Learn Prolog - 10 Code Examples & CST Typing Practice Test

Prolog (Programming in Logic) is a high-level, declarative programming language focused on logic programming and symbolic reasoning. It is widely used in artificial intelligence, natural language processing, and rule-based systems, enabling developers to express knowledge and relationships rather than step-by-step instructions.

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Prolog Counter Simulation Prolog Simple Addition Prolog Factorial Prolog Fibonacci Sequence Prolog Max of Two Numbers Prolog List Sum Prolog Even Numbers Filter Prolog Conditional Counter Increment Prolog Resettable Counter Prolog Theme Toggle Only

Learn PROLOG with Real Code Examples

Updated Nov 20, 2025

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Code Sample Descriptions

Prolog Counter Simulation

:- dynamic(count/1).
:- dynamic(isDark/1).

count(0).
isDark(false).

updateUI :-
    count(C),
    isDark(D),
    write('Counter: '), write(C), nl,
    write('Theme: '), (D = true -> write('Dark') ; write('Light')), nl.

increment :-
    count(C),
    C1 is C + 1,
    retract(count(C)),
    assert(count(C1)),
    updateUI.

decrement :-
    count(C),
    C1 is C - 1,
    retract(count(C)),
    assert(count(C1)),
    updateUI.

toggleTheme :-
    isDark(D),
    D1 is (D = false -> true ; false),
    retract(isDark(D)),
    assert(isDark(D1)),
    updateUI.

reset :-
    retract(count(_)),
    assert(count(0)),
    updateUI.

% Simulate actions
updateUI,
increment,
increment,
toggleTheme,
decrement,
reset.

Demonstrates a simple counter simulation with theme toggle using Prolog facts and rules.

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Prolog Simple Addition

add(A,B,Sum) :- Sum is A + B.

% Example
?- add(10,20,S), write(S), nl.

Adds two numbers and prints the result.

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Prolog Factorial

factorial(0,1).
factorial(N,F) :- N > 0, N1 is N-1, factorial(N1,F1), F is N * F1.

% Example
?- factorial(5,F), write(F), nl.

Calculates factorial using recursion.

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Prolog Fibonacci Sequence

fib(0,0).
fib(1,1).
fib(N,F) :- N > 1, N1 is N-1, N2 is N-2, fib(N1,F1), fib(N2,F2), F is F1 + F2.

% Example
?- fib(10,F), write(F), nl.

Generates Fibonacci numbers recursively.

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Prolog Max of Two Numbers

max(A,B,A) :- A >= B.
max(A,B,B) :- B > A.

% Example
?- max(10,20,M), write(M), nl.

Finds the maximum of two numbers.

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Prolog List Sum

sum_list([],0).
sum_list([H|T],Sum) :- sum_list(T,SumT), Sum is H + SumT.

% Example
?- sum_list([1,2,3,4,5],S), write(S), nl.

Sums elements of a list recursively.

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Prolog Even Numbers Filter

even(X) :- 0 is X mod 2.
print_even([]).
print_even([H|T]) :- even(H), write(H), nl, print_even(T).
print_even([H|T]) :- \+ even(H), print_even(T).

% Example
?- print_even([1,2,3,4,5]).

Prints even numbers from a list.

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Prolog Conditional Counter Increment

:- dynamic(count/1).
count(3).
increment_if_less_than_5 :- count(C), C < 5 -> C1 is C + 1, retract(count(C)), assert(count(C1)); true.

% Example
?- increment_if_less_than_5, count(C), write(C), nl.

Increments counter only if less than 5.

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Prolog Resettable Counter

:- dynamic(count/1).
count(0).
increment :- count(C), C1 is C + 1, retract(count(C)), assert(count(C1)).
reset :- retract(count(_)), assert(count(0)).

% Example
?- increment, increment, count(C), write(C), nl, reset, count(C2), write(C2), nl.

Counter that can increment and reset.

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Prolog Theme Toggle Only

:- dynamic(isDark/1).
isDark(false).
toggle_theme :- isDark(D), D1 is (D = false -> true ; false), retract(isDark(D)), assert(isDark(D1)).

% Example
?- write('Initial Theme: '), isDark(D), write(D), nl, toggle_theme, isDark(D1), write('Toggled Theme: '), write(D1), nl.

Toggles theme multiple times.

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Frequently Asked Questions about Prolog

What is Prolog?

What are the primary use cases for Prolog?

Expert systems and rule-based AI. Natural language processing. Automated theorem proving. Knowledge representation and reasoning. Constraint logic programming

What are the strengths of Prolog?

Concise expression of complex logic. Ideal for symbolic reasoning. Automatic search and inference. Supports rapid prototyping of AI systems. Good for teaching logic programming concepts

What are the limitations of Prolog?

Not ideal for numerical computation or low-level tasks. Performance can degrade on large datasets. Less mainstream than procedural or object-oriented languages. Debugging can be challenging due to implicit control flow. Limited standard library for modern applications

How can I practice Prolog typing speed?

CodeSpeedTest offers 10+ real Prolog code examples for typing practice. You can measure your WPM, track accuracy, and improve your coding speed with guided exercises.

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Learn Prolog - 10 Code Examples & CST Typing Practice Test

Code Sample Descriptions

Prolog Counter Simulation

:- dynamic(count/1).
:- dynamic(isDark/1).

count(0).
isDark(false).

updateUI :-
    count(C),
    isDark(D),
    write('Counter: '), write(C), nl,
    write('Theme: '), (D = true -> write('Dark') ; write('Light')), nl.

increment :-
    count(C),
    C1 is C + 1,
    retract(count(C)),
    assert(count(C1)),
    updateUI.

decrement :-
    count(C),
    C1 is C - 1,
    retract(count(C)),
    assert(count(C1)),
    updateUI.

toggleTheme :-
    isDark(D),
    D1 is (D = false -> true ; false),
    retract(isDark(D)),
    assert(isDark(D1)),
    updateUI.

reset :-
    retract(count(_)),
    assert(count(0)),
    updateUI.

% Simulate actions
updateUI,
increment,
increment,
toggleTheme,
decrement,
reset.

Demonstrates a simple counter simulation with theme toggle using Prolog facts and rules.

Let’s Try →

Prolog Simple Addition

add(A,B,Sum) :- Sum is A + B.

% Example
?- add(10,20,S), write(S), nl.

Adds two numbers and prints the result.

Let’s Try →

Prolog Factorial

factorial(0,1).
factorial(N,F) :- N > 0, N1 is N-1, factorial(N1,F1), F is N * F1.

% Example
?- factorial(5,F), write(F), nl.

Calculates factorial using recursion.

Let’s Try →

Prolog Fibonacci Sequence

fib(0,0).
fib(1,1).
fib(N,F) :- N > 1, N1 is N-1, N2 is N-2, fib(N1,F1), fib(N2,F2), F is F1 + F2.

% Example
?- fib(10,F), write(F), nl.

Generates Fibonacci numbers recursively.

Let’s Try →

Prolog Max of Two Numbers

max(A,B,A) :- A >= B.
max(A,B,B) :- B > A.

% Example
?- max(10,20,M), write(M), nl.

Finds the maximum of two numbers.

Let’s Try →

Prolog List Sum

sum_list([],0).
sum_list([H|T],Sum) :- sum_list(T,SumT), Sum is H + SumT.

% Example
?- sum_list([1,2,3,4,5],S), write(S), nl.

Sums elements of a list recursively.

Let’s Try →

Prolog Even Numbers Filter

even(X) :- 0 is X mod 2.
print_even([]).
print_even([H|T]) :- even(H), write(H), nl, print_even(T).
print_even([H|T]) :- \+ even(H), print_even(T).

% Example
?- print_even([1,2,3,4,5]).

Prints even numbers from a list.

Let’s Try →

Prolog Conditional Counter Increment

:- dynamic(count/1).
count(3).
increment_if_less_than_5 :- count(C), C < 5 -> C1 is C + 1, retract(count(C)), assert(count(C1)); true.

% Example
?- increment_if_less_than_5, count(C), write(C), nl.

Increments counter only if less than 5.

Let’s Try →

Prolog Resettable Counter

:- dynamic(count/1).
count(0).
increment :- count(C), C1 is C + 1, retract(count(C)), assert(count(C1)).
reset :- retract(count(_)), assert(count(0)).

% Example
?- increment, increment, count(C), write(C), nl, reset, count(C2), write(C2), nl.

Counter that can increment and reset.

Let’s Try →

Prolog Theme Toggle Only

:- dynamic(isDark/1).
isDark(false).
toggle_theme :- isDark(D), D1 is (D = false -> true ; false), retract(isDark(D)), assert(isDark(D1)).

% Example
?- write('Initial Theme: '), isDark(D), write(D), nl, toggle_theme, isDark(D1), write('Toggled Theme: '), write(D1), nl.

Toggles theme multiple times.

Let’s Try →

Frequently Asked Questions about Prolog

What is Prolog?

What are the primary use cases for Prolog?

Expert systems and rule-based AI. Natural language processing. Automated theorem proving. Knowledge representation and reasoning. Constraint logic programming

What are the strengths of Prolog?

Concise expression of complex logic. Ideal for symbolic reasoning. Automatic search and inference. Supports rapid prototyping of AI systems. Good for teaching logic programming concepts

What are the limitations of Prolog?

How can I practice Prolog typing speed?

CodeSpeedTest offers 10+ real Prolog code examples for typing practice. You can measure your WPM, track accuracy, and improve your coding speed with guided exercises.