diff a ruby string or array

Question

How do I do a diff of two strings or arrays in Ruby?

I have reworded this question in the hope that it can now be reopened. I think the question was valid, just worded in a way that made it sound like it would result in a lot of opinion answers. If you want to help reopen this, you should see a tiny "reopen" link just under the "ruby" and "diff" question tags. — Gerry, Jun 30 '16 at 23:38
this is about the 10th question that i've found this week that has been flagged as off topic. it's not off topic. it's a great question. please please, hall monitors, stop doing this. — user1130176, Feb 16 '17 at 11:34
For strings you can use this answer http://stackoverflow.com/a/42573286/745489 — Dzmitry, Mar 03 '17 at 07:44

score 35 · Answer 1 · answered Sep 17 '08 at 04:53

35

For arrays, use the minus operator. For example:

>> foo = [1, 2, 3]
=> [1, 2, 3]
>> goo = [2, 3, 4]
=> [2, 3, 4]
>> foo - goo
=> [1]

Here the last line removes everything from foo that is also in goo, leaving just the element 1. I don't know how to do this for two strings, but until somebody who knows posts about it, you could just convert each string to an array, use the minus operator, and then convert the result back.

answered Sep 17 '08 at 04:53

Chris Bunch

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This is incorrect. foo = [1,2,3] bar = [4,5,6] foo - bar # => [1,2,3] Ruby's implementation is correct, but I don't thin this is what the question asked for. – Chris Lloyd Aug 24 '10 at 06:59
6

To get around Chris L's point, you can do (foo - bar) + (bar - foo). To get all differences between the arrays (or strings if you use .to_a) – diedthreetimes Jul 21 '11 at 00:05

samg · Answer 2 · 2010-11-23T02:46:38.863

25

I got frustrated with the lack of a good library for this in ruby, so I wrote http://github.com/samg/diffy. It uses diff under the covers, and focuses on being convenient, and providing pretty output options.

edited Nov 23 '10 at 02:46

answered Jun 30 '10 at 03:40

samg

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I am using diffy for one of my projects. Simply works. Thanks. – ardsrk Feb 29 '12 at 07:13

score 21 · Answer 3 · edited Jun 24 '14 at 17:50

21

For strings, I would first try out the Ruby Gem that @sam-saffron mentioned below. It's easier to install: http://github.com/pvande/differ/tree/master

gem install differ

irb
require 'differ'

one = "one two three"
two = "one two 3"

Differ.format = :color
puts Differ.diff_by_word(one, two).to_s

Differ.format = :html
puts Differ.diff_by_word(one, two).to_s

edited Jun 24 '14 at 17:50

dgo.a

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answered Jan 06 '09 at 11:11

da01

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score 21 · Accepted Answer · edited Sep 27 '17 at 02:02

21

diff.rb is what you want, which is available at ~~http://users.cybercity.dk/~dsl8950/ruby/diff.html~~ via internet archive:

http://web.archive.org/web/20140421214841/http://users.cybercity.dk:80/~dsl8950/ruby/diff.html

edited Sep 27 '17 at 02:02

Gavin Miller

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answered Sep 17 '08 at 04:52

Brian Mitchell

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For those that don't grok the implications of GPL: it includes a 'copyleft' concept which demands that any proprietary code you incorporate it into forces you to open source and GPL-give-away your previously-proprietary code. – New Alexandria Apr 25 '14 at 15:21
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Commenting to say that link is still dead http://www.downforeveryoneorjustme.com/http://users.cybercity.dk/~dsl8950/ruby/diff.html – Btuman May 09 '17 at 18:39

Grant Hutchins · Answer 5 · 2011-09-29T13:43:33.820

5

There is also diff-lcs which is available as a gem. ~~It hasn't been updated since 2004 but~~ we have been using it without any problem.

Edit: A new version was released in 2011. Looks like it's back in active development.

http://rubygems.org/gems/diff-lcs

edited Sep 29 '11 at 13:43

answered Jan 07 '09 at 02:40

Grant Hutchins

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Has anybody been successful in using this for HTML? – aaandre Feb 28 '12 at 02:55

score 5 · Answer 6 · edited Jan 05 '19 at 20:26

5

t=s2.chars; s1.chars.map{|c| c == t.shift ? c : '^'}.join

This simple line gives a ^ in the positions that don't match. That's often enough and it's copy/paste-able.

edited Jan 05 '19 at 20:26

Michal Lonski

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answered Jul 26 '18 at 17:50

Steve

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Thanks for the edit Michal, that's clearly better than what I wrote! – Steve Jan 06 '19 at 22:46

Brian Armstrong · Answer 7 · 2009-04-10T23:40:05.443

The HTMLDiff that @da01 mentions above worked for me.

script/plugin install git://github.com/myobie/htmldiff.git

# bottom of environment.rb
require 'htmldiff'

# in model
class Page < ActiveRecord::Base
  extend HTMLDiff
end

# in view
<h1>Revisions for <%= @page.name %></h1>
<ul>
<% @page.revisions.each do |revision| %>
  <li>
    <b>Revised <%= distance_of_time_in_words_to_now revision.created_at %> ago</b><BR>
      <%= Page.diff(
        revision.changes['description'][0],
        revision.changes['description'][1]
      ) %>
      <BR><BR>
  </li>
<% end %>

# in style.css
ins.diffmod, ins.diffins { background: #d4fdd5; text-decoration: none; }
del.diffmod, del.diffdel { color: #ff9999; }

Looks pretty good. By the way I used this with the acts_as_audited plugin.

score 2 · Answer 8 · answered Jul 31 '09 at 05:10

2

I just found a new project that seems pretty flexible:

http://github.com/pvande/differ/tree/master

Trying it out and will try to post some sort of report.

answered Jul 31 '09 at 05:10

Sam Saffron

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score 2 · Answer 9 · answered Oct 02 '09 at 21:27

2

I had the same doubt and the solution I found is not 100% ruby, but is the best for me. The problem with diff.rb is that it doesn't have a pretty formatter, to show the diffs in a humanized way. So I used diff from the OS with this code:

 def diff str1, str2
   system "diff #{file_for str1} #{file_for str2}"
 end

 private
 def file_for text
   exp = Tempfile.new("bk", "/tmp").open
   exp.write(text)
   exp.close
   exp.path
 end

answered Oct 02 '09 at 21:27

Daniel Cukier

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using temporary files is generally a bad idea when you have an in memory option available. – epochwolf Oct 15 '09 at 19:15
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@epochwolf How are you making diff read from memory? It only supports diffing files as far as I'm aware. – Gerry Apr 03 '16 at 01:17

score 2 · Answer 10 · answered Feb 03 '12 at 17:53

2

Just for the benefit of Windows people: diffy looks brilliant but I belive it will only work on *nix (correct me if I'm wrong). Certainly it didn't work on my machine.

Differ worked a treat for me (Windows 7 x64, Ruby 1.8.7).

answered Feb 03 '12 at 17:53

russthegibbon

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There is some Windows support baked in now. There's some details about what kind of setup you need to do in the README. https://github.com/samg/diffy#on-windows – samg Dec 28 '12 at 07:23

dimus · Answer 11 · 2017-08-08T15:58:59.363

To get character by character resolution I added a new function to damerau-levenshtein gem

require "damerau-levenshtein"
differ = DamerauLevenshtein::Differ.new
differ.run "Something", "Smothing"
# returns ["S<ins>o</ins>m<subst>e</subst>thing", 
#  "S<del>o</del>m<subst>o</subst>thing"]

or with parsing:

require "damerau-levenshtein"
require "nokogiri"

differ = DamerauLevenshtein::Differ.new
res = differ.run("Something", "Smothing!")
nodes = Nokogiri::XML("<root>#{res.first}</root>")

markup = nodes.root.children.map do |n|
  case n.name
  when "text"
    n.text
  when "del"
    "~~#{n.children.first.text}~~"
  when "ins"
    "*#{n.children.first.text}*"
  when "subst"
    "**#{n.children.first.text}**"
  end
end.join("")

puts markup

score 1 · Answer 12 · answered Jul 07 '11 at 12:43

1

Maybe Array.diff via monkey-patch helps...

http://grosser.it/2011/07/07/ruby-array-diffother-difference-between-2-arrays/

answered Jul 07 '11 at 12:43

grosser

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diff a ruby string or array

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