f



'ab\c' and 'ab\\c'

I encountered a rather peculiar behavior of strings today.
Here is my irb session:
-------------------------------------------------------------------
irb(main):001:0> 'ab\c'
irb(main):002:0' '
SyntaxError: compile error
(irb):2: unterminated string meets end of file
        from (irb):2
irb(main):003:0> 'ab\\c'
=> "ab\\c"
irb(main):004:0> 'a\b'
=> "a\\b"
irb(main):005:0>
-------------------------------------------------------------------

As you can see when I typed 'ab\c'  my irb didn't return. It was
expecting some thing more. Then I typed a single quote ( ' ) to
terminate the string and it gave me this SyntaxError. The same thing
happens even I type 'ab\\c'

I was trying this after reading section 3.2.1.1 of "The Ruby
Programming Language". This is what it says about backslashes in
single quoted strings:

"In single-quoted strings, a backslash is not special if the character
that follows it is anything other than a quote or a backslash. "

But at the same time if I type 'a\b' it returns 'a\\b' which is inline
with the book. Any ideas what's happening here?

Thanks
subbu

0
loveajax (6)
7/9/2008 4:53:58 AM
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I think you stumbled on another special character, \c, which seems to 
have some weird properties:

-in single quotes, must be followed by another character, but when it 
is, just swaps the \ for a \\

-in double quotes, again must be followed by another character, which it 
translates into a number, though by what scheme i have no idea:

>> "ab\ca"
=> "ab\001"
>> "ab\cb"
=> "ab\002"
>> "ab\cA"
=> "ab\001"
>> "ab\cz"
=> "ab\032"

I'm quite intrigued by this as well.


-- 
Posted via http://www.ruby-forum.com/.

0
7/9/2008 9:00:09 AM
From: "Max Williams" <toastkid.williams@gmail.com>
>
>>> "ab\ca"
> => "ab\001"
>>> "ab\cb"
> => "ab\002"
>>> "ab\cA"
> => "ab\001"
>>> "ab\cz"
> => "ab\032"
> 
> I'm quite intrigued by this as well.

control characters.

puts "\cG"

...should beep.

puts "\cH"

...should backspace.

puts "boolean\cH\cH\cH\cHbies"

...is left as an exercise for the reader.  ;)


Regards,

Bill



0
billk (827)
7/9/2008 9:14:45 AM
On Wed, Jul 9, 2008 at 11:00 AM, Max Williams
<toastkid.williams@gmail.com> wrote:
> I think you stumbled on another special character, \c, which seems to
> have some weird properties:
>
> -in single quotes, must be followed by another character, but when it
> is, just swaps the \ for a \\
>
> -in double quotes, again must be followed by another character, which it
> translates into a number, though by what scheme i have no idea:

In a double-quoted string, \c followed by another character forms a
control character. Control characters are character codes 0 to 31 in
the ASCII character set. When you call inspect on a string (as irb
does for you), Ruby escapes these control characters and those are the
number you are seeing.

In a single-quoted string, \c doesn't do anything, just like "The Ruby
programming language" says. Irb gets confused here and does interpret
the \c, thinks the string goes on while it doesn't, asks more input
until a quote is given, which really starts a new string which is then
unterminated and that's the error you get.

It's a bug in irb.

Peter

0
7/9/2008 9:25:33 AM
Ah...

There's no index entry for \c in pickaxe (unless i'm being dumb), which 
seems like an omission.
-- 
Posted via http://www.ruby-forum.com/.

0
7/9/2008 11:38:24 AM
Interesting findings. Thanks all for the explanations.

On Jul 9, 2:25 pm, Calamitas <calamita...@gmail.com> wrote:
> On Wed, Jul 9, 2008 at 11:00 AM, Max Williams
>
> <toastkid.willi...@gmail.com> wrote:
> > I think you stumbled on another special character, \c, which seems to
> > have some weird properties:
>
> > -in single quotes, must be followed by another character, but when it
> > is, just swaps the \ for a \\
>
> > -in double quotes, again must be followed by another character, which it
> > translates into a number, though by what scheme i have no idea:
>
> In a double-quoted string, \c followed by another character forms a
> control character. Control characters are character codes 0 to 31 in
> the ASCII character set. When you call inspect on a string (as irb
> does for you), Ruby escapes these control characters and those are the
> number you are seeing.
>
> In a single-quoted string, \c doesn't do anything, just like "The Ruby
> programming language" says. Irb gets confused here and does interpret
> the \c, thinks the string goes on while it doesn't, asks more input
> until a quote is given, which really starts a new string which is then
> unterminated and that's the error you get.
>
> It's a bug in irb.
>
> Peter

0
loveajax (6)
7/9/2008 11:46:08 AM
On Jul 9, 2008, at 7:38 AM, Max Williams wrote:
> Ah...
>
> There's no index entry for \c in pickaxe (unless i'm being dumb),  
> which
> seems like an omission.


In my 2nd ed. pickaxe's index:

String
   control characters \n etc.  321

You just have to know what to look for. ;-)

-Rob

Rob Biedenharn		http://agileconsultingllc.com
Rob@AgileConsultingLLC.com



0
Rob7461 (594)
7/9/2008 11:58:50 AM
Rob Biedenharn wrote:
> On Jul 9, 2008, at 7:38 AM, Max Williams wrote:
>> Ah...
>>
>> There's no index entry for \c in pickaxe (unless i'm being dumb),  
>> which
>> seems like an omission.
> 
> 
> In my 2nd ed. pickaxe's index:
> 
> String
>    control characters \n etc.  321
> 
> You just have to know what to look for. ;-)
> 
> -Rob
> 
> Rob Biedenharn    http://agileconsultingllc.com
> Rob@AgileConsultingLLC.com

You would have to know what \c means to find this out, though!  That's 
like saying the answer is in the index but the question isn't...if you 
know what i mean.
-- 
Posted via http://www.ruby-forum.com/.

0
7/9/2008 4:19:17 PM
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