assert

assert(Clause)

• Will add Clause to the current program, and will always succeed
• The Clause is added at the end of the clauses associated with its predicate name
• Clause may be a fact or a rule
• Has arity of 1

Facts should be given in the form :

windy 
sunny 
miles(new_album)

and rules

(Head :- C1, C2, _, Cn)

• Note : the rule must be enclosed in parenthesis
• Since Prolog has no global variables, assert must be used to save the value of a variable computed during the execution of a program

assert(value(x,5))

Q: value(x, Value) 
A: Value = 5

similarly

Q: crisis 
A: no

Q: assert(crisis)
A: yes

Q: crisis
A: yes

• Assert can be used as part of the program - (program modifying itself!!)
• A way perhaps to deal with changing situations
• One program being able to modify another one - even the internal structure of a program through the use of assert and retract

Q: assert((likes(keith,Person) :- likes(Person, jazz))).
A: yes
Q: assert(likes(james,coltrane)).
A: yes

The above will generate the following definition of likes

likes(keith,Person) :-
 likes(Person, jazz).

likes(james,coltrane).

• Assert of two types asserta and assertz
• asserta : Adds a clause at the beginning of the clauses associated with its predicate name

Q: asserta((likes(keith,Person) :- likes(Person, jazz))).
A: yes
Q: asserta(likes(james,coltrane)).
A: yes

The above will generate the following definition of likes

likes(james,coltrane).
likes(keith,Person) :-
 likes(Person, jazz).

• Similarly assertz : add a clause at the end of the clauses associated with its predicate name
• Something that LPA MacProlog offers in addition to the above two is assertx/2 - which allows a clause to be inserted at a particular position

Q: assert(foo(a),0), assert(foo(b),1), 
assert(foo(c),999),assert(foo(d),2)

foo(b)
foo(d)
foo(a)
foo(c)

• If position = 0 or greater than number of clauses acts as assertz