Re^3: Changing quoted strings spanning more than one line (modifying constants)

That's cute, but doesn't help explaining why $1 =~ tr/\n// doesn't attempt to modify $1, which is read-only.

Interesting, though.

local $, = ", ";
local $\ = "\n";

for ($foo,$bar) {
   print map { ++$_ } 0..4;
}
[download]

1, 2, 3, 4, 5
2, 3, 4, 5, 6
[download]

The outer $_ isn't related to map's $_. But it seems like map's $_ isn't being reset each time through the loop. What gives? Bug or some (undocumented) feature?

--shmem

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}

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Re^4: Changing quoted strings spanning more than one line (modifying constants) by ikegami (Patriarch) on Sep 19, 2007 at 21:01 UTC
That's cute, but doesn't help explaining why $1 =~ tr/\n// doesn't attempt to modify $1, which is read-only I know. Someone already explained that `tr/\n//` doesn't modify the variable on which acts because it wasn't asked to. But it seems like map's $_ isn't being reset each time through the loop. Apparently, Perl expands `0..4` into the 5 item list only once, possibly as an optimisation since 0..4 is constant. However, it doesn't set the read-only flag on the members of the expanded list. The `map` I constructed modifies this list that should be constant, but isn't. Change `0..4` to `0,1,2,3,4` and you'll get a read-only violation. Change `++$_` to `1+$_` and you won't modify the list.	[reply] [d/l] [select]

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Re^4: Changing quoted strings spanning more than one line (modifying constants)
by ikegami (Patriarch) on Sep 19, 2007 at 21:01 UTC

That's cute, but doesn't help explaining why $1 =~ tr/\n// doesn't attempt to modify $1, which is read-only

I know. Someone already explained that tr/\n// doesn't modify the variable on which acts because it wasn't asked to.

But it seems like map's $_ isn't being reset each time through the loop.

Apparently, Perl expands 0..4 into the 5 item list only once, possibly as an optimisation since 0..4 is constant. However, it doesn't set the read-only flag on the members of the expanded list. The map I constructed modifies this list that should be constant, but isn't.

Change 0..4 to 0,1,2,3,4 and you'll get a read-only violation.
Change ++$_ to 1+$_ and you won't modify the list.

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