Re: faulty expression prob

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Re^2: faulty expression prob by perl-diddler (Chaplain) on Apr 03, 2008 at 23:07 UTC
That's were I was going (where I wanted to end up) as I was trying to remove the duplication in the code. The 2 mentions of the L-value "$pkg->{DIST} were definite candidates for reduction, but I got stuck along the way when the less 'reduced' expression I initially posted wasn't working as expected...Perhaps someone can point out the "logical error" from the following change of "logic" (I'm paying careful attention to op-precedence as others have suggested that might be the problem). BTW, "?:" has higher precedence than assignment "=". "and" is lower than both. From the orig (dropping unnecessary parens around assign before 'and' (numbering for greater easy in referring back to which step is causing problem) (0): `return ( $rh=$pkg->{'RPM_HDR'} and $dist=$rh->tag('DISTRIBUTION') ) ? $pkg->{'DIST'}=$dist : $pkg->{'DIST'}=$undef;` [download] Logically substituting "X" for the conditional expression in parens, "( $rh=$pkg->{'RPM_HDR'} and $dist=$rh->tag('DISTRIBUTION') )", I get (1): `return X ? $pkg->{'DIST'}=$dist : $pkg->{'DIST'}=$undef;` [download] AHAH!....I see the prob...that "=" on the "end" (before $undef)... It is lower precedence than the ?: (as mentioned above...but wasn't sure which '=' the '?:' was interfering with). So the above (using D for "$pkg->{'DIST'}) is really (2?): `return (X ? D=$dist : D ) = $undef;` [download] Um...Why wouldn't that be a syntax error? the "(X?D=$dist:D)" part isn't a valid address that can be assigned to. (Note. "$undef" != undef, it's equal to a string of length 5 characters, 'undef'). I can see why what I wrote would be wrong -- but why wouldn't Perl flag it as illegal syntax?	[reply] [d/l] [select]
Re^3: faulty expression prob by jwkrahn (Abbot) on Apr 04, 2008 at 02:41 UTC
Why wouldn't that be a syntax error? Because ?: can be used on either the right hand side of an assignment: `my $var = $test ? $value_one : $value_two;` [download] Or on the left hand side of an assignment: `( $test ? $var_one : $var_two ) = $value;` [download] And because `$pkg->{'DIST'}` can be assigned to it is not a syntax error.	[reply] [d/l] [select]