Ruby: if statement using regexp and boolean operator

Question

I'm learning Ruby and have failed to make a compound 'if' statement work. Here's my code (hopefully self explanatory)

commentline = Regexp.new('^;;') 
blankline = Regexp.new('^(\s*)$')

if (line !~ commentline || line !~ blankline)
  puts line
end

the variable 'line' is gotten from reading the following file:

;; alias filename backupDir

Prog_i  Prog_i.rb ./store
Prog_ii Prog_ii.rb ./store

This fails and I'm not sure why. Basically I want the comment lines and blank lines to be ignored during the processing of the lines in the file. Thanks for your help.

This really belongs on SO. A good rule to follow is if you are in front of your IDE, the question belongs on SO. If you are in front of your whiteboard it belongs on Programmers. — Walter
– Walter, Commented Dec 7, 2012 at 0:12
Walter, I believe you, but I'm at a loss to understand why it doesn't belong on "Programmers". I've read the docs, so could you help me out? — bev
– bev, Commented Dec 7, 2012 at 1:22
For reference, Ruby has a standard baked-in syntax for creating regexps. commentline = /^;;/ and blankline = /^(\s*)$/ respectively in your case. — KChaloux
– KChaloux, Commented Dec 7, 2012 at 13:35
@sawa - wow you studied logics in elementary school!!! I think I was studying manners then. — bev
– bev, Commented Dec 7, 2012 at 20:39

ratchet freak · Accepted Answer · 2012-12-07 18:50:52Z

6

you need to use an AND

basically you want not (blank or comment) which turns into not blank and not comment after applying DeMorgan

if (line !~ commentline && line !~ blankline)
  puts line
end

or

unless(line ~= commentline || line ~= blankline)
  puts line
end

depending on which you find more readable

edited Dec 7, 2012 at 18:50

answered Dec 7, 2012 at 0:08

ratchet freak

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4 Comments

tadman Over a year ago

There's also the unless approach.

KChaloux Over a year ago

To expand on what @tadman said, the syntax would become unless (line !~ commentLine || line !~ blankline. It's the same as negating the whole if statement (as @ratchet freak has done in his example here) - Edit: Oop, looks like Rachet's gone and updated his answer.

ratchet freak Over a year ago

@KChaloux added that in the answer (I'm not really familiar with ruby syntax)

bev Over a year ago

Thanks for the logic reminder, ratchet freak. You're correct.

akuhn · Accepted Answer · 2012-12-07 06:37:39Z

1

You can write this much more terse, as

puts DATA.readlines.reject{|each|each =~ /^;;|^\s*$/}

__END__
;; alias filename backupDir

Prog_i  Prog_i.rb ./store
Prog_ii Prog_ii.rb ./store

edited Dec 7, 2012 at 6:37

answered Dec 7, 2012 at 1:15

akuhn

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2 Comments

bev Over a year ago

Thanks akuhn. I put a modified version of this and it worked. The example I used was an MWE. When I stuck the various versions from the answers into my code a version of this was what I used. BTW, do you know why my question was migrated here from programmers? Thx.

akuhn Over a year ago

@bev I guess it is because you ask very API specific questions, programmers is more about how to prepare for a job talk or on best practices for version control, etcetera.

the Tin Man · Accepted Answer · 2012-12-07 02:08:38Z

1

This is your code:

commentline = Regexp.new('^;;') 
blankline = Regexp.new('^(\s*)$')

if (line !~ commentline || line !~ blankline)
  puts line
end

and how I'd write the same thing:

[
  ';; alias filename backupDir',
  '',
  'Prog_i  Prog_i.rb ./store',
  'Prog_ii Prog_ii.rb ./store'
].each do |line|

  puts line if (!line[/^(?:;;)?$/])

end

Which outputs:

;; alias filename backupDir
Prog_i  Prog_i.rb ./store
Prog_ii Prog_ii.rb ./store

answered Dec 7, 2012 at 2:08

the Tin Man

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5 Comments

user2398029 Over a year ago

+1. Didn't know you could use a Regexp within braces like that.

sawa Over a year ago

Notice what OP is doing. It is not the same as what you are doing. Yours is correct. The OP's condition always returns true because no string can match commentline and blankline at the same time.

the Tin Man Over a year ago

Yes, or use a fixed-string search, or even capture a single element in a regex.

the Tin Man Over a year ago

@sawa: Oh. Oops. Maybe I should "fix" mine to not work too? :-)

akuhn Over a year ago

Even more Ruby, you could use a multiline string or a DATA segment.

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