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Version: v11.0.0

assertz/1

Description​

assertz/1 is a predicate that asserts a clause into the database as the last clause of the predicate.

Signature​

assertz(+Clause)

Where:

  • Clause is the clause to assert into the database.

Examples​

Assert a fact into the database​

This scenario demonstrates the process of asserting a new fact into a Prolog database. In Prolog, asserting a fact means adding a new piece of information or knowledge into the database, allowing it to be referenced in subsequent queries. This is particularly useful when you want to dynamically extend the knowledge base with facts or rules based on conditions or interactions during runtime.

Here are the steps of the scenario:

  • Given the program:
assert_fact :- assertz(father(john, pete)).
  • Given the query:
assert_fact, father(X, Y).
  • When the query is run
  • Then the answer we get is:
height: 42
gas_used: 3977
answer:
  has_more: false
  variables: ["X", "Y"]
  results:
  - substitutions:
    - variable: X
      expression: john
    - variable: 'Y'
      expression: pete

Only dynamic predicates can be asserted​

This scenario demonstrates that only dynamic predicates can be asserted. In Prolog, dynamic predicates are those that can be modified during runtime. This is in contrast to static predicates, which are fixed and cannot be modified.

Here are the steps of the scenario:

  • Given the program:
parent(jane, alice).
  • Given the query:
assertz(parent(john, alice)).
  • When the query is run
  • Then the answer we get is:
height: 42
gas_used: 3975
answer:
  has_more: false
  results:
  - error: "error(permission_error(modify,static_procedure,parent/2),assertz/1)"

Show that the fact is asserted at the end of the database​

This scenario demonstrates that the assertz/1 predicate adds the fact to the end of the database. This means that the fact is the last fact to be matched when a query is run.

This is in contrast to the asserta/1 predicate, which adds the fact to the beginning of the database.

Here are the steps of the scenario:

  • Given the program:
:- dynamic(parent/2).

parent(jane, alice).

assert_fact :- assertz(parent(john, alice)).
  • Given the query:
assert_fact, parent(X, alice).
  • When the query is run (limited to 2 solutions)
  • Then the answer we get is:
height: 42
gas_used: 3977
answer:
  has_more: false
  variables: ["X"]
  results:
  - substitutions:
    - variable: X
      expression: jane
  - substitutions:
    - variable: X
      expression: john

Add and remove items in an inventory​

This scenario demonstrates how to maintain a dynamic list of items (like in an inventory system) by representing each item as a fact in the Prolog knowledge base. By using dynamic predicates, we can add items to the inventory and remove them on demand.

Here are the steps of the scenario:

  • Given the program:
:- dynamic(inventory/1).

add_item(Item) :- assertz(inventory(Item)).
remove_item(Item) :- retract(inventory(Item)).
  • And the query:
add_item('apple'),
add_item('banana'),
add_item('orange'),
remove_item('banana'),
findall(I, inventory(I), CurrentInventory).
  • When the query is run
  • Then the answer we get is:
height: 42
gas_used: 3984
answer:
  has_more: false
  variables: ["I","CurrentInventory"]
  results:
  - substitutions:
    - variable: CurrentInventory
      expression: "[apple,orange]"