Page 215 - DCAP506_ARTIFICIAL_INTELLIGENCE

Unit 14: Prolog

to be the similar term as the argument inside X. In Prolog’s output this is indicated through the Notes
use of the variable _G177. This variable has been produced by Prolog during execution time. Its
particular name, _G177 here, may be different each time the query is submitted.

Self Assessment

Fill in the blanks:
11. Two terms are defined to ...................... if they are either indistinguishable or if they can be
made indistinguishable by variable instantiation.

12. The similar variable has to be instantiated with the similar ...................... all through an
expression.

14.8 Backtracking

The backtracking predicate provides an alternative path for a program when the current path
fails.
The response to a query is generated after executing and evaluating various conditional
statements in the program. During execution, the control goes to the first conditional statement
and tests it with a variable. If the condition is FALSE, the execution moves back and tries to
prove the condition with another variable. This looping back to the previous statement is called
backtracking.

Listing 3 shows a sample ProLog program that uses the backtracking predicate:
Listing 3: Program with Backtracking
/* The program specifies that the weather can be sunny on Friday, Saturday, or Sunday and the
weekend is on Saturday and Sunday.*/
/* Facts */
weather(friday, sunny).
weather(saturday, sunny).

weather(sunday, sunny).
weekend(saturday).
weekend(sunday).
/* The rule states that outings are possible on weekends, when the weather is sunny */

/* Write is a built-in predicate to display on screen */
outing(Day) :- weather(Day, sunny),
weekend(Day),
write(Day),

nl.
/* End of Program */

Backtracking allows Prolog to find all alternative solutions to a given query.

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