问题描述
这是事实
%first line
connected(new_elmarg,elmarg).
connected(elmarg,ezbet_elnakhl).
connected(ezbet_elnakhl,ain_shams).
connected(ain_shams,elmatareyya).
connected(elmatareyya,helmeyet_elzaitoun).
connected(helmeyet_elzaitoun,hadayeq_elzaitoun).
connected(hadayeq_elzaitoun,saray_elqobba).
connected(saray_elqobba,hammamat_elqobba).
connected(hammamat_elqobba,kobri_elqobba).
connected(kobri_elqobba,manshiet_elsadr).
connected(manshiet_elsadr,eldemerdash).
connected(eldemerdash,ghamra).
connected(ghamra,alshohadaa).
connected(alshohadaa,urabi).
connected(urabi,nasser).
connected(nasser,sadat).
connected(sadat,saad_zaghloul).
connected(saad_zaghloul,alsayyeda_zeinab).
connected(alsayyeda_zeinab,elmalek_elsaleh).
connected(elmalek_elsaleh,margirgis).
connected(margirgis,elzahraa).
connected(elzahraa,dar_elsalam).
connected(dar_elsalam,hadayeq_elmaadi).
connected(hadayeq_elmaadi,maadi).
connected(maadi,thakanat_elmaadi).
connected(thakanat_elmaadi,tora_elbalad).
connected(tora_elbalad,kozzika).
connected(kozzika,tora_elasmant).
connected(tora_elasmant,elmaasara).
connected(elmaasara,hadayeq_helwan).
connected(hadayeq_helwan,wadi_hof).
connected(wadi_hof,helwan_university).
connected(helwan_university,ain_helwan).
connected(ain_helwan,helwan).
%second line
connected(shobra_elkheima,koliet_elzeraa).
connected(koliet_elzeraa,mezallat).
connected(mezallat,khalafawy).
connected(khalafawy,sainte_teresa).
connected(sainte_teresa,road_elfarag).
connected(road_elfarag,massara).
connected(massara,ataba).
connected(ataba,naguib).
connected(naguib,opera).
connected(opera,dokki).
connected(dokki,bohooth).
connected(bohooth,cairo_university).
connected(cairo_university,faisal).
connected(faisal,giza).
connected(giza,omm_elmisryeen).
connected(omm_elmisryeen,sakiat_mekki).
connected(sakiat_mekki,elmounib).
我必须在不使用内置谓词(findall ...)的情况下计算直接连接到给定站的站数。
Input: are_connected(sadat,X).
Output: X = 4.
Input: are_connected(helwan,X).
Output: X = 1.
我的代码:
are_connected(A,0) :-
connected(_,A).
are_connected(A,0) :-
connected(A,_).
are_connected(A,S) :-
are_connected(A,News),!,S is News + 1.
注意:打印的“0”的数量应该是输出,但我不知道怎么做!
注意:在 connected(,) 中,遗漏的原子是下划线 (_)。
解决方法
如果不允许使用 findall/3
,则可以使用动态谓词来存储计数:
:- dynamic(myflag/2).
myflag(Name,Old,New) :-
( retract(myflag(Name,Old)),!
; true ),Val is New,assertz(myflag(Name,Val)).
示例:
?- myflag(counter,_,0),myflag(counter,A,A+1),B,B+1),C,C).
A = 0,B = 1,C = 2.
然后,使用谓词myflag/3
,您可以定义以下谓词:
are_connected(A,N) :-
myflag(counter,% initialize counter
( ( connected(_,A)
; connected(A,_)
),V,V+1),% increment counter
fail % backtracks to find another solution
; myflag(counter,N,N) % get final counter value
).
示例:
?- are_connected(sadat,X).
X = 4.
?- are_connected(helwan,X).
X = 1.
在 SWI-Prolog 中,您可以使用内置谓词 flag/3
,而不是 myflag/3
。
are_connected(A,N) :-
flag(counter,flag(counter,% increment counter
fail % backtracks to find another solution
; flag(counter,N) % get final counter value
).