Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>The <a href="http://en.wikipedia.org/wiki/DPLL_algorithm">DPLL</a> is essentially a <strong>backtracking algorithm</strong>, and that's the main idea behind the recursive calls. </p> <p>The algorithm is building solution while trying assignments, you have a partial solution which might prove successful or not-successful as you go on. The geniusity of the algorithm is how to build the partial solution.</p> <p>First, let's define what a <strong>unit clause</strong> is: </p> <p>A unit clause is a clause which has exactly one literal which is still unassigned, and the other (assigned) literals - are all assigned to false. <br>The importance of this clause is that if the current assignment is valid - you can determine what is the value of the variable which is in the unassigned literal - because the literal must be true.</p> <p>For example: If we have a formula: </p> <pre><code>(x1 \/ x2 \/ x3) /\ (~x1 \/ ~x4 \/ x5) /\ ( ~x1 \/ x4) </code></pre> <p>And we already assigned: </p> <pre><code>x1=true, x4=true </code></pre> <p>Then <code>(~x1 \/ ~x4 \/ x5)</code> is a unit clause, because you must assign <code>x5=true</code> in order to satisfy this clause in the current partial assignment.</p> <p><strong>The basic idea of the algorithm is:</strong></p> <ol> <li>"Guess" a variable</li> <li>Find all unit clauses created from the last assignment and assign the needed value</li> <li>Iteratively retry step 2 until there is no change (found transitive closure)</li> <li>If the current assignment cannot yield true for all clauses - fold back from recursion and retry a different assignment</li> <li>If it can - "guess" another variable (recursively invoke and return to 1)</li> </ol> <p><strong>Termination:</strong></p> <ol> <li>There is nowhere to go "back to" and change a "guess" (no solution)</li> <li>All clauses are satisfied (there is a solution, and the algorithm found it)</li> </ol> <p>You can also have a look at <a href="http://webcourse.cs.technion.ac.il/236342/Winter2011-2012/ho/WCFiles/Tut14.pdf">these lecture slides</a> for more information and an example. (Ignore the first Hebrew slide, the next are all English; You might also want to ignore the 'learning' at the end until you understand the basics better)</p> <p><strong>Usage example and importance:</strong> <br>The DPLL, though 50 years old - is still the basis for most SAT solvers. <br>SAT Solvers are very useful for solving hard problems, one example in software verification - where you represent your model as a set of formulas, and the condition you want to verify - and invoke the SAT solver over it. Although exponential worst case - the average case is fast enough and is widely used in the industry (mainly for verifying Hardware components)</p>
    singulars
    1. This table or related slice is empty.
    1. POHow does the DPLL algorithm work?
      singulars
      1. PTQuestion
    1. PTAnswer
    1. USEliot Ball
    1. USamit
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. POHow does the DPLL algorithm work?
      singulars
      1. PTQuestion
    1. This table or related slice is empty.
    1. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    3. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTAcceptedByOriginator
    1. CO"mainly for verifying hardware components" -- not _exactly_ true. DPLL forms the basis for any SMT solver, which see more absolute widespread use (by number of people) inside static analysis software than hardware verification. Though it is certainly true that they see more use in the hardware industry than software industry!
      singulars
      1. PO
      1. USKristopher Micinski
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload