between them. Use of `defined`, and of `$value eq ""` [download] are typical idioms to deal with that. And no, you're not supposed to make a distinction between `0` and `"0"`. Thank you for your time.	[reply] [d/l] [select]