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Predicate arg/3
Availability:built-in
[ISO]arg(?Arg, +Term, ?Value)
Term should be instantiated to a term, Arg to an integer between 1 and the arity of Term. Value is unified with the Arg-th argument of Term. Arg may also be unbound. In this case Value will be unified with the successive arguments of the term. On successful unification, Arg is unified with the argument number. Backtracking yields alternative solutions.117The instantiation pattern (-, +, ?) is an extension to‘standard’Prolog. Some systems provide genarg/3 that covers this pattern. The predicate arg/3 fails silently if Arg = 0 or Arg > arity and raises the exception domain_error(not_less_than_zero, Arg) if Arg < 0.
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LogicalCaptain said (2020-12-19T12:37:48):0 upvotes 0 0 downvotes
Picture of user LogicalCaptain.

Also raises type_error if Arg is not compound:

?-
arg(1,foo,X).

ERROR: Type error: `compound' expected, found `foo' (an atom)

An example for "backtracking yields alternative solutions"

?-
arg(Position,f(a,b,c),Term).

Position = 1, Term = a ;
Position = 2, Term = b ;
Position = 3, Term = c.

The operator to decide whether "it's the same" is unification, not "==":

?-
arg(Position,f(A,B,C),X).

Position = 1, A = X ;
Position = 2, B = X ;
Position = 3, C = X.

?-
arg(Position,f(A,B,C),B).

Position = 1, A = B ;
Position = 2 ;
Position = 3, B = C.

A bit abstrusely:

?-
arg(Position,f(a,K,h(K)),K).

Position = 1, K = a ;
Position = 2 ;
Position = 3, K = h(K).
?-
set_prolog_flag(occurs_check,true), arg(Position,f(a,K,h(K)),K).

Position = 1, K = a ;
Position = 2 ;
false.

The above also means you can search:

?-
arg(Position,f(a,b,c),b).

Position = 2 ;
false.

Excellent!