Please work in SWISH or locally, using swipl command.
A list is an ordered collection of items. An element can be any Prolog structure (term).
We denote the list:
[a,b,c] [2,4,6, ala, has, a, cat] []
Each list consists of:
The head and tail are separated by the operator | (vertical bar), e.g .:
? - [X | Y] = [a, b, c, d]. X = a Y = [b, c, d];
The decomposition and structuring of lists is mainly carried out by the unification mechanism. For example:
? - [X, Y | Z] = [a, b, c, d]. X = a Y = b Z = [c, d]; ? - [X, Y, a] = [Z, b, Z]. X = a Y = b Z = a;
Exercise
Please check the following unification:
? - X = [a, b, c]. ? - [X | Y] = [a, b, c]. ? - [[a, b], c] = [X, Y]. ? - [a(b), c(X)] = [Z, c(a)]. ? - [X | Y] = [a].
Selecting an Item:
third([A, B, C | Rest], C).
Define a predicate that compares 2 selected list items, e.g. 3 and 4.
Example of use:
? - compare([a, b, c, d]). false ? - compare([a, b, c, c]). true
Define a predicate that replaces 2 selected list items, e.g. 3 and 4.
? - replace ([a, b, c, d], X). X = [a, b, d, c] true
belongs(X, [X | _]). belongs(X, [_ | Ytail]) :- belongs(X, Ytail).
Exercise
Add the predicate belongs to your program.
Check and think about the operation of the following:
? - belongs(c, [a, b, c, d]). ? - belongs(x, [a, b, c, d]). ? - belongs(X, [a, b, c, d]). ? - belongs(x, a). ? - belongs(X, a).
list_length([], 0). list_length([_ | Tail], Length):- list_length(Tail, X), Length is X + 1.
Exercise
Add the predicate list_length to the file.
Check and think about the operation of the following:
? - list_length ([a, b, c], X).
(source: LearnPrologNow))
a2b([], []). a2b([a | Ta], [b | Tb]): - a2b(Ta, Tb).
Exercise
Add the predicate a2b to the file.
Check and think about the operation of the following:
? - a2b([a, a, a], [b, b, b]). ? - a2b([a, a, a, a], [b, b, b]). ? - a2b([a, s, d], [b, s, d]). ? - a2b([a, a, a, a], X). ? - a2b(X, [b, b]). ? - a2b(X, Y).
Note: this predicate does “something interesting” only for lists starting with 'a' and 'b'!
list_merge([], X, X). list_merge([X | L1], L2, [X | L3]): - list_merge(L1, L2, L3).
Exercise
Add the predicate list_merge to the file.
Check and think about the operation of the following:
? - list_merge([a, b], [c, d], X). ? - list_merge([a, b], X, [c, d]). ? - list_merge([a, b], X, [a, b, c, d]). ? - list_merge(A, B, [a, b, c, d, e]). ? - list_merge([1,2,3], [a, b, c], X). ? - list_merge([1,2,3], [a (e), b (f), c (d, g)], X). ? - list_merge(Before, [5 | After], [1,2,3,4,5,6,7,8,9]). ? - list_merge(_, [A, 5, B, _], [1,2,3,4,5,6,7,8,9]). ? - list_merge(A, [x, x, x | _], [a, b, x, x, c, x, x, x, d, e]).
Note: add and test the predicate:
belongs(X, L) :- list_merge(_, [X | _], L).
Define a predicate that checks if the Item is the last element of the List:
last(Item, List).
With and without the use of “list_merge”.
list_add(X, L, [X | L]).
In practice, an additional predicate would not be needed here.
Exercise
Add the predicate list_add to the file.
Check and think about the operation of the following:
? - list_add(a, [c, d], X). ? - list_add([a, b], [c, d], X).
list_remove(X, [X | Remainder], Remainder). list_remove(X, [Y | Tail], [Y | Rest]) :- list_remove(X, Tail, Rest).
Exercise
Add the predicate list_remove to the file.
Check and think about the operation of the following:
? - list_remove(a, [a, b, a, c, a, a], X). ? - list_remove(a, [a, b, c, d], X). ? - list_remove(c, [a, b, c, d], X). ? - list_remove(c, X, [a, b, c, d]). ? - list_remove(1, X, [a, b, c, d]).
Please find all solutions (;).
Note: add and test the predicate:
list_insert(X, L, Large) :- list_remove(X, Large, L).
Note: add and test the predicate:
belongs2 (X, L) :- list_remove(X, L, _).
list_contains(S, L) :- list_merge(_, L2, L), list_merge(S, _, L2).
Exercise
Add the predicate list_contains to the file.
Check and think about the operation of the following:
? - list_contains(a, [a, b, c]). ? - list_contains([a], [a, b, c]). ? - list_contains(X, [a, b, c]). ? - list_contains([X], [a, b, c]). ? - list_contains([X, Y], [a, b, c]). ? - list_contains([X, Y, Z], [a, b, c]).
permutation([],[]). permutation([X | L], P) :- permutation(L, L1), list_insert(X, L1, P). permutation2([], []). permutation2(L, [X | P]) :- list_remove(X, L, L1), permutation2(L1, P).
Exercise
Add the predicate permutation to the file.
Check and think about the operation of the following:
? - permutation([a, b, c], X). ? - permutation2([a, b, c], X).
list_reverse([], []). list_reverse([H | T], L): - list_reverse(T, R), list_merge(R, [H], L).
Exercise
Add the predicate list_reverse to the file.
Check and think about the operation of the following:
? - list_reverse([a, b, c, d], X). ? - list_reverse([a, b, c, d], [d, c, b, a]).
A string in Prolog may be represented by:
List representation increases the string processing possibilities.
Useful predicate:
write_string([H | T]) :- put(H), write_string(T). write_string([]).
Another situation: An example of using the built-in predicates name and append to transform strings.
Predicate plural (Noun, Pl) - converts an English regular noun from singular to plural.
plural(Noun, PL) :- name(Noun, L), name(s, T), append(L, T, NL), name(PL, NL).
Exercise
Check and think about the operation of the following:
? - write('ala'). ? - write('ala has a cat'). ? - write("ala"). ? - write("ala has a cat"). ? - X = 'a', put(X). ? - X = 'ala', put(X). ? - X = "a", put(X). ? - put(65), put(66), put(67).
Add the predicates write_string and plural to the file.
Check and think about the operation of the following:
? - write_string("ala has a cat"). ? - permutation ("abc", X), write_string(X), nl, fail. ? - list_insert("", "abcdefgh", Z), write_string(Z), nl, fail.
? - plural(day, PL).
