Programming Paradigms Declarative Programming Paradigm 1 Learning Objectives

Programming Paradigms Declarative Programming Paradigm 1

Learning Objectives Describe with the aid of examples, the characteristics of a variety of programming paradigms. Explain the terms declarative and procedural as applied to high-level languages. Discuss the concepts and, using examples, show an understanding of backtracking, instantiation and satisfying goals. 2

Programming Paradigms Methods of programming. 3

Procedural Programming Paradigm Expect the user to write the instructions in sequence telling the computer exactly how to solve a problem. n n The way you have used Visual Basic up to know has been in the procedural paradigm form. Other examples are C++, FORTRAN, COBOL, BASIC etc. 4

Declarative Programming Paradigm A database of facts / rules is first created. The user writes programs as queries with criteria (rules) which are inputted into a search engine which searches the database for the answers and returns them to the user. The user / program specifies what is to be searched for but does not provide the computer with a procedure, detailing how to search, for the computer to follow as in procedural languages e. g. Visual Basic etc. . User / Program Search Engine Database 5

Declarative Programming Paradigm languages are used: When solving problems in artificial intelligence such as medical diagnosis, fault finding in equipment and oil exploration. In robot control. An example of a declarative language is Prolog. 6

Example Database male(X). states that X is male female(X). states that X is female parent(X, Y). states that X is a parent of Y female(jane). female(anne). female(sandip). male(charnjit). male(jaz). male(tom). parent(jane, mary). parent(jane, rajinder). parent(charnjit, mary). parent(charnjit, rajinder). parent(jaz, atif). parent(sandip, atif). Example of Backtracking

Instantiation A particular variable is assigned a value. n e. g. If male(X) and male(jaz) then jaz is an instance of X or X is instantiated to jaz 8

A goal The intention to find all instances that satisfy a query with criteria (rule/s). n Answer a query with criteria written by the user by using a search engine to search a database to find and report items which satisfy the criteria of the query. 9

Example Goal Query: n n What are the names of all the males? In prolog: male(X). Answer: n n n X = charnjit X = jaz X = tom Note that the user does not have to tell Prolog how to search for the values of X that satisfy the query. 10

Backtracking If a query consists of two or more criteria (rules) and an item in a database satisfies the first criteria (rule) but does not satisfy another criteria (rule), then the search engine must go back to find another item which satisfies the first criteria and then try again to see if the other criteria (rule/s) are also satisfied. n This is repeated until all items have been searched (note in some searches only one item would be expected so the search would stop if one is found). 11

Example of Backtracking Query: n n What is the name of the mother of Atif? mother(X, Y): - parent(X, atif), female(X). When the query is inputted into the search engine all parents are searched first until one satisfies the first criteria (rule). Example Database Ignore: n n parent(jane, mary). parent(jane, rajinder). parent(charnjit, mary). parent(charnjit, rajinder). because Y<> atif n Remember that the definition parent(X, Y) states that 12 X is a parent of Y.

Example of Backtracking The first parent to satisfy the 1 st criteria is parent(jaz, atif) so X = jaz However, male(jaz) so the 2 nd criteria (rule) is not satisfied so this item (X = jaz) is rejected. Now the search engine has to go back and search for another parent of atif (backtracking). This reveals parent(sandip, atif) so X = sandip As female(sandip) the 2 nd criteria is also satisfied so report sandip is the mother. 13

Note: Backtracking can only occur if the query has two or more criteria and if the answer is not found the first time. n i. e. The first item which satisfies the first criteria does not satisfy the other criteria, otherwise the answer has been found and backtracking is unnecessary. 14

Plenary male(X). states that X is male female(X). states that X is female parent(X, Y). states that X is a parent of Y mother(X, Y): - parent(X, Y), female(X). states that X is mother of Y if X is parent of Y and X is female. 15

Plenary Sto, Dis, May, David, Minah, John are all members of one family. The following facts apply: female(sto). female(may). female(minah). male(john). male(dis). male(david). parent(john, dis). parent(john, may). parent(dis, sto). parent(dis, david). parent(minah, dis). parent(minah, may). 16

Plenary (a) By using examples from the facts given, explain what is meant by: n n n (i) instantiation. (ii) a goal. (iii) backtracking. 17

Plenary (i) -A particular variable is assigned a value n -e. g. if male(X) then dis is an instance of X / X is instantiated to dis (ii) -The intention to find all instances that satisfy a rule/set of facts n -e. g. If rule is male(X) then the goal is to find dis and david and john (iii) -If the result of one rule does not apply in a second rule, then go back to find another result of the first rule n -e. g. parent (john, dis) is found if we are searching for mother of dis. This fails the second part of the rule for mother because john is male, so backtracking is used to return to the next example satisfying the first part of the rule. 18

Plenary (b) A new rule states grandparent (X, Y): parent(X, Z), parent(Z, Y). n n (i) Write down a rule to define grandmother. (ii) Explain how this new rule is used to find the grandmother of david. 19

Plenary (i) grandmother (x, y) : - grandparent (x, y), female (x) (ii) n n n n Ignores parent (john, dis) parent (john, may) parent (dis, sto) because Y<> david finds parent (dis, david) searches for parent (X, dis), finds X = john finds male (john), rejects X = john because not female backtracks to find next occurrence of (X, dis) finds parent (minah, dis) finds female (minah), reports minah is grandmother 20