# swipl 5.6.x

Version of implementation SWI-Prolog of programming language Prolog

A version of SWI-Prolog compiler.

## Examples:

### Hello, World! - Prolog (49):

This example doesn’t need any facts or rules to be evaluated. The query is executed in interactive mode, and results in the following output:

`Hello, World!`
`yes`

First line is the actual output of `write` predicate, and second line is the result of query evaluation.

Note that replacing single-quotes with double-quotes makes Prolog output the string as an array of ASCII-codes of individual characters:

`| ?- write("Hello, World!").`
`[72,101,108,108,111,44,32,87,111,114,108,100,33]`

`yes`

``````write('Hello, World!'), nl.
``````

### Quadratic equation - Prolog (241):

This is an ISO Prolog example, using standard read/1 predicate for reading input. Note that when using read/1, you have to put full stop . after each value you input.

``````q :- write('A = '),
(   A = 0, write('Not a quadratic equation');
write('B = '),
write('C = '),
D is B*B-4*A*C,
(   D = 0, write('x = '), X is -B/2/A, write(X);
D > 0, write('x1 = '), X1 is (-B+sqrt(D))/2/A, write(X1), nl, write('x2 = '), X2 is (-B-sqrt(D))/2/A, write(X2);
R is -B/2/A, I is abs(sqrt(-D)/2/A),
write('x1 = ('), write(R), write(', '), write(I), write(')'), nl,
write('x1 = ('), write(R), write(', -'), write(I), write(')')
)
).
``````

### Factorial - Prolog (238):

Almost identical to Poplog Prolog example, except for the syntax of compiling a file (doesn’t have a `-` before file name). However, the results of execution depend on the implementation. SWI-Prolog handles large numbers just fine, while in GNU Prolog and B-Prolog 12! overflows the numeric data type, so all values after 11! are incorrect.

`| ?- [fact].`
compiling /home/nickolas/Desktop/progopedia/prolog/fact.pl for byte code…
/home/nickolas/Desktop/progopedia/prolog/fact.pl compiled, 3 lines read — 1372 bytes written, 5 ms

yes
`| ?- fact(16,X).`
0! = 1
1! = 1
2! = 2
3! = 6
4! = 24
5! = 120
6! = 720
7! = 5040
8! = 40320
9! = 362880
10! = 3628800
11! = 39916800
12! = -57869312
13! = -215430144
14! = 205203456
15! = -143173632
16! = -143294464

X = -143294464 ?`

``````% fact.pl
fact(X, F) :-
( X=0, F=1;
Y is X-1, fact(Y, Z), F is X*Z),
write(X), write('! = '), write(F), nl.

% interactive
[fact].
fact(16,X).
``````

### Fibonacci numbers - Prolog (239):

Once again, the example is almost identical to Poplog Prolog one, except for the syntax of compiling/consulting a file.

``````% fibonacci.pl
:- dynamic(stored/1).

memo(Goal) :-
stored(Goal) -> true;
Goal, assertz(stored(Goal)).

fib(1,1) :- !, write('1, ').
fib(2,1) :- !, write('1, ').
fib(N,F) :-
N1 is N-1, memo(fib(N1,F1)),
N2 is N-2, memo(fib(N2,F2)),
F is F1 + F2,
write(F), write(', ').

% interactive
[fibonacci].
fib(16,X), write('...'), nl.
``````