Ruby problem with conditional ranges

ruby-talk
1

I'm running into a problem doing a conditional range in ruby. The sample
code is:

    #!/usr/bin/ruby

    ## one
    ## two
    ## three
    ## four
    ## five

    File.open($0, 'r') { |f|
  while f.gets
      print $_.sub(/^##/, '') if /^## two/ .. /^## four/
  end
    }

I'm expecting:

    two
    three
    four

but I get the whole file with the leading pound signs stripped off.
What's wrong here?

···

--
"Oh, look: rocks!"
  -- Doctor Who, "Destiny of the Daleks"

2

Hi,

At Mon, 4 Jun 2007 15:56:31 +0900,
Todd A. Jacobs wrote in [ruby-talk:254228]:

      print $_.sub(/^##/, '') if /^## two/ .. /^## four/

       print $_.sub(/^##/, '') if ~/^## two/ .. ~/^## four/

but I get the whole file with the leading pound signs stripped off.
What's wrong here?

Because Range object isn't nil nor false, and you'd see a
warning if you run with -w option.

  warning: range literal in condition

···

--
Nobu Nakada

3

Hi,

Implicit match against "$_" is not anymore supported in version 1.8.
You cas still use it in command line form (with "-e" option).

In regular file, you have to write the match explicitly :

## one
## two
## three
## four
## five
File.open($0, 'r') { |f|
  while f.gets
    print $_.sub(/^##/, '') if ($_ =~ /^## two/) .. ($_ =~ /^## four/)
  end
}

···

On 4 juin, 08:56, "Todd A. Jacobs" <tjacobs-sndr-019...@codegnome.org> wrote:

I'm running into a problem doing a conditional range in ruby. The sample
code is:

    #!/usr/bin/ruby

    ## one
    ## two
    ## three
    ## four
    ## five

    File.open($0, 'r') { |f|
        while f.gets
            print $_.sub(/^##/, '') if /^## two/ .. /^## four/
        end
    }

I'm expecting:

    two
    three
    four

but I get the whole file with the leading pound signs stripped off.
What's wrong here?

--
"Oh, look: rocks!"
        -- Doctor Who, "Destiny of the Daleks"

4

Thanks. Your explanation was very helpful. It works better now, but the
range operator *still* catches more than it should. For example:

    #!/usr/bin/ruby -w

    ## one
    ## two
    ## three
    ## four
    ## five

    File.open($0) do |file|
  while file.gets
      print $_.sub(/^## /, '') if
    ($_ =~ /^## two/)...($_ =~ /^## four/)
  end
    end

prints up to "four" when (as I understand it) it should stop at "three"
since "## four" would make the if-statement false. Am I still missing
something obvious?

···

On Mon, Jun 04, 2007 at 04:35:25PM +0900, come wrote:

Implicit match against "$_" is not anymore supported in version 1.8.

--
"Oh, look: rocks!"
  -- Doctor Who, "Destiny of the Daleks"

5

prints up to "four" when (as I understand it) it should stop at "three"
since "## four" would make the if-statement false. Am I still missing
something obvious?

Yes. That will never work, as this is not Perl nor are you creating a
range of regexp (which cannot be done, btw).
See what happens when the match does happen as you want:

a = 'four'
(a =~ /two/)...(a =~ /four/)

ArgumentError: bad value for range

[result is: (nil...'four') ]

Plus the uncommented # in the regex are probably making ruby assume a
comment.

This is another way of writing what you want to do:

#!/usr/bin/ruby -w

## one
## two
## three
## four
## five

File.open($0) do |file|
  range = [ '$_ =~ /^## two/', '$_ !~ /^## four/' ]
  while file.gets
    if eval("#{range[0]}")
      print $_.sub(/^## /, '');
      range.shift
    end
  end
end

···

On Jun 4, 9:51 pm, "Todd A. Jacobs" <tjacobs- sndr-019...@codegnome.org> wrote:

On Mon, Jun 04, 2007 at 04:35:25PM +0900, come wrote:

6

Sorry. Buggy code posted. Here's a correct implementation.

#!/usr/bin/ruby -w

## one
## two
## three
## five
## six
## four
## five

File.open($0) do |file|
  while file.gets
    if $_ =~ /^## two/
      loop {
        print $_.sub(/^## /, '')
        file.gets
        break if not $_ or $_ =~ /^## four/
      }
    end
  end
end

7

Yes, because (as with Perl) there are two forms of conditional range:
".." and "...".

The three dots form evaluate the condition after the code. Therefore,
the "four" is printed, then the condition is evaluated to false.

The two dots form evaluates the condition before the code. So it
should works as you expect : "($_ =~ /^## two/)..($_ =~ /^## four/)".

On 5 juin, 02:51, "Todd A. Jacobs"

···

range operator *still* catches more than it should. For example:

    #!/usr/bin/ruby -w

    ## one
    ## two
    ## three
    ## four
    ## five

    File.open($0) do |file|
        while file.gets
            print $_.sub(/^## /, '') if
                ($_ =~ /^## two/)...($_ =~ /^## four/)
        end
    end

prints up to "four" when (as I understand it) it should stop at "three"
since "## four" would make the if-statement false. Am I still missing
something obvious?

--
"Oh, look: rocks!"
        -- Doctor Who, "Destiny of the Daleks"