In this chapter, we will discuss loops and decision making in Prolog.

Loops

Loop statements are used to execute the code block multiple times. In general, for, while, do-while are loop constructs in programming languages (like Java, C, C++).

Code block is executed multiple times using recursive predicate logic. There are no direct loops in some other languages, but we can simulate loops with few different techniques.

Program

count_to_10(10) :- write(10),nl.
count_to_10(X) :-
   write(X),nl,
   Y is X + 1,
   count_to_10(Y).

Output

| ?- [loop].
compiling D:/TP Prolog/Sample_Codes/loop.pl for byte code...
D:/TP Prolog/Sample_Codes/loop.pl compiled, 4 lines read - 751 bytes written, 16 ms

(16 ms) yes
| ?- count_to_10(3).
3
4
5
6
7
8
9
10

true ?
yes
| ?-

Now create a loop that takes lowest and highest values. So, we can use the between() to simulate loops.

Program

Let us see an example program −

count_down(L, H) :-
   between(L, H, Y),
   Z is H - Y,
   write(Z), nl.
   
count_up(L, H) :-
   between(L, H, Y),
   Z is L + Y,
   write(Z), nl.

Output

| ?- [loop].
compiling D:/TP Prolog/Sample_Codes/loop.pl for byte code...
D:/TP Prolog/Sample_Codes/loop.pl compiled, 14 lines read - 1700 bytes written, 16 ms

yes
| ?- count_down(12,17).
5

true ? ;
4

true ? ;
3

true ? ;
2

true ? ;
1

true ? ;
0

yes
| ?- count_up(5,12).
10

true ? ;
11

true ? ;
12

true ? ;
13

true ? ;
14

true ? ;
15

true ? ;
16

true ? ;
17

yes
| ?-

Decision Making

The decision statements are If-Then-Else statements. So when we try to match some condition, and perform some task, then we use the decision making statements. The basic usage is as follows −

If <condition> is true, Then <do this>, Else 

In some different programming languages, there are If-Else statements, but in Prolog we have to define our statements in some other manner. Following is an example of decision making in Prolog.

Program

% If-Then-Else statement

gt(X,Y) :- X >= Y,write(''X is greater or equal'').
gt(X,Y) :- X < Y,write(''X is smaller'').

% If-Elif-Else statement

gte(X,Y) :- X > Y,write(''X is greater'').
gte(X,Y) :- X =:= Y,write(''X and Y are same'').
gte(X,Y) :- X < Y,write(''X is smaller'').

Output

| ?- [test].
compiling D:/TP Prolog/Sample_Codes/test.pl for byte code...
D:/TP Prolog/Sample_Codes/test.pl compiled, 3 lines read - 529 bytes written, 15 ms

yes
| ?- gt(10,100).
X is smaller

yes
| ?- gt(150,100).
X is greater or equal

true ?

yes
| ?- gte(10,20).
X is smaller

(15 ms) yes
| ?- gte(100,20).
X is greater

true ?

yes
| ?- gte(100,100).
X and Y are same

true ?

yes
| ?-