Forum: Ruby simple one-liners: last line missing

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Tobi R. (Guest)
on 2006-03-13 18:48
(Received via mailing list)
Hi

Let's say I'd want to modify each line of a file. Here's the dummy
data:

  $ cat data.txt
  first foo
  second foo
  third foo
  $

This works:

  $ ruby -ne '$_ =~ /(\S+)\s+(\S+)/;puts "#{$2} #{$1}"' data.txt
  foo first
  foo second
  foo third
  $

I don't really need to achieve that modification. Instead I'd like to
understand why the following version one doesn't work:

  $ ruby -pe 'sub(/(\S+)\s+(\S+)/,"#{$2} #{$1}")' data.txt

  foo first
  foo second
  $

TIA,
Tobi
ts (Guest)
on 2006-03-13 19:13
(Received via mailing list)
>>>>> "T" == Tobi R. <removed_email_address@domain.invalid> writes:

T>   $ ruby -pe 'sub(/(\S+)\s+(\S+)/,"#{$2} #{$1}")' data.txt

 The replacement string is build when #sub is called

 for the first call $1 = $2 = nil ==> the replacement string is ' '

 for the second call, $1 = 'first', $2 = 'foo' ==> the replacement
string
 is 'foo first'

 etc,


Guy Decoux
Andrew J. (Guest)
on 2006-03-13 19:19
(Received via mailing list)
On Tue, 14 Mar 2006 01:46:50 +0900, Tobi R. 
<removed_email_address@domain.invalid>
wrote:
[snip]
> I don't really need to achieve that modification. Instead I'd like to
> understand why the following version one doesn't work:
>
>   $ ruby -pe 'sub(/(\S+)\s+(\S+)/,"#{$2} #{$1}")' data.txt
>
>   foo first
>   foo second
>   $


When doing substitutions with a string replacement, you do not get what
you might expect because the interpolation of #{$1} in the replacement
string happens when the arguments are evaluated and passed into the
sub()
or gsub() methods, not during the match. That means they will have
whatever
value they had from the last successful match (or nil, as in your case
with
the first line printed).

To access backreferences within a replacement string, you may use the
same
notation as you do for backreferences within the pattern itself: \1, \2,
etc. For your example:

  ruby -pe 'sub(/(\S+)\s+(\S+)/,%q(\2 \1))' data.txt

Alternatively, you may use the block form of replacement, in which case
the
block is evaluated at match-time and $1, $2, etc are available and refer
to
their respective backreferences within the current match:

  ruby -pe 'sub(/(\S+)\s+(\S+)/){"#{$2} #{$1}"}' data.txt

cheer's
andrew
Lou V. (Guest)
on 2006-03-13 19:22
(Received via mailing list)
I think you want,

 >ruby -pe 'sub(/(\S+)\s+(\S+)/,"\\2 \\1")' data.txt
Tobi R. (Guest)
on 2006-03-13 19:28
(Received via mailing list)
On Tue 2006-03-14 ts wrote:
>
>  etc,

Thanks. I guess my (current) opinion is that it should work.

Tobi
Tobi R. (Guest)
on 2006-03-13 19:40
(Received via mailing list)
On Tue 2006-03-14 Tobi R. wrote:
> >  is 'foo first'
> >
> >  etc,
>
> Thanks. I guess my (current) opinion is that it should work.
>
> Tobi

Now I understand that it can't work.

Thanks again,
Tobi
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