问题
I need to swap two elements in list with specified indices using predicate:
swap(List, index1, index2).
I tried this way:
change_value([X|List], 0, Y, [Y|List2]) :-
copy_rest([X|List], List2).
change_value([], P, Y, []).
change_value([X|List], Pos, Y, [X|List2]) :-
X \== Y,
Pos > 0,
NewPos is Pos - 1,
change_value(List, NewPos, Y, List2).
copy_rest([], []).
copy_rest([X|List], [X|List2]) :-
copy_rest(List, List2).
Is there any better solution?
Thank you very much!
回答1:
No need to write recursive code!
Simply use the builtin predicates length/2, same_length/2, and append/3 like so:
list_i_j_swapped(As,I,J,Cs) :-
same_length(As,Cs),
append(BeforeI,[AtI|PastI],As),
append(BeforeI,[AtJ|PastI],Bs),
append(BeforeJ,[AtJ|PastJ],Bs),
append(BeforeJ,[AtI|PastJ],Cs),
length(BeforeI,I),
length(BeforeJ,J).
Done! Let's put it to use!
?- list_i_j_swapped([e0,e1,e2,e3,e4,e5],5,1,Ys). Ys = [e0,e5,e2,e3,e4,e1] ; false.
OK! Does it work in the "other direction", too?
?- list_i_j_swapped(Xs,5,1,[e0,e5,e2,e3,e4,e1]). Xs = [e0,e1,e2,e3,e4,e5] ; false.
Alright! What about the following quite general query?
?- list_i_j_swapped([A,B,C],I,J,Ys).
I = 0, J = 0, Ys = [A,B,C]
; I = 0, J = 1, Ys = [B,A,C]
; I = 0, J = 2, Ys = [C,B,A]
; I = 1, J = 0, Ys = [B,A,C]
; I = 1, J = 1, Ys = [A,B,C]
; I = 1, J = 2, Ys = [A,C,B]
; I = 2, J = 0, Ys = [C,B,A]
; I = 2, J = 1, Ys = [A,C,B]
; I = J, J = 2, Ys = [A,B,C]
; false.
It worked! At last, we run the most general query:
?- list_i_j_swapped(Xs,I,J,Ys).
I = 0, J = 0, Xs = [_A] , Ys = [_A]
; I = 0, J = 0, Xs = [_A,_B] , Ys = [_A,_B]
; I = 0, J = 1, Xs = [_A,_B] , Ys = [_B,_A]
; I = 1, J = 0, Xs = [_A,_B] , Ys = [_B,_A]
; I = 1, J = 1, Xs = [_A,_B] , Ys = [_A,_B]
; I = 0, J = 0, Xs = [_A,_B,_C], Ys = [_A,_B,_C]
...
Fair enumeration out-of-the-box? What's not to like?
来源：https://stackoverflow.com/questions/26834387/swap-two-elements-from-list-with-specified-indices