Interpreter

The interpreter consists of the Interpreter defined in this file that can run a Program and the Evaluator that can evaluate an Expression.

The interpreter implements the semantics defined by the function sem: Prog * V -> V, where V = Id -> Z is the set of all variable valuations to the set Z set of integers. sem is inductively defined as follows:

sem(x ":=" e, v) = v.update(x, eval(e, v))
sem(c1 ";" c2) = sem(c2, sem(c1, v))
sem("if" "(" e ")" "then" "{" c1 "}" else "{" c2 "}") =
  sem(c1, v)    if eval(e, v) != 0
  sem(c2, v)    else
sem("while" "(" e ")" "{" c "}", v) =
  sem(c ";" "while" "(" e ")" "{" c "}", v)   if eval(e, v) != 0
  v                                           else

with

a variable valuation v,
an expression e,
an identifier x and
programs c, c1, c2.

The evaluation function eval is described at the Evaluator.

------------------Show Header Code ( lines)------------------

package interpreter;

import program.*;

import java.util.HashMap;
import java.util.Map;

The Interpreter implements the semantic function defined above with the help of the Visitor. The Interpreter runs the given Program in the constructor and can be used as follows on a given program of type Program.

Interpreter interpreter = new Interpreter(program);
System.out.println(interpreter.getValuation());

The semantic function sem passes along the variable valuation v. In the Interpreter the valuation is realized as global variable valuation. Because of the in-order execution this global variable always represents the state of the v passed to the semantic function sem.

public class Interpreter extends Visitor {
    private final Map<String, Integer> valuation = new HashMap<String, Integer>();

    public Map<String, Integer> getValuation() {
        Map<String, Integer> result = new HashMap<String, Integer>();
        result.putAll(valuation);
        return result;
    }

    public Interpreter(Program program) {
        visit(program);
    }

sem(x ":=" e, v) = v.update(x, eval(e, v))

    public void visitAssignment(Assignment assignment) {
        Evaluator evaluator = new Evaluator(assignment.expression, valuation);
        valuation.put(assignment.identifier.name, evaluator.getValue());
    }

sem(c1 ";" c2) = sem(c2, sem(c1, v))

    public void visitComposition(Composition composition) {
        visit(composition.first);
        visit(composition.second);
    }

sem("if" "(" e ")" "then" "{" c1 "}" else "{" c2 "}") =
  sem(c1, v)    if eval(e, v) != 0
  sem(c2, v)    else

    public void visitConditional(Conditional conditional) {
        Evaluator evaluator = new Evaluator(conditional.condition, valuation);
        if (evaluator.getValue() != 0) {
            visit(conditional.thenCase);
        } else {
            visit(conditional.elseCase);
        }
    }

sem("while" "(" e ")" "{" c "}", v) =
  sem(c ";" "while" "(" e ")" "{" c "}", v)   if eval(e, v) != 0
  v                                           else

    public void visitLoop(Loop loop) {
        Evaluator evaluator = new Evaluator(loop.condition, valuation);
        if (evaluator.getValue() != 0) {
            visit(new Composition(loop.program, loop));
        }
    }
}