prolog - Getting all nodes that have atleast 2 nodes connected to them -


having issues prolog question:

the following clauses represent directed graph, nodes atoms, , edges denoted connected predicate. given following clauses in database, answer 2 questions below.

connected(a,b). connected(b,d). connected(b,e). connected(b,f). connected(a,c). connected(c,f). connected(c,g). connected(h,c). path(x,y) :- connected(x,y). path(x,z) :- connected(x,y), path(y,z).

show prolog query returns nodes having 2 or more different incoming edges (i.e., @ least 2 different nodes connected it). also, show result of entering query (asking every solution). query may return same node multiple times, , may printout values of other variables in query. variable denoting node in question should called dnode.

so far have:

path(dnode,__) , path(__,dnode).

but give me b , c
think letters more 1 nodes a, b, c, f.
tried a, b , c:

path(__,dnode),path(dnode,__) ; path(dnode,__) , path(dnode,__).

but got a, b, c , h.
assuming i'll have letters want:

path(__,dnode),path(dnode,__) ; path(dnode,__) , path(dnode,__) ; path(__,dnode) , path(__,dnode).

it gives me a, b, c, e, f, g , h though.

any advice how 4 letters want appreciated.

if display graph, perhaps graphviz

enter image description here

?- writeln('digraph g {'), forall(connected(a,b), writeln(a->b)), writeln('}').

you can see [c,f] have 2 incoming edges, , don't need path/2. it's sufficient join on second argument of connected/2, test first arguments different (operator (\=)/2).


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