http://qs321.pair.com?node_id=496032

in reply to True or False? A Quick Reference Guide

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]```

-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.

Replies are listed 'Best First'.
Re^2: True or False? A Quick Reference Guide
by demerphq (Chancellor) on Sep 29, 2005 at 11:21 UTC

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.