Hmmm, here are some test cases where your code appears to fail.

    [ 7, 100, 204, 312 ],
    [ 8, 102, 313, 409 ],
    [ 9, 205, 206, 315 ],
    [ 10, 207, 210, 314 ],

    [ 100, 204, 312 ],
    [ 102, 313, 409 ],
    [ 205, 206, 315 ],
    [ 207, 210, 314, 401, 402, 403, 404 ],

    [ 1, 6, 11 ],
    [ 2, 7, 12 ],
    [ 3, 8, 13 ],
    [ 4, 9, 14 ],

[download]

I'm off out now, so I've got no time to look for a pattern (or to attempt to understand your code :-)...

Quick update: Limbic~Region's code, above, gets these right.

In reply to Re^3: Searching parallel arrays. by Not_a_Number
in thread Searching parallel arrays. by BrowserUk

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