Added note on the style thing.. becareful with &&, it has a low high* enough precedence that it gets interpreted early. From perlop...

As more readable alternatives to `&&' and `||' when used for control flow, Perl provides `and' and `or' operators (see below). The short- circuit behavior is identical. The precedence of "and" and "or" is much lower, however, so that you can safely use them after a list oper- ator without the need for parentheses:

Update:* Tnx davido for the correction.

Play that funky music white boy..

The precedence rules are usually sensible. It's fairly common to combine comparisons with logical operators; hence comparisons have higher precedence.

You cannot do a double-comparison as a single expression. The answer is to use the logical and operator to chain two expressions.

if ( $X > 9 and $X < 100 ) {
    $Y = $X + 1900;
} elsif ( $X >= 0 and $X < 10 ) {
    $Y = $X + 2000;
}
[download]

By the way, that's just one way to do it.

Seems a little suboptimal to handle it with yacc, rather than something that could give a better error message.

Range comparisons are practically unknown among programming languages. As far as I know, Perl 6 will be the first widely used language to allow range checking.

Older languages use the excuse of implementation to forbid something which is a natural mathematical statement. Comparisons like < return a boolean value, a truth value: Is 9 less than $X? yes or no?. Languages such as C would then use the result of that comprison in stage two: Is (9<$X) less than 100?. Obviously unlikely to be what you want, so Perl detects and reports the error.

By optimizing your comparisons a little, you can eliminate the need for range testing:

if ( $X < 0 ) {
    handle_X_under_0();
} elsif ( $x < 10 ) {
    $Y = $X + 2000;
} elsif ( $X < 100 ) {
    $Y = $X + 1900;
} else {
    handle_X_over_100();
}
[download]

$Y = ($X + 2000) if ($X >=0 && $X < 10) ;
$Y = ($X + 1900) if ($X >=10 && $X < 100) ;
[download]

$Y += ($X < 10) ? 2000 : 1900;
[download]

Ay yi yi...as a beginner at Perl that just makes my brain hurt. ;-)

Can someone parse that out, or give me a link to where it's explained. Thanks

This all stuff used in many languages and not specific to Perl.

The ternary operator works in the form of $condition_expr ? $true_expr : $false_expr. The conditional expression (here: ($X < 10)) is evaluated first. If it tests true, the $true_expr is then evaluated and its result is returned. Likewise with $false_expr if the conditional expression evaluated to false.

Then you have a shortcut operator $foo op= $bar;, where op stands for a mathematical, bitwise, or logical operator - here +. It is a shortcut for $foo = $foo op $bar;. So $foo += $bar; is a concise way to say "increase $foo by $bar".

So a sort-of token-by-token translation of the expression in my post to English would read something like "$Y is increased by either (if $X is less than 10) 2000 or by 1900 (otherwise)".

Obviously the first time you see such a construct it'll look strange. And it certainly isn't the point to use them wherever possible. Brevity is not a goal for its own sake. But assuming familiarity with such constructs, readability is maximized by choosing the briefest possible form that follows the structure of a natural language sentence describing the same thought. In this case this is statement is closest to how I'd explain the intent: "add the century depending on the year". It expresses exactly that in a straightforward manner: there's one condition, one addition/assignment, and the numbers, without any artificial extra verbiage.

Makeshifts last the longest.

I know that feeling! It's a common brain-response when getting questions answered here. I highly recommend getting a copy of Learning Perl. Basic stuff like this is covered well with many exercises to give you a feel for how to use the language.

-Theo-
(so many nodes and so little time ... )