94 lines
1.9 KiB
Prolog
94 lines
1.9 KiB
Prolog
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% iterate over list in sucessive order
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% prints each element
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%
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% example: iter_nth([1,2,3,4,5,6,7,8,9])
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% output: [1,2,3,4,5,6,7,8,9]
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iter([]).
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iter([H|T]):-writeln(H), iter(T).
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% iterate over list in reverse order
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% prints each element
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%
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% example: iter_nth([1,2,3,4,5,6,7,8,9])
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% output: [9,8,7,6,5,4,3,2,1]
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iter_rev([A]):-writeln(A).
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iter_rev([H|T]):-iter_rev(T), writeln(H).
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% prints every 2nd element from the supplied list
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iter_every_2nd([]).
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iter_every_2nd([_|[H2|T]]):-writeln(H2), iter_every_2nd(T).
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% iterate over every nth element in the list
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%
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% example: iter_nth([1,2,3,4,5,6,7,8,9], 3)
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% output: [3, 6, 9]
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iter_nth([], _, _).
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iter_nth([H|T], E, 0):-
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R is E - 1,
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iter_nth(T, E, R),
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writeln(H).
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iter_nth([_|T], E, L):-
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R is L - 1,
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iter_nth(T, E, R).
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iter_nth(L, N):-
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R is N - 1,
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iter_nth(L, N, R).
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% iterate over all elements with an even index
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iter_even(L):-iter_nth(L, 2).
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% iterate over all elements with an odd index
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iter_odd(L):-iter_nth(L, 2, 0).
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% counts the amount of items in the supplied list
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%
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% example: len([1,2,3,4,5,6], X)
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% output: X = 6
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len([], L):-L is 0.
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len([_|T], L):-
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len(T, Tmp),
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L is Tmp + 1.
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% count the amounts an element `E` is present in the supplied list
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%
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% example: count([a,b,c,a], a, X)
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% output: X = 2
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count([], _, L):-L is 0.
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count([H|T], E, L):-
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H == E,
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count(T, E, R),
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L is R + 1.
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count([_|T], E, L):-
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count(T, E, R),
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L is R.
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% return the nth element of the supplied list.
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% indexing starts with 0
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%
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% example: nth([a,b,c,d,e], 3, X)
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% output: X = d
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nth([],_).
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nth([H|_], 0, O):-O is H.
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nth([_|T], C, O):-
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C1 is C - 1,
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nth(T,C1, K),
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O is K.
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% return the index of the first occurance of the element
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% in a list
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% indexing starts with 0
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%
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% example: indexof([0,1,3,4,5,6,7,8,9], 3, X)
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% output: X = 2
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indexof([],_,C):-C is 0.
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indexof([H|_],E,C):-
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H == E,
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C is 0.
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indexof([_|T],E,C):-
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indexof(T,E,R),
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C is R + 1.
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