Shell Programming and Scripting

I'm not yet sure I entirely grasp it, but that "grinding itself down to 0 lines" confirms my gut feeling about the processing. Unfortunately the few sample limes don't allow for a thorough testing. How about

Code:

awk '
$1 in X  || $2 in X     {next
                        }

$3 >  $4 ||
$3 == $4 && $5 < $6     {TMP = $2
                        }

$3 <  $4 ||
$3 == $4 && $5 > $6     {TMP = $1
                        }

                        {X[TMP]
                         print TMP
                        }
' file
ID1
ID4
ID2

I think that's worked.. thank you!!

If that logic works, you might want to try

Code:

awk '
!($1 in X  || $2 in X)  {TMP = 1 + ($3 >  $4 || $3 == $4 && $5 < $6)
                         X[$TMP]
                         print $TMP
                        }
' file
ID1
ID4
ID2

You may want to post a bit larger sample so the test can be built on a larger basis.

What if $3 == $4 && $5 == $6?

That's even more elegant and gives the same output. I'm struggling to understand how you managed to condense my four criteria into :

Code:

($3 >  $4 || $3 == $4 && $5 < $6)

Re: if

Code:

$3==$4 && $5==$6

, there is one occurrence of this in the whole file, but one of the IDs has already been 'eliminated' by the time we get to that row (hence that row no longer exists), rendering it not a problem..

In fact, what I applied is your logic from post#1, deploying the fact that the two expressions are mutually exclusive except for the $5 == $6 case. The result of a boolean expression is either 1 (TRUE), or 0 (FALSE), and by adding 1 the target field (1 or 2) is calculated (In a strict language like e.g. PASCAL you can't do that (arithmetics with logical values), but C or awk allow for it).
If you can make very sure the $5 == $6 case doesn't exist (or disappears in the process), you can leave its consideration out, but good programming style would require it to be covered, and be it with an error condition.

Thank you for the explanation - v. helpful.