Solving Logic Puzzles in Prolog: Puzzle 1 of 3

Recently I played around with Prolog again to solve 3 small logic puzzles. This post is about solving the first puzzle. This first puzzle consists of 2 individual puzzles that follow the exact same internal structure, so can be solved the exact same way of stating rules in Prolog (just with different content).

The small puzzle: problem and question

There are 4 students: Carrie, Erma, Ora und Tracy. Each has one scholarship and one major subject they study. The goal is to find out which student has which scholarship and studies which subject (with all scholarships and majors being different from each other) from the hints provided. The available scholarships are: 25000, 30000, 35000 and 40000 USD. The available majors are: Astronomy, English, Philosophy, Physics. The following hints are given to solve the problem:

1. The student who studies Astronomy gets a bigger scholarship than Ora.
2. Ora is either the one who studies English or the one who studies Philosophy.
3. Erna has a 10000 USB bigger scholarship than Carrie.
4. Tracy has a bigger scholarship than the student that studies English.

The small puzzle: the Prolog solution

One way of solving this puzzle in Prolog to ask for a solution (which in our example consists of the 4 students and their associated attributes) that fulfills the rules that we can derive from the 4 hints from above. This is actually pretty short and easy in Prolog syntax:

% ####################################################################
% Logic puzzle solver.
%
% Read definition to prolog:
%   ['puzzle.pl'].
%
% 2) Ask for solutions:
%   solve(Carrie,Erma,Ora,Tracy).
%
% Rainhard Findling
% 10/2015
% ####################################################################

all_members([H],L2) :- member(H,L2).
all_members([H|T],L2) :- member(H,L2), all_members(T, L2).

or([H]) :- H,!.
or([H|_]) :- H,!.
or([_|T]) :- or(T).

solve(Carrie,Erma,Ora,Tracy) :-
    % all students
    Carrie = [Carrie_scholarship, Carrie_major],
    Erma = [Erma_scholarship, Erma_major],
    Ora = [Ora_scholarship, Ora_major],
    Tracy = [Tracy_scholarship, Tracy_major],
    % grouping
    All = [Carrie,Erma,Ora,Tracy],
    % ensure all values exist once
    all_members([25, 30, 35, 40], [Carrie_scholarship, Erma_scholarship, Ora_scholarship, Tracy_scholarship]),
    all_members([astronomy, english, philosophy, physics], [Carrie_major, Erma_major, Ora_major, Tracy_major]),
    % clue 1
    member([C1_scholarship,astronomy], All),
    Ora = [Ora_scholarship,_],
    Ora_scholarship > C1_scholarship,
    % clue 2
    or([Ora_major = english, Ora_major = physics]),
    % clue 3
    member([C3_scholarship, physics], All),
    C3_scholarship - Carrie_scholarship =:= 5,
    % clue 4
    Erma_scholarship - Carrie_scholarship =:= 10,
    % clue 5
    member([C5_scholarship, english], All),
    Tracy_scholarship > C5_scholarship.

If we ask for the solution, the single possible solution is presented:

?- solve(Carrie,Erma,Ora,Tracy).
Carrie = [25, astronomy],
Erma = [35, english],
Ora = [30, physics],
Tracy = [40, philosophy] ;

The slightly bigger puzzle: problem and question

There are 5 car renting contracts. Each has a duration, a pickup location, a car brand and a customer associated to it. The goal is to find out for all contracts, what the customer, duration, car brand and pickup location is (again given the fact that each attribute is unique amongst the contracts). The pickup locations are: Brownfield, Durham, Iowa Falls, Los Altos and Redding. The car brands are: Dodge, Fiat, Hyundai, Jeep and Nissan. The contract duration are 2, 3, 4, 5 and 6 days. And the customer names are Freda, Opal, Penny, Sarah and Vicky. The following hints are given to solve the problem:

1. The contracts of Vicky, the one with pickup location in Los Altos, the one with pickup location in Durham and the one with the Fiat are all different contracts.
2. The contract with the Jeep is not in Iowa Falls.
3. The contract of Vicky and the one with the Nissan are picked up in either Los Altos or Redding.
4. Penny’s contract is not for 6 days.
5. The contract in Iowa Falls is for 5 days.
6. The contract with the Durham is 3 days longer than the contract of Opal.
7. Of the contract with Nissan and the 2 day contract, one is picked up in Redding and the other is Freda’s contract.
8. The contract with the Jeep is not for 6 days.
9. The contract of Opal is 1 day longer than the one with the Hyundai.

The slightly bigger puzzle: the Prolog solution

Again, we can define a solution to fulfill the stated rules:

% ####################################################################
% Logic puzzle solver.
%
% Read definition to prolog:
%   ['puzzle.pl'].
%
% 2) Ask for solutions:
%   solve(Freda, Opal, Penny, Sarah, Vicky).
%
% Rainhard Findling
% 10/2015
% ####################################################################

all_members([H],L2) :- member(H,L2).
all_members([H|T],L2) :- member(H,L2), all_members(T, L2).

all_not([H]) :- not(H).
all_not([H|T]) :- not(H), all_not(T).

all_not_members([H],L2) :- not(member(H,L2)).
all_not_members([H|T],L2) :- not(member(H,L2)), all_not_members(T, L2).

and([H]) :- H.
and([H|T]) :- H, and(T).
or([H]) :- H,!.
or([H|_]) :- H,!.
or([_|T]) :- or(T).

solve(Freda, Opal, Penny, Sarah, Vicky) :-
    % all runners
    Freda = [Freda_car, Freda_location, Freda_days],
    Opal = [Opal_car, Opal_location, Opal_days],
    Penny = [Penny_car, Penny_location, Penny_days],
    Sarah = [Sarah_car, Sarah_location, Sarah_days],
    Vicky = [Vicky_car, Vicky_location, Vicky_days],
    % grouping
    All = [Freda, Opal, Penny, Sarah, Vicky],
    % ensure all values exist once
    all_members([dodge, fiat, hyundai, jeep, nissan], [Freda_car, Opal_car, Penny_car, Sarah_car, Vicky_car]),
    all_members([brownfield, durham, iowafalls, losaltos, redding], [Freda_location, Opal_location, Penny_location, Sarah_location, Vicky_location]),
    all_members([2,3,4,5,6], [Freda_days, Opal_days, Penny_days, Sarah_days, Vicky_days]),
    % clues from easy (fast) to hard (slow)
    % clue 5
    member([_,iowafalls,5], All),
    % clue 9
    member([hyundai,_,C9_days], All),
    Opal_days - C9_days =:= 1,
    % clue 6
    member([_,durham,C6_days], All),
    C6_days - Opal_days =:= 3,
    % clue 4
    not(Penny_days = 6),
    % clue 2
    member([C2_car,iowafalls,_], All),
    not(C2_car = jeep),
    % clue 8
    member([C8_car,_,6], All),
    not(C8_car = jeep),
    % clue 3
    member([nissan,C3_location,_], All),
    or([and([C3_location = losaltos, Vicky_location = redding]), and([C3_location = redding, Vicky_location = losaltos])]),
    % clue 7
    member([nissan, C7_1_location,_], All),
    member([_,C7_2_location, 2], All),
    or([and([Freda_location = C7_1_location, C7_2_location = redding]), and([Freda_location = C7_2_location, C7_1_location = redding])]),
    % clue 1
    member([C1_1_car,losaltos,_], All),
    member([C1_2_car,durham,_], All),
    member([fiat,C1_3_location,_], All),
    all_not_members([C1_1_car, C1_2_car, Vicky_car],[fiat]), %car
    all_not_members([C1_3_location, Vicky_location],[losaltos, durham]). %location

… and ask for it, which again presents the single solution:

?- solve(Freda, Opal, Penny, Sarah, Vicky).
Freda = [nissan, losaltos, 4],
Opal = [jeep, brownfield, 3],
Penny = [fiat, iowafalls, 5],
Sarah = [dodge, durham, 6],
Vicky = [hyundai, redding, 2] ;