Interpreter.java

Below is the syntax highlighted version of Interpreter.java from §3.5 Searching Applications.

/******************************************************************************
 *  Compilation:  javac Interpreter.java
 *  Execution:    java Interpreter
 *  Dependencies: In.java ST.java
 *
 *  Parses simple arithmetic expressions of the form z = x and
 *  z = x + y, where x and y can be symbolic variables or real
 *  numbers. Uses a symbol table to store the mapping between
 *  variable names and their values.
 *
 *
 *  % java Interpreter
 *  >> x := 34
 *  x = 34.0
 *
 *  >> y := 23 * x
 *  y := 782.0
 *
 *  >> z := x ^ y
 *  z := Infinity
 *
 *  >> z := y ^ 2
 *  z := 611524.0
 *
 *  >> x
 *  x := 34.0
 *
 *  >> x := sqrt 2
 *  x := 1.4142135623730951
 *
 *  >> [Ctrl-d]
 *
 *  Remarks
 *  -------
 *
 *   - Currently allows values on the LHS, e.g., 17 = x + y,
 *     and treats "17" as a variable name.
 *
 *
 ******************************************************************************/

public class Interpreter {

    public static void main(String[] args) {

        ST<String, Double> st = new ST<String, Double>();

        // read in one line at a time and parse
        String line;
        In stdin = new In();
        System.out.print(">> ");
        while ((line = stdin.readLine()) != null) {
            String[] tokens = line.split("\\s");

            // singe variable - just print out its value
            if (tokens.length == 1)  {
                String zvar = tokens[0];
                System.out.println(zvar + " := " + st.get(zvar));
            }

            // z = x
            else if (tokens.length == 3) {
                String zvar = tokens[0];
                String eq   = tokens[1];
                String xvar = tokens[2];
                if (!eq.equals(":=")) throw new RuntimeException("Illegal assignment");
                double x;
                if (st.contains(xvar)) x = st.get(xvar);
                else                   x = Double.parseDouble(xvar);
                st.put(zvar, x);
                System.out.println(zvar + " := " + st.get(zvar));
            }

            // z = function x
            else if (tokens.length == 4) {
                String zvar = tokens[0];
                String eq   = tokens[1];
                String func = tokens[2];
                String xvar = tokens[3];
                if (!eq.equals(":=")) throw new RuntimeException("Illegal assignment");

                double x;
                if (st.contains(xvar)) x = st.get(xvar);
                else                   x = Double.parseDouble(xvar);

                if      (func.equals("sin")) st.put(zvar, Math.sin(x));
                else if (func.equals("cos")) st.put(zvar, Math.cos(x));
                else if (func.equals("sqrt")) st.put(zvar, Math.sqrt(x));
                else if (func.equals("-")) st.put(zvar, -x);
                else throw new RuntimeException("Illegal function");
                System.out.println(zvar + " := " + st.get(zvar));
            }

            // z = x + y
            else if (tokens.length == 5) {
                String zvar = tokens[0];
                String eq   = tokens[1];
                String xvar = tokens[2];
                String op   = tokens[3];
                String yvar = tokens[4];
                if (!eq.equals(":=")) throw new RuntimeException("Illegal assignment");

                double x;
                if (st.contains(xvar)) x = st.get(xvar);
                else                   x = Double.parseDouble(xvar);

                double y;
                if (st.contains(yvar)) y = st.get(xvar);
                else                   y = Double.parseDouble(xvar);

                if      (op.equals("+")) st.put(zvar, x + y);
                else if (op.equals("-")) st.put(zvar, x - y);
                else if (op.equals("*")) st.put(zvar, x * y);
                else if (op.equals("/")) st.put(zvar, x / y);
                else if (op.equals("^")) st.put(zvar, Math.pow(x, y));
                else throw new RuntimeException("Illegal operator");
                System.out.println(zvar + " = " + st.get(zvar));
            }

            else throw new RuntimeException("Illegal expression");
            System.out.println();
            System.out.print(">> ");
        }

    }
}