Re^2: Changing quoted strings spanning more than one line (modifying constants)

I wonder how the $1 =~ tr/\n// in your solution doesn't die with "modification of a readonly value attempted"?

Have a peek at this:

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

for (1..2) {
   print map { ++$_ } 0..4;
}
[download]

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

Comment on Re^2: Changing quoted strings spanning more than one line (modifying constants) Select or Download Code

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Re^3: Changing quoted strings spanning more than one line (modifying constants) by shmem (Chancellor) on Sep 19, 2007 at 20:46 UTC
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}	[reply] [d/l] [select]
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]