Chapter 3

Question 1

Constants

Answer 1

Must start with a lower letter which can be followed by letters, underscores or digits.
It can also be a quoted string.

Question 2

Quoted string

Answer 2

Any string of characters (other than single quote) enclosed within single quotes

Question 3

Are these allowed constants in Prolog?

1. George
2. granny_sarah
3. ‘Hello world’
4. ’ _ ‘

Answer 3

1. No
2. Yes
3. Yes
4. Yes

Question 4

Variables

Answer 4

Can start with uppercase and can be followed by any number of letters, underscores and digits

Question 5

Atomic sentences (or atoms)

Answer 5

they have the following form:
predicate(argument1, …., argumentN)

where predicate = Prolog constant and
arguments = Prolog constants/variables

We always need parenthesis after the predicate unless it has 0 arguments

Question 6

Think of two examples of correct Prolog atomic sentences in the KB.

Answer 6

1. child(john, sue)

2. my_car(red, ‘bmw’)

Question 7

Conditional sentences (clauses)

Answer 7

structure –> head :- body1, … , bodyN
head –> Prolog atom (then..)
body –> Prolog atom (if…)

Question 8

Think of an example of a Prolog conditional sentence

Answer 8

sister(X, Y) :- female(X), person(Y), sibling(X, Y)

Question 9

What would be a case where an atomic sentence is just a special case of a conditional sentence?

Answer 9

A case where a conditional sentence has an empty body (“if” part)

Example:
sister(X, Y) :- nothing –> in this case, sister(X, Y) is just a special case of a conditional sentence.

Question 10

A Prolog program is a sequence of …

Answer 10

clauses

Question 11

Prolog clauses can be …

Answer 11

atomic/conditional sentences

Question 12

Where can spaces, newlines and comments be inserted in a Prolog program?

Answer 12

1. at the end of a program or

2. just before a constant or a variable

Question 13

Comments can be added followed by the character…

Answer 13

%

Question 14

What is the goal of Prolog by answering a query?

Answer 14

To establish whether there is a relationship between the query and the KB

Question 15

Query

Answer 15

Atom with/without variables terminated by a period.

Question 16

What are the 3 possible outputs when posing a query WITHOUT variables?

Answer 16

Yes - the atom is established by back-chaining
No - the atom cannot be established by back-chaining
Nothing - Prolog is still looking for a relationship

Question 17

What are the 3 possible outputs when posing a query WITH variables?

Answer 17

1. Prolog displays the values of the variables for which the query can be established.
2. No - the aim cannot be established for any values of the variable
3. No answer, still looking

Question 18

If the posed query contains variables and Prolog displays the values of the variables for which the query can be established, the user has two options:

Answer 18

1. press space/return, which will output “Yes” and complete the query answering
2. Type semicolon, which will prompt Prolog to look for further variables outcomes

Question 19

Conjunctive queries

Answer 19

sequences of atoms separated by commas (which play the role of “and”) and terminated by a period. All atoms need to be established in order to return “Yes” by Prolog.

Question 20

When are conjunctive queries particularly helpful?

Answer 20

When a variable appears in more than one atom in the query

Question 21

What is this symbol used for in Prolog? “+”

Answer 21

Negation in queries

Question 22

If a line in Prolog starts with “Call”, what does it mean?

Answer 22

Prolog starts to work on an atomic sentence

Question 23

If a line in Prolog starts with “Fail”, what does it mean?

Answer 23

An atomic query has failed and Prolog needs to look for alternatives

Question 24

If a line in Prolog starts with “Exit”, what does it mean?

Answer 24

An atomic query has tentatively succeeded, pending the remaining conjunctive query

Question 25

If a line in Prolog starts with “Redo”, what does it mean?

Answer 25

Prolog has gone back to a choice point to reconsider an atomic query

Question 26

When dealing with a negated query, will Prolog consider the negation during the back-chaining or will it drop it?

Answer 26

It will drop it and attempt to look for a matching atomic sentence: if it does, the it will return SUCCEED and the negated query will fail. Go back to the most recent choice point and reconsider.

Question 27

Uninstantiated variable

Answer 27

It does not have a tentative value yet

Question 28

Instantiated variable

Answer 28

Has a tentative value

Question 29

What should we make sure to do when variables appear in a negated query?

Answer 29

We should make sure that they are already instantiated at an earlier stage of the back-tracking. Practically, use another atom in the query BEFORE the negated part.

Question 30

When does an equality hold in Prolog?

Answer 30

When the two terms (constants or variables) can be made equal by instantiating any variables

Question 31

Is an equality or inequality more useful in Prolog?

Answer 31

Inequality

Question 32

How can we interpret this program?

male(X), male(Y), male(Z) , + X = Y, + X = Z, + Y = Z

Answer 32

A program that defines a new predicate “male” that holds when three people X, Y and Z are different people and are all males.

Question 33

Unification

Answer 33

Clauses in a program are selected during back-chaining via a matching process called unification.

Question 34

Two atoms whose variables are distinct are said to “unify” when…

Answer 34

there is a substitution of values for the variables that makes the atoms identical.

Question 35

Will these atoms unify?

1. p(b, X, b) and p(Y, a, b) for X = a, Y = b
2. p(X, b, X) and p(a, b, Y) for X = a, Y = a
3. p(X, b, X, a) and p(Y, Z, Z, Y)

Answer 35

yes
yes
no ( the 2 occurrences of X force Y and Z to be equal, but they must also match a and b respectively)

Question 36

Why is it important to rename a variable in a query with a name that is not in the KB?

Answer 36

Because we might lose some unifications that would be possible if the name of the variables were different.