
arg(+N, +Term, -Arg)

   Succeeds if Arg is the Nth argument of the compound term Term.



Arguments
   N                   Integer not greater than the arity of Term, or a list.
   Term                Compound term.
   Arg                 Prolog term.

Type
   Term Manipulation

Description
   If Term is a structure, unifies Arg with the Nth argument of a structure
   Term.


   If Term is a list (N must be either the integer 1 (for the head) or 2
   (for the tail), unifies Arg with the head or tail of the list.  This is
   a consequence of the fact that ./2 is the list functor and
   .(a,.(b,.(c,[]))) is the same as [a,b,c].


   If N is a list of integers and Term is a nested structure, then Arg
   is the subterm of Term described by this list of integers.
   E.g. arg([2,1,3], Term, Arg) is the same as arg(2, Term, T1),
   arg(1, T1, T2), arg(3, T2, Arg).




Modes and Determinism
   arg(+, +, -) is det

Exceptions
     4 --- Either N or Term (or both) is not instantiated    (non-coroutine mode only).
     5 --- N is instantiated, but not to an integer or list of integers.
     5 --- Term is instantiated, but not to a compound term.
     6 --- N is an integer less than 1 or greater than the arity of    Term.

Examples
   
Success:
      arg(2,foo(boo,moo),moo).
      arg(2,.(a,b,c),b).
      arg(2,.(a,b),b).
      arg(2,term1(term2(a,b),c),c).
      arg(2,f(a,f(a,b)),f(X,Y)).        (gives X=a; Y=b).
      arg(2,[a,b,c],[b,c]).
      arg(2,.(a,.(b,.(c,[]))),[b,c]).
      arg(2,[1],[]).
      arg([2,1], f(a,g(b,c)), X).       (gives X=b).
Fail:
      arg(2,f(a,f(a,b)),f(X,X)).
Error:
      arg(N,f(1,2),1).         (Error 4).
      arg(N,[],X),             (Error 5).
      arg(0,foo(boo,moo),moo). (Error 6).
      arg(3,foo(boo,moo),moo). (Error 6).




See Also
   functor / 3, =.. / 2, subscript / 3
