Learn Prolog in One Video

Posted by Derek Banas on Aug 12, 2015 in Web Design | 0 comments

Numerous people have asked if I can help them work through the numerous poorly written books on Prolog and so today I’ll do my best to teach Prolog in one video. I paid special attention to focus on those topics that people have told me they struggle with.

Also in this video I’ll show how not only to install Prolog, but also numerous other languages like C++ on Windows. Also I’ll specifically cover the following topics in Prolog Facts, Rules, Variables, If, Complex Terms / Structures, Format, Comparison, Trace, Recursion, Math, Reading / Writing to the Console, File I/O, Looping, Changing the Database, Lists, Strings and much more. All of the code follows the video.

If you like videos like this, it helps to tell Google with a click here [googleplusone]

Prolog Cheat Sheet

% Prolog programs are a collection of Facts, and Rules that we can
% Query.

% Prolog focuses on describing facts and relationships about problems
% rather then on creating a series of steps to solve that problem.

% These Facts and Rules are stored in a file called a Database
% or Knowledge Base

% You load a knowledge base like this [knowledge]. or this
% consult('knowledge.pl').
% halt. exits the prolog system
% listing. Displays the contents of the database
% All these commands are called predicates

% ---------- INTRODUCTION ----------
% write prints text between quotes to the screen
% nl stands for new line and \'s allows you to use quotes
% write('Hello World'),nl,write('Let\'s Program').

% This is a fact where loves is a predicate and romeo and
% juliet are atoms (constants) and loves arguments
loves(romeo, juliet).

% This is a rule where :- (if) says if the item on the right is
% true, then so is the item on the left
loves(juliet, romeo) :- loves(romeo, juliet).

% Evaluating whether the goal was met in the terminal
% loves(juliet, romeo). = yes

% Facts and Rules are called clauses

% A Variable is an object we can't name at the time of execution
% Variables are uppercase while atoms are lowercase
% loves(romeo, X). = X = juliet

% ---------- FACTS ----------
% Write the relationship first followed by the objects between
% parenthese followed by a dot

% albert, male, female are atom constants that must begin with a
% lowercase letter unless they are between single quotes
% An atom can contain letters, numbers, +, -, _, *, /, <, >, :, ., ~, &
% AN ATOM CANNOT START WITH _

% The name before parenthese is called the predicate
% The names in parenthese are called arguments

% Let's define information about the people above

male(albert).
male(bob).
male(bill).
male(carl).
male(charlie).
male(dan).
male(edward).

female(alice).
female(betsy).
female(diana).

% We can find out if alice is a woman with
% female(alice). = yes
% listing(male). = list all clauses defining the predicate male
% male(X), female(Y). = Show all combinations of male and female

% ---------- RULES ----------
% Rules are used when you want to say that a fact depends on a group of facts

% NOTE : You'll get the discontiguous predicate warning if you
% don't keep your predicates together

happy(albert).
happy(alice).
happy(bob).
happy(bill).
with_albert(alice).

% We can define the Fact that when Bob is happy he runs
% :- stands for if
runs(albert) :- happy(albert).
% runs(albert). = yes

% We can check if 2 conditions are true by putting a comma (and)
% between questions (CONJUCTIONS)
dances(alice) :-
  happy(alice),
  with_albert(alice).

% We can define predicates to keep commands brief
does_alice_dance :- dances(alice),
       write('When Alice is happy and with Albert she dances').
% Just type does_alice_dance. in the terminal

% Both rules must be true to get a yes result
swims(bob) :-
  happy(bob),
  near_water(bob).
% swims(bob). = no

% We can create 2 instances and if either comes back true the result
% will be yes
swims(bill) :-
  happy(bill).

swims(bill) :-
  near_water(bill).
% swims(bill). = yes

% ---------- VARIABLES ----------
% A variable is an object we are unable to name when writing a program.
% An instantiated variable is one that stands for an object.
% A variable begins with an uppercase letter or _ and can contain
% the same symbols as atoms.
% The same variable name used in 2 different questions represents 2
% completely different variables.

% An uninstantiated variable can be used to search for any match.

% Return all females (Type ; to cycle through them)
% female(X). X = alice X = betsy X = diana

parent(albert, bob).
parent(albert, betsy).
parent(albert, bill).

parent(alice, bob).
parent(alice, betsy).
parent(alice, bill).

parent(bob, carl).
parent(bob, charlie).

% When you are cycling through the results the no at the end signals
% that there are no more results
% parent(X, bob). X = albert, X = alice

% parent(X, bob), dances(X). X = alice

% Who is Bobs parent? Does he have parents?
% parent(Y, carl), parent(X, Y). = X = albert, Y = bob, X = alice
% Y = bob

% Find Alberts grandchildren
% Is Albert a father? Does his children have any children?
% parent(albert, X), parent(X, Y). = X = bob, Y = carl, X = bob,
% Y = charlie

% Use custom predicate for multiple results
get_grandchild :- parent(albert, X), parent(X, Y),
              write('Alberts grandchild is '),
              write(Y), nl.

% Do Carl and Charlie share a parent
% Who is Carls parent? Is this same X a parent of Charlie
% parent(X, carl), parent(X, charlie). = X = bob

% Use format to get the results
% ~w represents where to put each value in the list at the end
% ~n is a newline
% ~s is used to input strings
get_grandparent :- parent(X, carl),
                parent(X, charlie),
                format('~w ~s grandparent~n', [X, "is the"]).

% Does Carl have an Uncle?
% Who is Carls parent? Who is Carls fathers brother?
brother(bob, bill).
% parent(X, carl), brother(X, Y). = X = bob, Y = bill

% Demonstrate axioms and derived facts
% We can also use variables in the database
% If you get the singleton warning, that means you defined a variable
% that you didn't do anything with. (This is ok sometimes)
grand_parent(X, Y) :-
  parent(Z, X),
  parent(Y, Z).
% grand_parent(carl, A). = A = albert, A = alice

% X blushes if X is human
blushes(X) :- human(X).
human(derek).

% If we say one thing is true when somehing else is true, we can also
% find that match if we only assign one thing to be true here.
% blushes(derek). = yes

% Another example on cause and effect
stabs(tybalt,mercutio,sword).
hates(romeo, X) :- stabs(X, mercutio, sword).
% hates(romeo, X). = X = tybalt

% We can use _ (anonymous variable) if we won't use the variable
% more than once
% The value of an anonymous var is not output
% Check if any males exist in the database : male(_). = yes

% ---------- WHERE IS IF? ----------
% You can use a type of case statement instead

what_grade(5) :-
  write('Go to kindergarten').
what_grade(6) :-
  write('Go to first grade').
what_grade(Other) :-
  Grade is Other - 5,
  format('Go to grade ~w', [Grade]).

% ---------- COMPLEX TERMS / STRUCTURES ----------
% A Structure is an object made up from many other objects (components)
% Structures allow us to add context about what an object is to avoid
% confusion. has(albert,olive) Does Albert have a pet named Olive?
% Does Albert have the food named Olive?

% Structures have a functor followed by a list of arguments
% The number of arguments a Structure has is its arity
% female(alice). has an arity of one

% Albert owns a pet cat named Olive
% This is a recursive definition

owns(albert, pet(cat, olive)).

% owns(albert, pet(cat, X)). : X = olive

customer(tom, smith, 20.55).
customer(sally, smith, 120.55).

% An anonymous variable is used when we don't want a value returned
% Is there a customer named sally and what is her balance
% customer(sally,_,Bal).

% tab puts the defined number of spaces on the screen
% ~2f says we want a float with 2 decimals
get_cust_bal(FName, LName) :- customer(FName, LName, Bal),
  write(FName), tab(1),
  format('~w owes us $~2f ~n', [LName, Bal]).

% Use a complex term to define what it means to be a vertical
% versus a horizontal line
vertical(line(point(X, Y), point(X, Y2))).
horizontal(line(point(X, Y), point(X2, Y))).

% vertical(line(point(5, 10), point(5, 20))). = yes
% horizontal(line(point(10, 20), point(30, 20))).

% We can also ask what the value of a point should be to be vertical
% vertical(line(point(5, 10), point(X, 20))). = X = 5

% We could also ask for the X and Y points
% vertical(line(point(5, 10), X)). = X = point(5,_)

% ---------- COMPARISON ----------
% alice = alice. = yes
% 'alice' = alice. = yes (Prolog considers these to be the same)
% \+ (alice = albert). = yes (How to check for not equal)

% 3 > 15. = no
% 3 >= 15. = no
% 3 =< 15. = yes

% W = alice. = yes
% This says that we can assign the value of alice to W and not that
% W is equal to alice

% Rand1 = Rand2. = yes
% This says that any variable can be assigned anything and one of
% those things is another variable

% If variables can be matched up between 2 complex terms and the
% functors are equal then the complex terms are equal
% rich(money, X) = rich(Y, no_debt).

% ---------- TRACE ----------
% Using trace we can see how Prolog evaluates queries one at a time

warm_blooded(penguin).
warm_blooded(human).

produce_milk(penguin).
produce_milk(human).

have_feathers(penguin).
have_hair(human).

mammal(X) :-
  warm_blooded(X),
  produce_milk(X),
  have_hair(X).


% trace.
% mammal(human).
%       1    1  Call: mammal(human) ?
%       2    2  Call: warm_blooded(human) ?
%       2    2  Exit: warm_blooded(human) ?
%       3    2  Call: produce_milk(human) ?
%       3    2  Exit: produce_milk(human) ?
%       4    2  Call: have_hair(human) ?
%       4    2  Exit: have_hair(human) ?
%       1    1  Exit: mammal(human) ?
% yes

% mammal(penguin).
%       1    1  Call: mammal(penguin) ?
%       2    2  Call: warm_blooded(penguin) ?
%       2    2  Exit: warm_blooded(penguin) ?
%       3    2  Call: produce_milk(penguin) ?
%       3    2  Exit: produce_milk(penguin) ?
%       4    2  Call: have_hair(penguin) ?
%       4    2  Fail: have_hair(penguin) ?
%       1    1  Fail: mammal(penguin) ?
% no
%
% notrace. Turns off trace

% Output what ever matches the clauses
% warm_blooded(X), produce_milk(X), write(X),nl.

% ---------- RECURSION ----------

/*
parent(albert, bob).
parent(albert, betsy).
parent(albert, bill).

parent(alice, bob).
parent(alice, betsy).
parent(alice, bill).

parent(bob, carl).
parent(bob, charlie).
*/

% Works for exact matches
related(X, Y) :- parent(X, Y).
% related(albert, bob). = true

% Cycles through possible results until related returns a true
related(X, Y) :-
  parent(X, Z),
  related(Z, Y).

% related(albert,carl). = true

% 1. parent(albert, Z). = true = Z = bob, betsy, bill
% 2. related(Z, carl). = true when Z = bob

% ---------- MATH ----------
% Prolog provides 'is' to evaluate mathematical expressions
% X is 2 + 2. = X = 4

% You can use parenthese
% X is 3 + (2 * 10). =  X = 23

% You can also make comparisons
% 50 > 30. = yes
% (3*10) >= (50/2). = yes
% \+ (3 = 10). = yes (How to check for not equal)
% 5+4 =:= 4+5. = yes (Check for equality between expressions)
% 5+4 =\= 4+5. = yes (Check for non-equality between expressions)
% 5 > 10 ; 10 < 100. (Checks if 1 OR the other is true)

% X is mod(7,2). = X = 1 (Modulus)

double_digit(X,Y) :- Y is X*2.
% double_digit(4,Y). = Y = 8
% Take the 1st argument, multiply it times 2 and return it as the
% 2nd argument

% Get random value between 0 and 10
% random(0,10,X).

% Get all values between 0 and 10
% between(0,10,X).

% Add 1 and assign it to X
% succ(2,X).

% Get absolute value of -8
% X is abs(-8).

% Get largest value
% X is max(10,5).

% Get smallest value
% X is min(10,5).

% Round a value
% X is round(10.56).

% Convert float to integer
% X is truncate(10.56).

% Round down
% X is floor(10.56).

% Round up
% X is ceiling(10.56).

% 2^3
% X is 2** 3.

% Check if a number is even
% 10//2 = 5 (is 10 = 2 * 5)
is_even(X) :- Y is X//2, X =:= 2 * Y.

% sqrt, sin, cos, tan, asin, acos, atan, atan2, sinh, cosh, tanh,
% asinh, acosh, atanh, log, log10, exp, pi, e

% ---------- INPUT / OUTPUT ----------
% write('You saw me'), nl.

% writeq('I show quotes'), nl.

% You can read data with read
say_hi :-
  write('What is your name? '),
  read(X),
  write('Hi '),
  write(X).

% say_hi.
% What is your name 'Derek'.
% Hi Derek

fav_char :-
  write('What is your favorite character? '),

  % Receives a char and saves its ascii value to X
  get(X),
  format('The Ascii value ~w is ', [X]),

  % Outputs Ascii value as the char
  put(X),nl.

% Write to a file by defining the file, text to write, connection
% to the file (Stream)
write_to_file(File, Text) :-
  open(File, write, Stream),
  write(Stream, Text), nl,
  close(Stream).

% Read from a file
read_file(File) :-
        open(File, read, Stream),

        % Get char from the stream
        get_char(Stream, Char1),

        % Outputs the characters until end_of_file
        process_stream(Char1, Stream),
        close(Stream).

% Continue getting characters until end_of_file
% ! or cut is used to end backtracking or this execution
process_stream(end_of_file, _) :- !.

process_stream(Char, Stream) :-
        write(Char),
        get_char(Stream, Char2),
        process_stream(Char2, Stream).

% ---------- HOW TO LOOP ----------

% Use recursion to loop
count_to_10(10) :- write(10), nl.

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

% Receives Low (lowest value) and High (highest value)
count_down(Low, High) :-
  % Assigns values between Low and High to Y
  between(Low, High, Y),
  % Assigns the difference to Z
  Z is High - Y,
  write(Z),nl,
  % Continue looping until Y = 10
  Y = 10.

count_up(Low, High) :-
  between(Low, High, Y),
  Z is Y + Low,
  write(Z), nl,
  Y = 10.

% Loop until they guess a number
% start is a dummy value used to start the looping
guess_num :- loop(start).

% When they guess 15 they execute this message and exit
loop(15) :- write('You guessed it!').

loop(X) :-
  x \= 15,
  write('Guess Number '),
  read(Guess),
  write(Guess),
  write(' is not the number'), nl,
  loop(Guess).

% guess_num.
% Guess Number 12.
% 12 is not the number
% Guess Number 15.
% 15 is not the number
% You guessed it!

% ---------- CHANGING THE DATABASE ----------
% Any predicate you plan to motify should be marked as dynamic before
% this predicate is used in any way
:- dynamic(father/2).
:- dynamic(likes/2).
:- dynamic(friend/2).
:- dynamic(stabs/3).

father(lord_montague,romeo).
father(lord_capulet,juliet).

likes(mercutio,dancing).
likes(benvolio,dancing).
likes(romeo,dancing).
likes(romeo,juliet).
likes(juliet,romeo).
likes(juliet,dancing).

friend(romeo,mercutio).
friend(romeo,benvolio).
% friend(X, romeo) :- friend(romeo, X).

stabs(tybalt,mercutio,sword).
stabs(romeo,tybalt,sword).

% Add new clause to the database at the end of the list for the same
% predicate
% assertz(friend(benvolio, mercutio)).
% friend(benvolio, mercutio). = yes

% Add clause at the start of the predicate list
% asserta(friend(mercutio, benvolio)).
% friend(mercutio, benvolio). = yes

% Delete a clause
% retract(likes(mercutio,dancing)).
% likes(mercutio,dancing). = no

% Delete all clauses that match
% retractall(father(_,_)).
% father(lord_montague,romeo). = no

% Delete all matching clauses
% retractall(likes(_,dancing)).
% likes(_,dancing). = no

% ---------- LISTS ----------
% You can store atoms, complex terms, variables, numbers and other
% lists in a list
% They are used to store data that has an unknown number of elements

% We can add items to a list with the | (List Constructor)
% write([albert|[alice, bob]]), nl.

% Get the length of a list
% length([1,2,3], X).

% We can divide a list into its head and tail with |
% [H|T] = [a,b,c].

% H = a
% T = [b,c]

% We can get additional values by adding more variables to the left
% of |

%[X1, X2, X3, X4|T] = [a,b,c,d].

% We can use the anonymous variable _ when we need to reference a
% variable, but we don't want its value
% Let's get the second value in the list
% [_, X2, _, _|T] = [a,b,c,d].

% We can use | to access values of lists in lists
% [_, _, [X|Y], _, Z|T] = [a, b, [c, d, e], f, g, h].

% Find out if a value is in a list with member
% List1 = [a,b,c].
% member(a, List1). = yes

% We could also get all members of a list with a variable
% member(X, [a, b, c, d]).

% Reverse a list
% reverse([1,2,3,4,5], X).

% Concatenate 2 lists
% append([1,2,3], [4,5,6], X).

% Write items in list on separate line
write_list([]).

write_list([Head|Tail]) :-
  write(Head), nl,
  write_list(Tail).
% write_list([1,2,3,4,5]). = Outputs the list

% ---------- STRINGS ----------
% Convert a string into an Ascii character list
% name('A random string', X).

% Convert a Ascii character list into a string
% name(X, [65,32,114,97,110,100,111,109,32,115,116,114,105,110,103]).

% Append can join strings
join_str(Str1, Str2, Str3) :-

  % Convert strings into lists
  name(Str1, StrList1),
  name(Str2, StrList2),

  % Combine string lists into new string list
  append(StrList1, StrList2, StrList3),

  % Convert list into a string
  name(Str3, StrList3).

% join_str('Another ', 'Random String', X). = X = 'Another Random String'

% get the 1st char from a string
/*
name('Derek', List),
nth0(0, List, FChar),
put(FChar).
*/

% Get length of the string
atom_length('Derek',X).

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% Prolog programs are a collection of Facts, and Rules that we can

% Query.

% Prolog focuses on describing facts and relationships about problems

% rather then on creating a series of steps to solve that problem.

% These Facts and Rules are stored in a file called a Database

% or Knowledge Base

% You load a knowledge base like this [knowledge]. or this

% consult('knowledge.pl').

% halt. exits the prolog system

% listing. Displays the contents of the database

% All these commands are called predicates

% ---------- INTRODUCTION ----------

% write prints text between quotes to the screen

% nl stands for new line and \'s allows you to use quotes

% write('Hello World'),nl,write('Let\'s Program').

% This is a fact where loves is a predicate and romeo and

% juliet are atoms (constants) and loves arguments

loves(romeo, juliet).

% This is a rule where :- (if) says if the item on the right is

% true, then so is the item on the left

loves(juliet, romeo) :- loves(romeo, juliet).

% Evaluating whether the goal was met in the terminal

% loves(juliet, romeo). = yes

% Facts and Rules are called clauses

% A Variable is an object we can't name at the time of execution

% Variables are uppercase while atoms are lowercase

% loves(romeo, X). = X = juliet

% ---------- FACTS ----------

% Write the relationship first followed by the objects between

% parenthese followed by a dot

% albert, male, female are atom constants that must begin with a

% lowercase letter unless they are between single quotes

% An atom can contain letters, numbers, +, -, _, *, /, <, >, :, ., ~, &

% AN ATOM CANNOT START WITH _

% The name before parenthese is called the predicate

% The names in parenthese are called arguments

% Let's define information about the people above

male(albert).

male(bob).

male(bill).

male(carl).

male(charlie).

male(dan).

male(edward).

female(alice).

female(betsy).

female(diana).

% We can find out if alice is a woman with

% female(alice). = yes

% listing(male). = list all clauses defining the predicate male

% male(X), female(Y). = Show all combinations of male and female

% ---------- RULES ----------

% Rules are used when you want to say that a fact depends on a group of facts

% NOTE : You'll get the discontiguous predicate warning if you

% don't keep your predicates together

happy(albert).

happy(alice).

happy(bob).

happy(bill).

with_albert(alice).

% We can define the Fact that when Bob is happy he runs

% :- stands for if

runs(albert) :- happy(albert).

% runs(albert). = yes

% We can check if 2 conditions are true by putting a comma (and)

% between questions (CONJUCTIONS)

dances(alice) :-

happy(alice),

with_albert(alice).

% We can define predicates to keep commands brief

does_alice_dance :- dances(alice),

write('When Alice is happy and with Albert she dances').

% Just type does_alice_dance. in the terminal

% Both rules must be true to get a yes result

swims(bob) :-

happy(bob),

near_water(bob).

% swims(bob). = no

% We can create 2 instances and if either comes back true the result

% will be yes

swims(bill) :-

happy(bill).

swims(bill) :-

near_water(bill).

% swims(bill). = yes

% ---------- VARIABLES ----------

% A variable is an object we are unable to name when writing a program.

% An instantiated variable is one that stands for an object.

% A variable begins with an uppercase letter or _ and can contain

% the same symbols as atoms.

% The same variable name used in 2 different questions represents 2

% completely different variables.

% An uninstantiated variable can be used to search for any match.

% Return all females (Type ; to cycle through them)

% female(X). X = alice X = betsy X = diana

parent(albert, bob).

parent(albert, betsy).

parent(albert, bill).

parent(alice, bob).

parent(alice, betsy).

parent(alice, bill).

parent(bob, carl).

parent(bob, charlie).

% When you are cycling through the results the no at the end signals

% that there are no more results

% parent(X, bob). X = albert, X = alice

% parent(X, bob), dances(X). X = alice

% Who is Bobs parent? Does he have parents?

% parent(Y, carl), parent(X, Y). = X = albert, Y = bob, X = alice

% Y = bob

% Find Alberts grandchildren

% Is Albert a father? Does his children have any children?

% parent(albert, X), parent(X, Y). = X = bob, Y = carl, X = bob,

% Y = charlie

% Use custom predicate for multiple results

get_grandchild :- parent(albert, X), parent(X, Y),

write('Alberts grandchild is '),

write(Y), nl.

% Do Carl and Charlie share a parent

% Who is Carls parent? Is this same X a parent of Charlie

% parent(X, carl), parent(X, charlie). = X = bob

% Use format to get the results

% ~w represents where to put each value in the list at the end

% ~n is a newline

% ~s is used to input strings

get_grandparent :- parent(X, carl),

parent(X, charlie),

format('~w ~s grandparent~n', [X, "is the"]).

% Does Carl have an Uncle?

% Who is Carls parent? Who is Carls fathers brother?

brother(bob, bill).

% parent(X, carl), brother(X, Y). = X = bob, Y = bill

% Demonstrate axioms and derived facts

% We can also use variables in the database

% If you get the singleton warning, that means you defined a variable

% that you didn't do anything with. (This is ok sometimes)

grand_parent(X, Y) :-

parent(Z, X),

parent(Y, Z).

% grand_parent(carl, A). = A = albert, A = alice

% X blushes if X is human

blushes(X) :- human(X).

human(derek).

% If we say one thing is true when somehing else is true, we can also

% find that match if we only assign one thing to be true here.

% blushes(derek). = yes

% Another example on cause and effect

stabs(tybalt,mercutio,sword).

hates(romeo, X) :- stabs(X, mercutio, sword).

% hates(romeo, X). = X = tybalt

% We can use _ (anonymous variable) if we won't use the variable

% more than once

% The value of an anonymous var is not output

% Check if any males exist in the database : male(_). = yes

% ---------- WHERE IS IF? ----------

% You can use a type of case statement instead

what_grade(5) :-

write('Go to kindergarten').

what_grade(6) :-

write('Go to first grade').

what_grade(Other) :-

Grade is Other - 5,

format('Go to grade ~w', [Grade]).

% ---------- COMPLEX TERMS / STRUCTURES ----------

% A Structure is an object made up from many other objects (components)

% Structures allow us to add context about what an object is to avoid

% confusion. has(albert,olive) Does Albert have a pet named Olive?

% Does Albert have the food named Olive?

% Structures have a functor followed by a list of arguments

% The number of arguments a Structure has is its arity

% female(alice). has an arity of one

% Albert owns a pet cat named Olive

% This is a recursive definition

owns(albert, pet(cat, olive)).

% owns(albert, pet(cat, X)). : X = olive

customer(tom, smith, 20.55).

customer(sally, smith, 120.55).

% An anonymous variable is used when we don't want a value returned

% Is there a customer named sally and what is her balance

% customer(sally,_,Bal).

% tab puts the defined number of spaces on the screen

% ~2f says we want a float with 2 decimals

get_cust_bal(FName, LName) :- customer(FName, LName, Bal),

write(FName), tab(1),

format('~w owes us $~2f ~n', [LName, Bal]).

% Use a complex term to define what it means to be a vertical

% versus a horizontal line

vertical(line(point(X, Y), point(X, Y2))).

horizontal(line(point(X, Y), point(X2, Y))).

% vertical(line(point(5, 10), point(5, 20))). = yes

% horizontal(line(point(10, 20), point(30, 20))).

% We can also ask what the value of a point should be to be vertical

% vertical(line(point(5, 10), point(X, 20))). = X = 5

% We could also ask for the X and Y points

% vertical(line(point(5, 10), X)). = X = point(5,_)

% ---------- COMPARISON ----------

% alice = alice. = yes

% 'alice' = alice. = yes (Prolog considers these to be the same)

% \+ (alice = albert). = yes (How to check for not equal)

% 3 > 15. = no

% 3 >= 15. = no

% 3 =< 15. = yes

% W = alice. = yes

% This says that we can assign the value of alice to W and not that

% W is equal to alice

% Rand1 = Rand2. = yes

% This says that any variable can be assigned anything and one of

% those things is another variable

% If variables can be matched up between 2 complex terms and the

% functors are equal then the complex terms are equal

% rich(money, X) = rich(Y, no_debt).

% ---------- TRACE ----------

% Using trace we can see how Prolog evaluates queries one at a time

warm_blooded(penguin).

warm_blooded(human).

produce_milk(penguin).

produce_milk(human).

have_feathers(penguin).

have_hair(human).

mammal(X) :-

warm_blooded(X),

produce_milk(X),

have_hair(X).

% trace.

% mammal(human).

% 1 1 Call: mammal(human) ?

% 2 2 Call: warm_blooded(human) ?

% 2 2 Exit: warm_blooded(human) ?

% 3 2 Call: produce_milk(human) ?

% 3 2 Exit: produce_milk(human) ?

% 4 2 Call: have_hair(human) ?

% 4 2 Exit: have_hair(human) ?

% 1 1 Exit: mammal(human) ?

% yes

% mammal(penguin).

% 1 1 Call: mammal(penguin) ?

% 2 2 Call: warm_blooded(penguin) ?

% 2 2 Exit: warm_blooded(penguin) ?

% 3 2 Call: produce_milk(penguin) ?

% 3 2 Exit: produce_milk(penguin) ?

% 4 2 Call: have_hair(penguin) ?

% 4 2 Fail: have_hair(penguin) ?

% 1 1 Fail: mammal(penguin) ?

% no

% notrace. Turns off trace

% Output what ever matches the clauses

% warm_blooded(X), produce_milk(X), write(X),nl.

% ---------- RECURSION ----------

parent(albert, bob).

parent(albert, betsy).

parent(albert, bill).

parent(alice, bob).

parent(alice, betsy).

parent(alice, bill).

parent(bob, carl).

parent(bob, charlie).

% Works for exact matches

related(X, Y) :- parent(X, Y).

% related(albert, bob). = true

% Cycles through possible results until related returns a true

related(X, Y) :-

parent(X, Z),

related(Z, Y).

% related(albert,carl). = true

% 1. parent(albert, Z). = true = Z = bob, betsy, bill

% 2. related(Z, carl). = true when Z = bob

% ---------- MATH ----------

% Prolog provides 'is' to evaluate mathematical expressions

% X is 2 + 2. = X = 4

% You can use parenthese

% X is 3 + (2 * 10). = X = 23

% You can also make comparisons

% 50 > 30. = yes

% (3*10) >= (50/2). = yes

% \+ (3 = 10). = yes (How to check for not equal)

% 5+4 =:= 4+5. = yes (Check for equality between expressions)

% 5+4 =\= 4+5. = yes (Check for non-equality between expressions)

% 5 > 10 ; 10 < 100. (Checks if 1 OR the other is true)

% X is mod(7,2). = X = 1 (Modulus)

double_digit(X,Y) :- Y is X*2.

% double_digit(4,Y). = Y = 8

% Take the 1st argument, multiply it times 2 and return it as the

% 2nd argument

% Get random value between 0 and 10

% random(0,10,X).

% Get all values between 0 and 10

% between(0,10,X).

% Add 1 and assign it to X

% succ(2,X).

% Get absolute value of -8

% X is abs(-8).

% Get largest value

% X is max(10,5).

% Get smallest value

% X is min(10,5).

% Round a value

% X is round(10.56).

% Convert float to integer

% X is truncate(10.56).

% Round down

% X is floor(10.56).

% Round up

% X is ceiling(10.56).

% 2^3

% X is 2** 3.

% Check if a number is even

% 10//2 = 5 (is 10 = 2 * 5)

is_even(X) :- Y is X//2, X =:= 2 * Y.

% sqrt, sin, cos, tan, asin, acos, atan, atan2, sinh, cosh, tanh,

% asinh, acosh, atanh, log, log10, exp, pi, e

% ---------- INPUT / OUTPUT ----------

% write('You saw me'), nl.

% writeq('I show quotes'), nl.

% You can read data with read

say_hi :-

write('What is your name? '),

read(X),

write('Hi '),

write(X).

% say_hi.

% What is your name 'Derek'.

% Hi Derek

fav_char :-

write('What is your favorite character? '),

% Receives a char and saves its ascii value to X

get(X),

format('The Ascii value ~w is ', [X]),

% Outputs Ascii value as the char

put(X),nl.

% Write to a file by defining the file, text to write, connection

% to the file (Stream)

write_to_file(File, Text) :-

open(File, write, Stream),

write(Stream, Text), nl,

close(Stream).

% Read from a file

read_file(File) :-

open(File, read, Stream),

% Get char from the stream

get_char(Stream, Char1),

% Outputs the characters until end_of_file

process_stream(Char1, Stream),

close(Stream).

% Continue getting characters until end_of_file

% ! or cut is used to end backtracking or this execution

process_stream(end_of_file, _) :- !.

process_stream(Char, Stream) :-

write(Char),

get_char(Stream, Char2),

process_stream(Char2, Stream).

% ---------- HOW TO LOOP ----------

% Use recursion to loop

count_to_10(10) :- write(10), nl.

count_to_10(X) :-

write(X),nl,

Y is X + 1,

count_to_10(Y).

% Receives Low (lowest value) and High (highest value)

count_down(Low, High) :-

% Assigns values between Low and High to Y

between(Low, High, Y),

% Assigns the difference to Z

Z is High - Y,

write(Z),nl,

% Continue looping until Y = 10

Y = 10.

count_up(Low, High) :-

between(Low, High, Y),

Z is Y + Low,

write(Z), nl,

Y = 10.

% Loop until they guess a number

% start is a dummy value used to start the looping

guess_num :- loop(start).

% When they guess 15 they execute this message and exit

loop(15) :- write('You guessed it!').

loop(X) :-

x \= 15,

write('Guess Number '),

read(Guess),

write(Guess),

write(' is not the number'), nl,

loop(Guess).

% guess_num.

% Guess Number 12.

% 12 is not the number

% Guess Number 15.

% 15 is not the number

% You guessed it!

% ---------- CHANGING THE DATABASE ----------

% Any predicate you plan to motify should be marked as dynamic before

% this predicate is used in any way

:- dynamic(father/2).

:- dynamic(likes/2).

:- dynamic(friend/2).

:- dynamic(stabs/3).

father(lord_montague,romeo).

father(lord_capulet,juliet).

likes(mercutio,dancing).

likes(benvolio,dancing).

likes(romeo,dancing).

likes(romeo,juliet).

likes(juliet,romeo).

likes(juliet,dancing).

friend(romeo,mercutio).

friend(romeo,benvolio).

% friend(X, romeo) :- friend(romeo, X).

stabs(tybalt,mercutio,sword).

stabs(romeo,tybalt,sword).

% Add new clause to the database at the end of the list for the same

% predicate

% assertz(friend(benvolio, mercutio)).

% friend(benvolio, mercutio). = yes

% Add clause at the start of the predicate list

% asserta(friend(mercutio, benvolio)).

% friend(mercutio, benvolio). = yes

% Delete a clause

% retract(likes(mercutio,dancing)).

% likes(mercutio,dancing). = no

% Delete all clauses that match

% retractall(father(_,_)).

% father(lord_montague,romeo). = no

% Delete all matching clauses

% retractall(likes(_,dancing)).

% likes(_,dancing). = no

% ---------- LISTS ----------

% You can store atoms, complex terms, variables, numbers and other

% lists in a list

% They are used to store data that has an unknown number of elements

% We can add items to a list with the | (List Constructor)

% write([albert|[alice, bob]]), nl.

% Get the length of a list

% length([1,2,3], X).

% We can divide a list into its head and tail with |

% [H|T] = [a,b,c].

% H = a

% T = [b,c]

% We can get additional values by adding more variables to the left

% of |

%[X1, X2, X3, X4|T] = [a,b,c,d].

% We can use the anonymous variable _ when we need to reference a

% variable, but we don't want its value

% Let's get the second value in the list

% [_, X2, _, _|T] = [a,b,c,d].

% We can use | to access values of lists in lists

% [_, _, [X|Y], _, Z|T] = [a, b, [c, d, e], f, g, h].

% Find out if a value is in a list with member

% List1 = [a,b,c].

% member(a, List1). = yes

% We could also get all members of a list with a variable

% member(X, [a, b, c, d]).

% Reverse a list

% reverse([1,2,3,4,5], X).

% Concatenate 2 lists

% append([1,2,3], [4,5,6], X).

% Write items in list on separate line

write_list([]).

write_list([Head|Tail]) :-

write(Head), nl,

write_list(Tail).

% write_list([1,2,3,4,5]). = Outputs the list

% ---------- STRINGS ----------

% Convert a string into an Ascii character list

% name('A random string', X).

% Convert a Ascii character list into a string

% name(X, [65,32,114,97,110,100,111,109,32,115,116,114,105,110,103]).

% Append can join strings

join_str(Str1, Str2, Str3) :-

% Convert strings into lists

name(Str1, StrList1),

name(Str2, StrList2),

% Combine string lists into new string list

append(StrList1, StrList2, StrList3),

% Convert list into a string

name(Str3, StrList3).

% join_str('Another ', 'Random String', X). = X = 'Another Random String'

% get the 1st char from a string

name('Derek', List),

nth0(0, List, FChar),

put(FChar).

% Get length of the string

atom_length('Derek',X).

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