Given the start position and length, I want to generate a sublist of that length. I did it below using an accumulator:
get_sublist(_,_,0,Sublist,Sublist):-!.
get_sublist(List,Position,Length,Sublist,Acc):-
nth1(Position,List,Element),
append([Element],Acc,Acc1),
Position1 is Position + 1,
Length1 is Length - 1,
get_sublist(List,Position1,Length1,Sublist,Acc1).
Is there a shorter/faster way to do this either by using more built in predicates or using an alternative method? Thanks.
I would go for:
sublist(List, Offset, Length, Sublist):-
length(Prefix, Offset),
append(Prefix, Rest, List),
length(Sublist, Length),
append(Sublist, _, Rest).
Example of use:
?- sublist([a,b,c,d,e,f,g,h,i,j,k], 3, 6, X).
X = [d, e, f, g, h, i].
Notice I'm using count-by-0 for the Offset argument here.
if I run your code, I get
?- get_sublist([a,b,c,d,e,f,g,h,i,j,k], 3, 6, X, []).
X = [h, g, f, e, d, c].
then, a reverse/2 is missing ? In case, I would propose the good old findall/3:
get_sublist(List,Position,Length,Sublist):-
Stop is Position+Length-1,
findall(X, (between(Position,Stop,P),nth1(P,List,X)), Sublist).
edit bagof/3, by means of declaration of scoped variables (just P here), will avoid problems introduced by findall:
get_sublist(List,Position,Length,Sublist):-
Stop is Position+Length-1,
bagof(X, P^(between(Position,Stop,P),nth1(P,List,X)), Sublist).
?- get_sublist([A,B,C,D,E,F,G,H,I,J,K], 3, 6, X).
X = [C, D, E, F, G, H].
Related
I have a list of functions
List = [segmentTime(red,a,c,2),segmentTime(green,c,e,3),segmentTime(green,e,h,4),segmentTime(blue,h,i,5)]
How do I find the sum of the integer part of the function of all elements in the list?
i.e
sum = 2+3+4+5
A snippet code of a predicate would be extremely useful.
Thanks in advance :)
You will be surprised how simple the answer is.
sumList([],0).
sumList([segmentTime(_,_,_,X)|T],Z):- sumList(T,Z1),Z is Z1+X.
OUTPUT
?-sumList([segmentTime(red,a,c,2),segmentTime(green,c,e,3),segmentTime(green,e,h,4),segmentTime(blue,h,i,5)],M).
M=14
Hope this helped you.
Another way is to use library(lambda).
:- use_module(library(lambda)).
getList(L):-
L = [segmentTime(red,a,c,2),segmentTime(green,c,e,3),segmentTime(green,e,h,4),segmentTime(blue,h,i,5)].
sumOfList(L, S) :-
foldl(\X^Y^Z^(X = segmentTime(_,_,_,V), Z is Y + V), L, 0, S).
Ouput :
?- getList(L), sumOfList(L, S).
L = [segmentTime(red, a, c, 2), segmentTime(green, c, e, 3), segmentTime(green, e, h, 4), segmentTime(blue, h, i, 5)],
S = 14.
I'm working on a Prolog program which should load every nth element of a list into another list. For example:
?- pred([a,b,c,d,e,f,g,h,i,j],3,R) =>
R = [c,f,i]
Where pred is the predicate I'm attempting to implement.
But I honestly don't know how to do it. I know that I need a counter, which represents the current position of my Head, so it's gonna be a /4 predicate summarized into a /3 one later one, like
nth(list,number,result) :- nth(list,number,result,counter) or similiar.
Though, I don't know how to give the head a position number that can reset itself. Because once it hits n (let's say n=3, which is the c in the list), it has to turn back to 1 logically and count again up to 3, give out the element, and so on.
How can I work around these specific problems in my implementation?
An example of how this could be implemented:
nth_pos(L, N, R):-
nth_pos(L, 1, N, [], R).
nth_pos([], I, N, Acc, Acc).
nth_pos([H|T], I, N, Acc, R):-
I =:= N,
append(Acc, [H], Acc2),
I2 is 1,
nth_pos(T, I2, N, Acc2, R).
nth_pos([H|T], I, N, Acc, R):-
I < N,
I2 is I + 1,
nth_pos(T, I2, N, Acc, R).
Test-run:
?- nth_pos([a,b,c,d,e,f,g,h,i,j],3,R).
R = [c, f, i] .
?- nth_pos([a,b,c,d,e,f,g,h,i,j],1,R).
R = [a, b, c, d, e, f, g, h, i|...]
But I honestly don't know how to do it. I know that I need a counter,
which represents the current position of my Head, so it's gonna be a
/4 predicate summarized into a /3 one later one, like
nth(list,number,result) :- nth(list,number,result,counter) or
similiar.
Yes you are on the right track, note that an accumulator is used as well to build up the list so we get pred/5. Hope it helps. Note that this is not the only way to solve it.
I'm currently working on a Prolog program and having a lot of trouble figuring out how to implement it.
I want to put a list such as [1,2,2,1] into a function. I want it to multiply into itself to make a new matrix by doing this example.
1 * [1,2,2,1] which would yield [1,2,2,1]
2 * [1,2,2,1] which would yield [2,4,4,2]
2 * [1,2,2,1] which would yield [2,4,4,2]
1 * [1,2,2,1] which would yield [1,2,2,1]
And I want it to create all those together in a matrix like:
[[1,2,2,1],[2,4,4,2],[2,4,4,2],[1,2,2,1]].
Last part would be I want to zero out when I multiply by itself. So the 2nd spot would zero out the second spot making the final matrix:
[[0,2,2,1],[2,0,4,2],[2,4,0,2],[1,2,2,0]].
I want to have a predicate that calls another which makes each list. So heres my thoughts:
main(A,O):-
second(A,A,O).
second([],_,[]).
second([A|As],B,[O|Os]):- %creates the list of lists.
third(A,B,O),
second(As,B,Os).
third(_,[],[]).
third(A,[B|Bs],[O|Os]):-
fourth(A,B,O),
third(A,Bs,Os). %multiplies single digit by list.
fourth(A,B,0):- A == B.
fourth(A,B,O):- O is A * B.
I am getting the correct matrix but can not get the zero diagonal.
I just cant figure out a correct way to get the matrix with zeros down the diagonal. Any thoughts?
You can do the zeroes by introducing indices that indicate row and column you are at and check for a match:
main(A, O) :-
second(A, A, 0, O).
second([], _, _, []).
second([A|As], B, R, [O|Os]) :- %creates the list of lists.
third(A, B, 0, R, O),
R1 is R + 1,
second(As, B, R1, Os).
third(_, [], _, _, []).
third(A, [B|Bs], C, R, [O|Os]) :-
fourth(A, B, C, R, O),
C1 is C + 1,
third(A, Bs, C1, R, Os). %multiplies single digit by list.
fourth(_, _, X, X, 0).
fourth(A, B, C, R, O) :- C \== R, O is A * B.
Check:
| ?- main([1,2,2,1], L).
L = [[0,2,2,1],[2,0,4,2],[2,4,0,2],[1,2,2,0]] ? ;
no
Another interesting approach would be to create a maplist_with_index predicate which works just like maplist but manages an index and implicitly assumes the given predicate accepts the index as its first argument:
maplist_with_index(Pred, L, M) :-
maplist_with_index_(Pred, 0, L, M).
maplist_with_index_(Pred, I, [H|T], [M|Ms]) :-
Pred =.. [P|Pt],
append([P,I|Pt], [H], NewPred),
Call =.. NewPred,
call(Call, M),
I1 is I + 1,
maplist_with_index_(Pred, I1, T, Ms).
maplist_with_index_(_, _, [], []).
Then, the matrix program, using this predicate, looks like:
main(A, O) :-
second(A, A, O).
second(M, A, O) :-
maplist_with_index(third(A), M, O).
third(R, A, E, O) :-
maplist_with_index(fourth(R, E), A, O).
fourth(X, X, _, _, 0).
fourth(C, R, A, B, O) :- C \== R, O is A * B.
I'm trying to simulate a product of a matrix with a vector using these two predicates:
eva([], [], []).
eva([A|A1], [W], [Res|R1]) :-
vectormultiplication(A, W, Res),
eva(A1, W, R1).
vectormultiplication([A], [W], [A*W]).
vectormultiplication([A|A1], [W|W1], [A*W|Out1]) :-
vectormultiplication(A1, W1, Out1).
Where the [A|A1] in eva is a matrix (or a list of lists), [W] is a vector (a list),and [Res|R1] is the resulting product. vectormultiplication is supposed to go multiply each list in the list with the vector W. However, this strategy just produces a false response. Is there anything apparent that I'm doing wrong here that prevents me from getting the desired product? I'm currently using SWI Prolog version 5.10
you have 2 other problems apart that evidenced by Daniel (+1): here a cleaned up source
eva([], _, []). % here [] was wrong
eva([A|A1], W, [Res|R1]) :- % here [W] was wrong
vectormultiplication(A, W, Res),
eva(A1, W, R1).
vectormultiplication([A], [W], [M]) :-
M is A*W.
vectormultiplication([A|A1], [W|W1], [M|Out1]) :-
M is A*W,
vectormultiplication(A1, W1, Out1).
test:
?- eva([[1,2],[3,5]],[5,6],R).
R = [[5, 12], [15, 30]]
when handling lists, it's worth to use maplist if available
eva(A, W, R) :-
maplist(vectormultiplication1(W), A, R).
vectormultiplication1(W, A, M) :-
maplist(mult, A, W, M).
mult(A, W, M) :-
M is A*W.
Note I changed the order of arguments of vectormultiplication1, because the vector is a 'constant' in that loop, and maplist append arguments to be 'unrolled'.
Well, your first problem is that you think A*W is going to do anything by itself. In Prolog, that's just going to create the expression A*W, no different from *(A,W) or foo(A, W)--without is/2 involved, no actual arithmetic reduction will take place. That's the only real problem I see in a quick glance.
Here's my problem (by example since that's quicker):
?- enum_list([alpha, beta, gamma, beta, beta, delta, epsilon, alpha], L).
L = [alpha1, beta1, gamma, beta2, beta3, delta, epsilon, alpha2].
The problem is simple when I am allowed to reorder the list (just sort the list, group same elements into lists, enum the lists if they are longer than 1). But I want to keep the order. Any ideas?
How about:
enum_list(L, E):-
enum_list(L, E, [], _).
enum_list([], [], B, B).
enum_list([X|Tail], [Y|NTail], B, NB):-
select(X-C, B, MB),
succ(C, C1),
atom_concat(X, C1, Y),
!,
enum_list(Tail, NTail, [X-C1|MB], NB).
enum_list([X|Tail], [Y|NTail], B, NB):-
enum_list(Tail, NTail, [X-1|B], NB),
(member(X-1, NB) -> Y=X ; atom_concat(X, 1, Y)).
It iterates through the list and keeps a set of count of occurrences of each item so it knows when and what to append for each item to get the name.