research how not to do it
DESCRIPTION
Research how not to do it. Quick Intro. A refresher Chronological Backtracking (BT) what’s that then? when/why do we need it?. Limited Discrepancy Search (lds) what’s that then. Then the story … how not to do it. An example problem (to show chronological backtracking (BT)). 1. 2. - PowerPoint PPT PresentationTRANSCRIPT
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Research
how not to do it
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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 … how not to do it
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An example problem (to show chronological backtracking (BT))
Colour each of the 5 nodes, such that if they are adjacent, they take different colours
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A Tree Trace of BT (assume domain ordered {R,B,G})
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A Tree Trace of BT (assume domain ordered {R,B,G})
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A Tree Trace of BT (assume domain ordered {R,B,G})
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A Tree Trace of BT (assume domain ordered {R,B,G})
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A Tree Trace of BT (assume domain ordered {R,B,G})
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A Tree Trace of BT (assume domain ordered {R,B,G})
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A Tree Trace of BT (assume domain ordered {R,B,G})
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A Tree Trace of BT (assume domain ordered {R,B,G})
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A Tree Trace of BT (assume domain ordered {R,B,G})
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A Tree Trace of BT (assume domain ordered {R,B,G})
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A Tree Trace of BT (assume domain ordered {R,B,G})
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A Tree Trace of BT (assume domain ordered {R,B,G})
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A Tree Trace of BT (assume domain ordered {R,B,G})
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Could do better
Improvements:
• when colouring a vertex with colour X• remove X from the palette of adjacent vertices
• when selecting a vertex to colour• choose the vertex with the smallest palette• tie break on adjacency with uncoloured vertices
An inferencing step
A heuristic (Brelaz)
Conjecture: our heuristic is more reliable as we get deeper in search
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What’s a heuristic?
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Limited Discrepancy Search (LDS)
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Motivation for lds
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Motivation for LDS
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LDS
• show the search process• assume binary branching• assume we have 4 variables only• assume variables have 2 values each
Limited Discrepancy Search
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Limited Discrepancy Search (LDS) Ginsberg & Harvey
Take no discrepancies (go with the heuristic)
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Limited Discrepancy Search (LDS) Ginsberg & Harvey
Take no discrepancies
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Limited Discrepancy Search (LDS) Ginsberg & Harvey
Take no discrepancies
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Limited Discrepancy Search (LDS) Ginsberg & Harvey
Take no discrepancies
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Limited Discrepancy Search (LDS) Ginsberg & Harvey
Take 1 discrepancy
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Limited Discrepancy Search (LDS) Ginsberg & Harvey
Take 1 discrepancy
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Limited Discrepancy Search (LDS) Ginsberg & Harvey
Take 1 discrepancy
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Limited Discrepancy Search (LDS) Ginsberg & Harvey
Take 1 discrepancy
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Limited Discrepancy Search (LDS) Ginsberg & Harvey
Take 1 discrepancy
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Limited Discrepancy Search (LDS) Ginsberg & Harvey
Take 1 discrepancy
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Limited Discrepancy Search (LDS) Ginsberg & Harvey
Take 1 discrepancy
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Limited Discrepancy Search (LDS) Ginsberg & Harvey
Take 1 discrepancy
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Limited Discrepancy Search (LDS) Ginsberg & Harvey
Take 1 discrepancy
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Limited Discrepancy Search (LDS) Ginsberg & Harvey
Take 1 discrepancy
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Limited Discrepancy Search (LDS) Ginsberg & Harvey
Take 1 discrepancy
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Limited Discrepancy Search (LDS) Ginsberg & Harvey
Take 1 discrepancy
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Now take 2 discrepancies
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Limited Discrepancy Search (LDS) Ginsberg & Harvey
Take 2 discrepancies
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Limited Discrepancy Search (LDS) Ginsberg & Harvey
Take 2 discrepancies
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Limited Discrepancy Search (LDS) Ginsberg & Harvey
Take 2 discrepancies
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First proposal
For discrepancies 0 to n
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First proposal
For discrepancies 0 to n
k is remaining discrepancies
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First proposal
For discrepancies 0 to n
k is remaining discrepancies
Go with heuristic
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First proposal
For discrepancies 0 to n
k is remaining discrepancies
Go with heuristic
Go against then go with
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The lds search process: how it goes (a cartoon)
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The lds search process: how it goes
NOTE: lds revisits search states with k discrepanciesWhen searching with > k discrepancies
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My pseudo code
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lds revisits nodes: Korf’s improvement (AAAI 96)
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Korf’s improvement
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Korf’s 1st mistake!
Woops!
Do you see it? He’s taking his discrepancies late/deep!
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Wrong way round.
Is that important?
Harvey & Ginsberg
Korf
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Korf’s 1st mistake!
Wrong way round Richard
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Richard, was that a bug?
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Yes, but so?
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Korf’s 2nd bug
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Woops!
Richard, you know there is another bug?
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Richard, you know there is another bug?
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Richard, are you there?
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My pseudo code
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Has anyone noticed Korf’s bug?
Have people been using Korf’s LDS?
Have people been using Harvey & Ginsberg’s LDS?
Has anyone remembered the motivation for LDS?
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Toby, do you do it late or early?
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Chris, late or early?
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Wafa, late or early?
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Wafa’s response
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My pseudo code
I think this has not been reported
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Does it make a difference if we take discrepancies late or early?
An empirical study
Tests Harvey & Ginsberg’s motivation for LDS
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Car Sequencing Problem
Assessed exercise 2
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My empirical study on car sequencing problemsUsing various search algorithms, heuristics.
Question:
• does the order (late/early) that we take discrepancies in lds matter?• is the order sensitive to the heuristics used?
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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?
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See anything?
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What does this tell us about how we do research?
• we can just follow on without question (read the most recent paper)• forget the basic/initial hypothesis• 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
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