Bramer M. Logic Programming with Prolog

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194 Logic Programming With Prolog

Practical Exercise 4

Question 1

The following program is a possible solution. There is no need to change the

names of the predicates to isa_dog etc., but it makes the program easier to read if

you do.

?-op(150,xf,isa_dog).

?-op(150,xf,isa_cat).

?-op(150,xf,is_large).

?-op(150,xf,is_small).

?-op(150,xf,isa_large_dog).

?-op(150,xf,isa_small_animal).

?-op(150,xfy,chases).

fido isa_dog. fido is_large.

mary isa_cat. mary is_large.

rover isa_dog. rover is_small.

jane isa_cat. jane is_small.

tom isa_dog. tom is_small.

harry isa_cat.

fred isa_dog. fred is_large.

henry isa_cat. henry is_large.

bill isa_cat.

steve isa_cat. steve is_large.

jim is_large.

mike is_large.

X isa_large_dog:- X isa_dog,X is_large.

A isa_small_animal:- A isa_dog,A is_small.

B isa_small_animal:- B isa_cat,B is_small.

X chases Y:-

X isa_large_dog,Y isa_small_animal,

write(X),write(' chases '),write(Y),nl.

Specimen Solutions to Practical Exercises 195

There are six possible ways of satisfying the goal X chases Y.

?- X chases Y.

fido chases rover

X = fido ,

Y = rover ;

fido chases tom

X = fido ,

Y = tom ;

fido chases jane

X = fido ,

Y = jane ;

fred chases rover

X = fred ,

Y = rover ;

fred chases tom

X = fred ,

Y = tom ;

fred chases jane

X = fred ,

Y = jane ;

Question 2

A possible definition is given below.

pred(A,B):-X is (A+B)/2,write('Average is: '),

write(X),nl,Y is sqrt(A*B),

write('Square root of product is: '),write(Y),nl,

Z is max(X,Y),write('Larger is: '),write(Z),nl.

?- pred(6,7).

Average is: 6.5

Square root of product is: 6.48074069840786

Larger is: 6.5

yes

196 Logic Programming With Prolog

?- pred(1,9).

Average is: 5

Square root of product is: 3

Larger is: 5

yes

Specimen Solutions to Practical Exercises 197

Practical Exercise 5

Question 1

A possible definition is as follows:

makelower:-get0(X),process(X).

process(13):-nl.

process(X):-X=\=13,convert(X,Y),put(Y),makelower.

convert(X,Y):-X>=65,X=<=90,Y is X+32.

convert(X,X):-X<65.

convert(X,X):-X>90.

Question 2

copyterms(Infile,Outfile):-

seeing(S),telling(T),

see(Infile),tell(Outfile),

copy,

seen,see(S),told,tell(T).

copy:-read(X),process(X).

process(end_of_file).

process(X):-

X\=end_of_file,

write(X),write('.'),nl,copy.

Note the use of write('.') to output a full stop after each term, so that the file may

subsequently be read in again as specified.

198 Logic Programming With Prolog

Question 3

A possible definition of readfile is given below.

readfile(F):-

seeing(S),see(F),

readchar,readchar,readchar,

seen,see(S).

readchar:-get0(X),write(X),nl.

If 'end of file' and 'end of record' are represented as suggested in Chapter 5, the

output from a call to readfile would be:

?- readfile('testa.txt').

100

101

102

103

104

105

106

-1

yes

If this is not the output produced on your system, it means that your version of

Prolog uses a different way of representing end of file, end of record or both.

Question 4

It will be assumed that fill in1.txt contains

Specimen Solutions to Practical Exercises 199

first.

second.

third.

fourth.

'fifth and last'.

end.

and file in2.txt contains

alpha.

beta.

gamma.

omega.

end.

A possible definition of combine is as follows.

combine(In1,In2,Out):-

seeing(S),telling(T),

tell(Out),see(In1),copyfile,seen,

see(In2),copyfile,seen,see(S),

write(end),nl,told,telling(T).

copyfile:-read(N),process(N).

process(end).

process(N):-write(N),nl,copyfile.

Testing this by

?- combine('in1.txt','in2.txt','out.txt').

produces the output file out.txt given below.

first

second

third

fourth

fifth and last

alpha

beta

gamma

omega

end

200 Logic Programming With Prolog

Question 5

It will be assumed that the file test1.txt contains

first.

second.

third.

fourth.

end.

and file test2.txt contains

first.

xxxx.

third.

fourthxxx.

end.

A possible definition of compare is:

compare(File1,File2):-

seeing(S),compfile(File1,File2),

see(File1),seen, see(File2),seen,see(S).

compfile(File1,File2):-

see(File1),read(X),see(File2),read(Y),

comp(X,Y),process(X,Y,File1,File2).

comp(A,A):-write(A),write(' is the same as '),

write(A),nl.

comp(A,B):-write(A),write(' is different from '),

write(B),nl.

process(end,end,_,_).

process(_,_,File1,File2):-compfile(File1,File2).

?- compare('test1.txt','test2.txt').

first is the same as first

second is different from xxxx

third is the same as third

fourth is different from fourthxxx

end is the same as end

yes

Specimen Solutions to Practical Exercises 201

Practical Exercise 6

Question 1

A possible definition using recursion is given below:

outsquare(N1,N2):-N1>N2.

outsquare(N1,N2):-

write(N1),write(' squared is '),Square is N1*N1,

write(Square),nl,M is N1+1,outsquare(M,N2).

?- outsquare(6,12).

6 squared is 36

7 squared is 49

8 squared is 64

9 squared is 81

10 squared is 100

11 squared is 121

12 squared is 144

yes

Question 2

The following definition uses the repeat predicate:

go:-repeat,get0(X),getrest(X).

getrest(13):-nl. /* newline */

getrest(63):-nl,repeat,get0(X),X=:=13.

getrest(X):-put(X),fail.

?- go.

: abcdef

abcdef

yes

?- go.

: abcde?wxyz

abcde

yes

202 Logic Programming With Prolog

Question 3

The following program uses backtracking with failure:

find:-

person(_,_,Age,_,Prof),Age>40,

write('Profession is '),write(Prof),nl,fail.

find.

person(john,smith,45,london,doctor).

person(martin,williams,33,birmingham,teacher).

person(henry,smith,26,manchester,plumber).

person(jane,wilson,62,london,teacher).

person(mary,smith,29,glasgow,surveyor).

?- find.

Profession is doctor

Profession is teacher

yes

Specimen Solutions to Practical Exercises 203

Practical Exercise 7

Question 1

The corrected version is as follows.

factorial(1,1):-!.

factorial(N,Nfact):-N1 is N-1,

factorial(N1,Nfact1),Nfact is N*Nfact1.

?- factorial(6,N).

N = 720

?- factorial(7,M).

M = 5040

Question 2

A possible completed program is given below. It uses the same method of testing

whether a number is even that was used in Chapter 4, i.e. use // to divide the

number by 2, discarding any remainder, then multiply by 2 and check if the result

is the original number.

go:-repeat,read_and_check(N,Type),

write(N),write(' is '),write(Type),nl,N=:=100.

read_and_check(N,Type):-

write('Enter next number'),read(N),

check(N,Type).

check(N,even):-N1 is N//2,N2 is 2*N1,N2=:=N,!.

check(N,odd).