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Jason is a fully-fledged interpreter for an extended version of AgentSpeak, a BDI agent-oriented logic programming language.
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package jason.stdlib;

import jason.JasonException;
import jason.asSemantics.DefaultInternalAction;
import jason.asSemantics.InternalAction;
import jason.asSemantics.TransitionSystem;
import jason.asSemantics.Unifier;
import jason.asSyntax.ASSyntax;
import jason.asSyntax.ListTerm;
import jason.asSyntax.NumberTerm;
import jason.asSyntax.Term;

import java.util.Iterator;

/**
Internal action: .nth.

Description: gets the nth term of a list.

Parameters:

-/+ index (integer): the position of the term (the first term is at position 0)

+ list (list): the list where to get the term from.

-/+ term (term): the term at position index in the list.



Examples:

 .nth(0,[a,b,c],X): unifies X with a.
 .nth(2,[a,b,c],X): unifies X with c.
 .nth(0,[a,b,c],d): false.
 .nth(0,[a,b,c],a): true.
 .nth(5,[a,b,c],X): error.
 .nth(X,[a,b,c,a,e],a): unifies X with 0 (and 3 if it backtracks).


  @see jason.stdlib.concat
  @see jason.stdlib.delete
  @see jason.stdlib.length
  @see jason.stdlib.member
  @see jason.stdlib.sort
  @see jason.stdlib.max
  @see jason.stdlib.min
  @see jason.stdlib.reverse


  @see jason.stdlib.difference
  @see jason.stdlib.intersection
  @see jason.stdlib.union
*/

public class nth extends DefaultInternalAction {

    private static InternalAction singleton = null;
    public static InternalAction create() {
        if (singleton == null)
            singleton = new nth();
        return singleton;
    }

    @Override public int getMinArgs() {
        return 3;
    }
    @Override public int getMaxArgs() {
        return 3;
    }

    @Override protected void checkArguments(Term[] args) throws JasonException {
        super.checkArguments(args); // check number of arguments
        if (!args[0].isNumeric() && !args[0].isVar()) {
            throw JasonException.createWrongArgument(this,"the first argument should be numeric or a variable -- not '"+args[0]+"'.");
        }
        if (!args[1].isList()) {
            throw JasonException.createWrongArgument(this,"the second argument should be a list and not '"+args[1]+"'.");
        }
    }

    @Override
    public Object execute(TransitionSystem ts, final Unifier un, final Term[] args) throws Exception {
        checkArguments(args);

        ListTerm list = (ListTerm)args[1];

        if (args[0].isNumeric()) {
            int index = (int)((NumberTerm)args[0]).solve();

            if (index < 0 || index >= list.size()) {
                throw new JasonException("nth: index "+index+" is out of bounds ("+list.size()+")");
            }

            return un.unifies(args[2], list.get(index));
        }

        if (args[0].isVar()) {

            final Iterator ilist = list.iterator();

            // return all indexes for thirds arg
            return new Iterator() {
                int index = -1;
                Unifier c = null; // the current response (which is an unifier)

                public boolean hasNext() {
                    if (c == null) // the first call of hasNext should find the first response
                        find();
                    return c != null;
                }

                public Unifier next() {
                    if (c == null) find();
                    Unifier b = c;
                    find(); // find next response
                    return b;
                }

                void find() {
                    while (ilist.hasNext()) {
                        index++;
                        Term candidate = ilist.next();
                        c = un.clone();
                        if (c.unifiesNoUndo( args[2], candidate)) {
                            c.unifies(args[0], ASSyntax.createNumber(index));
                            return; // found another response
                        }
                    }
                    c = null; // no more sublists found
                }

                public void remove() {}
            };

        }
        return false;
    }
}