3. lds revisited (aka chinese whispers)

5. Send reinforcements. We’re going to advance.

7. Send three and fourpence. We’re going to a dance!

8. Motivation (rooted in footnote 279/1998) lds (improved) for non-binary domains present the algorithm how does it perform

9. A refresher Chronological Backtracking (BT) what’s that then? when/why do we need it? Quick Intro Limited Discrepancy Search (lds) what’s that then Then the story

10. An example problem (to show bt) Colour each of the 5 nodes, such that if they are adjacent, they take different colours 12 3 4 5

11. A Tree Trace of BT (assume domain ordered {R,B,G}) 12 3 4 5 v1 v2 v3 v4 v5

12. A Tree Trace of BT (assume domain ordered {R,B,G}) 1 2 3 4 5 v1 v2 v3 v4 v5

13. A Tree Trace of BT (assume domain ordered {R,B,G}) 12 3 4 5 v1 v2 v3 v4 v5

14. A Tree Trace of BT (assume domain ordered {R,B,G}) 12 3 4 5 v1 v2 v3 v4 v5

15. A Tree Trace of BT (assume domain ordered {R,B,G}) v1 v2 v3 v4 v5 12 3 4 5

16. A Tree Trace of BT (assume domain ordered {R,B,G}) v1 v2 v3 v4 v5 12 3 4 5

17. A Tree Trace of BT (assume domain ordered {R,B,G}) v1 v2 v3 v4 v5 12 3 4 5

18. A Tree Trace of BT (assume domain ordered {R,B,G}) v1 v2 v3 v4 v5 12 3 4 5

19. A Tree Trace of BT (assume domain ordered {R,B,G}) v1 v2 v3 v4 v5 12 3 4 5

20. A Tree Trace of BT (assume domain ordered {R,B,G}) v1 v2 v3 v4 v5 12 3 4 5

21. A Tree Trace of BT (assume domain ordered {R,B,G}) v1 v2 v3 v4 v5 12 3 4 5

22. A Tree Trace of BT (assume domain ordered {R,B,G}) v1 v2 v3 v4 v5 12 3 4 5

23. A Tree Trace of BT (assume domain ordered {R,B,G}) v1 v2 v3 v4 v5 12 3 4 5

24. LDS show the search process assume binary branching assume we have 4 variables only

25. Limited Discrepancy Search (LDS)Ginsberg & Harvey Take no discrepancies (go with the heuristic) What’s a heuristic

26. Limited Discrepancy Search (LDS)Ginsberg & Harvey Take no discrepancies

27. Limited Discrepancy Search (LDS)Ginsberg & Harvey Take no discrepancies

28. Limited Discrepancy Search (LDS)Ginsberg & Harvey Take no discrepancies

29. Limited Discrepancy Search (LDS)Ginsberg & Harvey Take 1 discrepancy

30. Limited Discrepancy Search (LDS)Ginsberg & Harvey Take 1 discrepancy

31. Limited Discrepancy Search (LDS)Ginsberg & Harvey Take 1 discrepancy

32. Limited Discrepancy Search (LDS)Ginsberg & Harvey Take 1 discrepancy

33. Limited Discrepancy Search (LDS)Ginsberg & Harvey Take 1 discrepancy

34. Limited Discrepancy Search (LDS)Ginsberg & Harvey Take 1 discrepancy

35. Limited Discrepancy Search (LDS)Ginsberg & Harvey Take 1 discrepancy

36. Limited Discrepancy Search (LDS)Ginsberg & Harvey Take 1 discrepancy

37. Limited Discrepancy Search (LDS)Ginsberg & Harvey Take 1 discrepancy

38. Limited Discrepancy Search (LDS)Ginsberg & Harvey Take 1 discrepancy

39. Limited Discrepancy Search (LDS)Ginsberg & Harvey Take 1 discrepancy

40. Limited Discrepancy Search (LDS)Ginsberg & Harvey Take 1 discrepancy

41. Now take 2 discrepancies

42. Limited Discrepancy Search (LDS)Ginsberg & Harvey Take 2 discrepancies

43. Limited Discrepancy Search (LDS)Ginsberg & Harvey Take 2 discrepancies

44. Limited Discrepancy Search (LDS)Ginsberg & Harvey Take 2 discrepancies

49. And now for the Chinese whispers

51. Motivation for lds

53. First proposal For discrepancies 0 to n

54. First proposal For discrepancies 0 to n k is remaining discrepancies

55. First proposal For discrepancies 0 to n k is remaining discrepancies Go with heuristic

56. First proposal For discrepancies 0 to n k is remaining discrepancies Go with heuristic Go against then go with

57. The lds search process: how it goes

58. NOTE: lds revisits search states with k discrepancies When searching with > k discrepancies

59. My pseudo code

60. lds revisits nodes: Korf’s improvement (AAAI 96)

61. Korf’s improvement

62. Korf’s 1 st mistake! Woops! Do you see it? He’s taking his discrepancies late/deep!

63. Korf’s 1 st mistake! Wrong way round Richard

64. Richard, was that a bug?

65. Yip, but so?

66. Korf’s 2 nd bug

67. Richard, you know there is another bug?

68. Woops! Richard, you know there is another bug?

69. Richard, are you there?

71. My pseudo code

72. Toby, do you do it late or early?

73. Chris, late or early?

74. Wafa, late or early?

75. Wafa’s response

76. My pseudo code I think this has not been reported

77. Are all discrepancies the same? Does it make a difference if we take discrepancies late or early?

78. Measuring discrepancies

79. Putting a cost on a discrepancy

80. No experiments done yet

81. Car sequencing Does it matter if we take discrepancies late or early? (basically, is H&G’s motivation for lds correct?)

82. Car Sequencing Problem Assessed exercise 2

95. My empirical study on car sequencing problems Using various search algorithms, heuristics. Question: does the order (late/early) that we take discrepancies in lds matter?

97. Well, did you see a pattern? If there is no pattern what does this say about H&G’s hypothesis? And, if no pattern, why is lds any good?

98. See anything?

99. Is this work worthy of more effort?

100. What does this tell us about how we do research? we can just follow on without question we can forget the basic/initial hypothesis we can forget to really look at our results we can be frightened or disinterested in –ve results we can publish papers with multiple errors we are human