FormulaEvaluator.java

/*
 * Use the JavaScript Engine to employ calls to the eval() function.
 * This is much simpler than writing a full formula parser and lexer
 * with the capacity to tokenize, substitute, and evaluate general
 * mathematical expressions.
 *
 * Session example:
 * Enter a formula.
 * (y-3.25) / x
 * Enter the value of the variable x:
 * 10.35
 * Enter the value of the variable y:
 * 13.6
 * formula: (y-3.25) / x
 * with variables:
 *    x = 10.35
 *    y = 13.6
 * translates to: (13.6-3.25) / 10.35 and
 * evaluates to: 1.0
 */

import java.util.Hashtable;
import java.util.Scanner;
import java.util.Map.Entry;

import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;

public class FormulaEvaluator
{
    public static void main( String[] args ) {
        // Scanner to collect user input
        Scanner scan = new Scanner(System.in);

        // prompt user for formula string
        System.out.println( "Enter a formula.");
        String formula = scan.nextLine();
        // variables will be presumed to be single, lower-case
        // alphabetical characters - kill everything else
        String variables = kernel(
            formula.replaceAll( "[^a-z]", "" )
        );

        // to collect values or each variable
        String[] values = new String[ variables.length() ];
        // for each lower-case variable name, collect value
        for (int i = 0; i < variables.length(); i++) {
            System.out.println(
                "Enter the value of the variable " +
                variables.charAt( i ) + ": "
            );
            values[ i ] = scan.next();
        }

        // copy original formula
        String evaluable = formula;
        // substitute the corresponding value for each variable
        // in the original formula
        for (int i = 0; i < variables.length( ); i++) {
            evaluable = evaluable.replaceAll(
                "" + variables.charAt( i ), values[ i ]
            );
        }

        // get script engine manager instance
        ScriptEngineManager manager = new ScriptEngineManager();
        // javascript engine instance to use for evaluation, since
        // math ops are mostly the same
        ScriptEngine engine = manager.getEngineByName("js");
        Object result = null;
        try {
            result = engine.eval( evaluable );
        } catch (ScriptException e) {
            System.out.println( e.toString() );
        }

        // compose output to report
        String rpt = "formula: " + formula + "\nwith variables:";
        for (int i = 0; i < variables.length( ); i++) {
            rpt += "\n   " + variables.charAt( i ) + " = " +
                values[ i ];
        }
        rpt += "\ntranslates to: " + evaluable + " and " +
            "\nevaluates to: " + String.valueOf( result );
        System.out.println( rpt );
    }
    // Return unique elements of a on first dimension.
    public static String kernel( String s ) {
        // Set hashtable style associative array working storage.
        Hashtable<String, String> r = new Hashtable<String, String>();
        // For each element in s...
        for (int i = 0; i < s.length(); i++) {
            // ...assign key/value pair - repeated values have same key and
            // replace one another, leaving only single instance of value.
            r.put("" + s.charAt( i ), "" + s.charAt( i ));
        }
        // Initialize return.
        StringBuffer sb = new StringBuffer();
        // For each key in r...
        for (Entry<String, String> entry : r.entrySet()) {
            sb.append( entry.getKey() );
        }
        return sb.toString();
    };

}