PROLOG

1
PROLOG

• Chapters 3 and 4

2
COMPOUND QUERIES

• Small knowledge base for the in-class exercises.
• PROLOG works in exactly the same fashion with a
  large knowledge base, but you would not be able
  to identify every relationship due to the size.
• With a large knowledge base, PROLOG allows you to
  uncover relationships that you couldnt identify
  manually.

3
IN-CLASS PROBLEM

• Using the problem from last week
• Add the relationships according to the revised
  organization chart (handout).

4
NEW RELATIONSHIPS
5
PRACTICE QUERY

• Query PROLOG to find someone who reports to both
• the vice president of engineering, and
• the director of research.

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PROLOG QUERY
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HOW PROLOG SOLVES QUERIES

• PROLOGs method of solving queries is fairly
  "brute force via the following steps
• work from left to right
• take the first literal (up to first ,)
• find a match in the knowledge base
• mark the the location of the match
• the solution to the first literal becomes a
  constraint for solving the rest of the query
• attempt to solve the rest of the query

8
BACKTRACKING

• PROLOGs procedure to back up when a search path
  fails.
• e.g., if the second half of the query does not
  find a match
• Recall that PROLOG placed a marker at the
  location of the match on the first predicate.

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BACKTRACKING PROCEDURE

• If a search through the entire knowledge base
  fails to find a match on the second half of the
  query, BACKTRACK to the marker.
• Erase the "binding" of the variable to that
  value.
• Move forward through the knowledge base to the
  next match with the first part of the query.
• Repeat the search attempting to match the second
  part of the query.

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TO FIND ALL SOLUTIONS

• Expand the backtracking process.
• When a complete match is found, "undo" that match
  (backtrack to the last marker and remove it).
• Move forward to try to solve again.
• Moves forward from the earlier match so it will
  look for matches in the rest of the knowledge
  base.
• When it fails (e.g., no more matches), backtrack
  further to the match of an earlier predicate
  repeat the search.

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MORE ON BACKTRACKING

• Prolog uses "strict chronological backtracking".
  
• When it cannot solve the latest match (2nd, 3rd,
  4th, whatever)
• "unbinds" only the most recent solution
• moves forward to find the next solution
• If, after searching the entire knowledge base, a
  solution is not found, PROLOG backtracks further,
  "unbinding" the next most previous variable.

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MULTIPLE VARIABLES

• Use the same logic for multiple variables as for
  single variables.
• add new predicates to the data base
• aggravates(secretary, sales_secretary).
• aggravates(secretary, eng_secretary).
• aggravates(secretary, exec_vp).
• Is there someone who reports to the president who
  aggravates others in the organization?

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QUERY
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TRACE

• ALS-PROLOG has a trace capability
• Allows you to examine which predicates are
  executed and where system fails
• tools --gt debugger
• window should pop up
• Preface each predicate with the word TRACE and
  predicate calls will be shown in debugging window
• Tip manually clear or insert spaces between
  executions

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QUERY with TRACE

• trace reports_to(Someone,president), trace
  aggravates(Someone,Others).
• Result
• Someonesecretary
• Otherssales_secretary
• Someonesecretary
• Otherseng_secretary
• Someonesecretary
• Othersexecutive_vp
• no.

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TRACE RESULTS FIRST MATCH

• (1) 1 call userreports_to(_6665,president)
• (1) 1 exit userreports_to(executive_vp,president
  )
• (1) 1 call useraggravates(executive_vp,_6671)
• (1) 1 fail useraggravates(executive_vp,_6671)
• (1) 1 redo userreports_to(executive_vp,president
  )
• (1) 1 exit userreports_to(vp_manufact,president)
  
• (1) 1 call useraggravates(vp_manufact,_6671)
• (1) 1 fail useraggravates(vp_manufact,_6671)
• (1) 1 redo userreports_to(vp_manufact,president)
  
• (1) 1 exit userreports_to(vp_engineering,preside
  nt)
• (1) 1 call useraggravates(vp_engineering,_6671)
• (1) 1 fail useraggravates(vp_engineering,_6671)
• (1) 1 redo userreports_to(vp_engineering,preside
  nt)
• (1) 1 exit userreports_to(treasurer,president)
• (1) 1 call useraggravates(treasurer,_6671)
• (1) 1 fail useraggravates(treasurer,_6671)
• (1) 1 redo userreports_to(treasurer,president)
• (1) 1 exit userreports_to(secretary,president)
• (1) 1 call useraggravates(secretary,_6671)

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REMAINDER OF TRACE

• (1) 1 exit useraggravates(secretary,eng_secretar
  y)
• (1) 1 redo useraggravates(secretary,eng_secretar
  y)
• (1) 1 exit useraggravates(secretary,executive_vp
  )
• (1) 1 redo useraggravates(secretary,executive_vp
  )
• (1) 1 fail useraggravates(secretary,_6671)
• (1) 1 redo userreports_to(secretary,president)
• (1) 1 exit userreports_to(vp_sales,president)
• (1) 1 call useraggravates(vp_sales,_6671)
• (1) 1 fail useraggravates(vp_sales,_6671)
• (1) 1 redo userreports_to(vp_sales,president)
• (1) 1 fail userreports_to(_6665,president)

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DIFFERENCES

• To ensure that unique solutions are found, it is
  sometimes necessary to constrain the solution so
  that two variables are not assigned the same
  value.
• \ is used to indicate "not equal".

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QUERIES AND RULES

• Instead of typing complicated queries, we can
  build Rules.
• The format for rules is as follows
• head - body.
• - is interpreted as "lt---"
• "If the body is true, then the head is true "
• Once the rule is added to the knowledge base, it
  can be executed.

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EXAMPLE RULE

• Write a rule to define co-workers
• Where co-workers are
• Two people who work for the same person

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CO-WORKERS RULE
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PERSONNEL PROBLEMS RULE

• How about a rule to define personnel problems?
• Let's say you have a personnel problem when two
  people are co-workers and one of them aggravates
  the other.
• Hint use existing rule!

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PERSONNEL PROBLEM PREDICATE
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HOW PROLOG EXECUTES A SAVED RULE

• PROLOG substitutes the body of the rule for the
  head, and solves as it would any compound query.
• see p. 72 of text for a summary of how rules
  work.
• Use TRACE, note you can creep one predicate at
  a time, or leap to the next solution.

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MULTIPLE VERSIONS OF A RULE

• Frequently you will define multiple versions of
  the same rule.
• PROLOG will search the knowledge base, starting
  from the top and attempting to solve all versions
  of the rule.
• If any version of the rule is true (there exist
  values to satisfy all variables in that
  particular version of the rule), then the rule
  is true.
• Used to implement OR.

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EXAMPLE OF MULTIPLE VERSIONS OF A RULE

• Using the organization chart, what if different
  kind of personnel problems exists?
• For instance what if an employee aggravates
  their boss?
• Define this as a rule, also a personnel problem
  (i.e., use the same name as your last rule).

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NEW TYPE OF PERSONNEL PROBLEM
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HOW PROLOG WORKS

• Refer also to 4.4 in the text.
• Figure 4-11 - this is the simple version.
• A goal is empty when there is nothing to be done
  to solve it.
• Figure 4-12 - includes chronological
  backtracking.
• Note extra loop. If more rules for predicate,
  apply those rules.
• If no more rules for the predicate, check for
  previous choices, if any exist, undo those
  choices (backtrack) to find other possible
  solutions.

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CYCLIC ACTION OF PROLOG
Start Submit Query
Is Goal Empty?
Yes
Halt with Success
No
More Rules for Predicate?
No
Halt with Failure
Yes
Apply (next) rule for Predicate
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CYCLE with BACKTRACKING
Start Submit Query
Is Goal Empty?
Yes
Halt with Success
No
Previous DB Choices?
More Rules for Predicate?
No
No
Halt with Failure
Yes
Yes
Apply (next) rule for Predicate
Undo effects of application of most recently
chosen rule from the database
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UNIFICATION

• Unification is the process of finding successful
  matches. Prolog works by "unifying" expressions.
  
• Figure 4-19 in text shows unification algorithm
• Figure 4-20 shows examples - work through, make
  sure you understand these examples.
• Be aware that
• No such thing as a GLOBAL variable.
• Scope is limited usually just that instance of
  the predicate.

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ARITHMETIC

• See pages 192-194
• Usual arithmetic operators, functions, and
  comparisons are available.