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Re: Select data between a START and END pattern

by Thelonius (Priest)
on May 01, 2003 at 20:51 UTC ( #254808=note: print w/replies, xml ) Need Help??


in reply to Select data between a START and END pattern

Although dragonchild gives a good example of .., here's a more general explanation, along with the difference between .. and ...

First of all, there is use of .. in a list context:

@a = 1 .. 7;
Then there's the unrelated use in a scalar context. It only makes sense in a loop.
while (something) { if (EXPRSTART .. EXPREND) { doit(); } }
is equivalent to:
$inmatch = 0; while (something) { if (!$inmatch && EXPRSTART) { $inmatch = 1; } if ($inmatch) { doit(); } if ($inmatch && EXPREND) { $inmatch = 0; } }
Okay? Now for three dots:
while (something) { if (EXPRSTART ... EXPREND) { doit(); } }
is equivalent to:
$inmatch = 0; while (something) { $wasinmatch = $inmatch; if (!$inmatch && EXPRSTART) { $inmatch = 1; } if ($inmatch) { doit(); } if ($wasinmatch && EXPREND) { $inmatch = 0; } }
Subtle difference. To see it in action, compare:
while (<>) { print if /A/ .. /B/ }
with
while (<>) { print if /A/ ... /B/ }
on this input file
Here's some text before Some text with an A some lines in the middle 1 some lines in the middle 2 some lines in the middle 3 Some text with a B some useless lines 1 some useless lines 2 some useless lines 3 A line with both an A and a B some lines after the line with both 1 some lines after the line with both 2 some lines after the line with both 3 Once again, text with a B more useless lines 1 more useless lines 2 more useless lines 3

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Re2: Select data between a START and END pattern
by dragonchild (Archbishop) on May 01, 2003 at 21:12 UTC
    Excellent explanation! ++! Learn something new every day...

    ------
    We are the carpenters and bricklayers of the Information Age.

    Don't go borrowing trouble. For programmers, this means Worry only about what you need to implement.

    Please remember that I'm crufty and crochety. All opinions are purely mine and all code is untested, unless otherwise specified.

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[Corion]: Aaah - you should be able to do this with overload, but I would hit somebody really hard if they constructed objects that are true but the empty string, and you not knowing about the domain knowledge where this makes sense
[Eily]: you could tie a variable into not having the same value each time, if you like to make people who try to debug your code facepalm
[Corion]: perl -wle 'package o; use overload q("") => sub {warn "str"; ""}, bool => sub{warn "bool"; 1}; package main; my $o={}; bless $o => o; print "Yay" if ($o && !length($o))'
[Corion]: But people writing such code should document the objects they construct and why it makes sense for an object to be invisible as string while being true in a boolean context
[hippo]: That's equal parts clever and horrendous.
[Eily]: the overload version wouldn't return true with "$x" && !length $x though, I guess
[hippo]: The more I look at this code, the more $x is a plain old scalar and the more this condition will never be true. I'm calling it a bug at this point.
[hippo]: Thanks for your input which has soothed my sanity (a little)
[Corion]: Eily: Sure - if you force both things into stringy things, then you break that magic. But that would also mean that you changed the expression, as now $x = 0.00 will be true instead of false as it were before
[Corion]: Ah no, at least in my feeble experiments that doesn't change the meaning

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