[WJG] (16/Feb/06) A simple proc to compare the contents of two [list]s. 
It returns a list equal to the differences.

 #----------------
 # compare list a with b
 #----------------
 proc listcomp {a b} {
  set diff {}
  foreach i $a {
    if {[lsearch -exact $b $i]==-1} {
      lappend diff $i
    }
  }
  return $diff
 }

 catch {console show}
 set a {1 2 3 4 5 6}
 set b {2 4 6 8}

 puts "list a = \{$a\}\nlist b = \{$b\}\nThe difference between a and b is \{[listcomp $a $b]\}."

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[male] - 2006-02-16: reading the title I thought about something like "'''string compare'''".

So what's about this approach of a kind of list compare '''lcompare''':

 proc lcompare {list1 list2} {
     set length1 [llength $list1];
     set length2 [llength $list2];

     if {$length1 < $length2} {
         return [expr {double(-1)}];
     } elseif {$length1 > $length2} {
         return [expr {double(1)}];
     }

     if {[lindex $list1 0] eq $list1} {
         return [expr {double([string compare $list1 $list2])}];
     }

     set result [expr {double(0)}];

     foreach element1 $list1 element2 $list2 {
         set result [expr {$result + [lcompare $element1 $element2]}];
     }

     return [expr {$result / $length1}];
 }

[WJG] (16/Feb/06) [male], what is this proc supposed to do? 

[male] - 2006-02-16: sorry I didn't explain it.

'''lcompare''' returns a number between -1 and 1 to tell if the list1 is "lower" or 
"greater" than list2 - like in '''string compare'''.

It is about comparing each atom of a list or sub lists with '''string compare'''. 
The results are summed and diveded by the count of compared atoms.  
I assumed this as one way to do this. Perhaps this is the wrong way!

[WJG] (16/Feb/06) I fancied that the idea was to provide a simple decision. 
Running my clip above gives away the purpose. 
Its simply to compare one list with the other and then to return the differences.

[jcw] - The way the original was written, it really is sort of a "minus" or "except" set operator.

(Exercise for the reader: fix the error when this is given two identical lists)


[AK] Feb 16, 2006: [Tcllib]'s [struct] module has a package [struct::set] with all kinds of set operators. The original '''listcomp''' is simply '''struct::set difference'''.

[KBK] 2007-11-20 The "difference between two lists" is a trifle ambiguous.  If the lists do indeed represent sets of items, then
'''struct::set difference''' is what you want.  If the lists are, say, lines from two versions of a file, and you want to find the difference the way the '''diff''' command does, then you're probably after '''struct::list longestCommonSubsequence''' (The '''struct::list lcsInvert''' call repackages the results of '''longestCommonSubsequence''' in a more convenient form.)  See also [diff in Tcl].
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See also: [Additional list functions]



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%| [Category String Processing] |%
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