From: Rob Biedenharn <Rob@...>
Date: 2007-03-28T10:08:05+09:00
Subject: Re: String Intersection. A better way

On Mar 27, 2007, at 8:13 PM, Timothy Hunter wrote:
> Daniel N wrote:
>> Hi all,
>>
>> I have an issue with strings.  I want the intesection of two  
>> strings but I'm
>> not sure how to go about it.  I have two cases.
>>
>> Case 1:
>> String 1 has the tail end intersecting with the start of string  
>> 2..  eg:
>>
>> String 1: The quick brown fox jumped over the lazy dog.
>> String 2: fox jumped over the lazy dog.  And in other news bla bla  
>> bla
>>
>> The problem is I don't know how much is intersecting.
>> What I've done is to take string 1 and say psudo code
>>
>> until match found
>>   string2 =~ /^(value of string1)/
>>   if match found set flag
>>   else delete first char of string1
>> loop again
>>
>> What is a better way to do this?
>>
>> Case 2:
>>
>> The start of String1 intersects with the end of string2  eg:
>>
>> String1:   And here we are with another string.
>> String2:   bla bla bla.   And here we are
>>
>> I don't really know about this one yet...  I haven't gotten to it.
>>
>> Can anyone pls suggest what I might do?
>>
>> Thanx
>> Daniel
>>
> Take a look at the MatchData class, particularly the #begin and  
> #offset methods.
>

What you might do is take a TDD approach (Test Driven Development)...

Change the method body until the tests pass and you're all set!  Of  
course, seeing the tests might help you solidify what you expect the  
behavior to be.

class String
   # intersection of self and other
   def &(other)
     ''
   end
end

if __FILE__ == $0
   require 'test/unit'
   class StringIntersectionTest < Test::Unit::TestCase
     def setup
       @empty = ''
     end

     def test_empty_string
       assert_equal @empty, @empty & @empty, "empty with empty"
       assert_equal @empty, 'not' & @empty,  "non-empty with empty"
       assert_equal @empty, @empty & 'not',  "empty with non-empty"
     end

     def test_equal_strings
       s = "Some long string that I'm ready to take to the intersection"
       assert_equal s, s & s, "string with self"
     end

     def test_prefix
       assert_equal 'f', 'f' & 'fr', 'single char string as prefix of  
other'
       assert_equal 'f', 'fr' & 'f', 'other is single char prefix of  
string'
       assert_equal 'fred', 'fred' & 'fredrick', 'multi-char string  
as suffix of other'
       assert_equal 'fred', 'fredrick' & 'fred', 'other is multi-char  
suffix of string'
     end

     def test_suffix
       assert_equal 'r', 'r' & 'fr', 'single char string as suffix of  
other'
       assert_equal 'r', 'fr' & 'r', 'other is single char suffix of  
string'
       assert_equal 'rick', 'rick' & 'fredrick', 'multi-char string  
as suffix of other'
       assert_equal 'rick', 'fredrick' & 'rick', 'other is multi-char  
suffix of string'
     end

     def test_other_intersections
       assert_equal 'site', 'new web site' & 'site-wide policy'
     end

     def test_other_empty_intersections
       assert_equal @empty, 'fred' & 'ethel'
       assert_equal @empty, 'The quick brown fox jumps over the lazy  
dog.' & 'The lazy dog lies under the jumping fox.'
       assert_equal @empty, 'Blah, Blah, blah' & 'yada, yada, yada'
     end
   end
end


Note that two of the six tests already pass just by returning the  
empty string ;-)

-Rob

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