arg(+N, +Term, ?Arg)

+N

Integer not greater than the arity of Term, or a list.

+Term

Compound term.

?Arg

Prolog term.

Description

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).

Fail Conditions

Resatisfiable

Exceptions

(4) instantiation fault

Either N or Term (or both) is not instantiated (non-coroutine mode only).

(5) type error

N is instantiated, but not to an integer or list of integers.

(5) type error

Term is instantiated, but not to a compound term.

(6) out of range

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