Java Programming: Using the dictionary class , modify your infix expression evaluator so that it can use variables. Create a dictionary. Add some variable names and associated values to it, then modify your evaluator so that the text string can include the names of variables as well as constants. When you encounter a variable name, look it up in the dictionary and use the value you find there. Treat undefined variables as an error. 

Dictionary class:

public class Dictionary { private HashClass theDict[]; private int size; private int used; public Dictionary(int maxSize) { this.theDict = new HashClass[maxSize]; this.size = maxSize; this.used = 0; } private int hash(String s) { int retval = 0; for(int i = 0; i < s.length(); i++) { retval += (int)s.charAt(i); } return retval % size; } public boolean isFull() { return used >= size-1; } private int findWhere(String s) // finds s or where it should be { int where = this.hash(s); while (theDict[where] != null && !theDict[where].getKey().equals(s)) { System.out.println("Ouch"); where = (where + 1) % size; } return where; } public void delete(String s) throws HashException { int where = findWhere(s); if(theDict[where] != null) { theDict[where] = null; used--; } else { throw new HashException("Missing Key" + s); } } public double lookup(String s) throws HashException { int where = findWhere(s); if(theDict[where] != null) return theDict[where].getValue(); else throw new HashException("Missing Key" + s); } public void insert(String s, double val) throws HashException { if(this.isFull()) { throw new HashException("Hash table overflow"); } int where = findWhere(s); if (theDict[where] == null) { theDict[where] = new HashClass(s, val); used++; } else theDict[where].setValue(val); } public String toString() { String retval = ""; for(int i = 0; i < size; i++) { if (theDict[i] != null) retval += i + " " + theDict[i] + " "; } return retval; } public static void main(String args[]) throws Exception { String words[] = {"cow", "aardvark", "axolotl", "hardware", "software", "tick", "waterbug", "tiger", "caribou", "dog", "clavicle", "patella", "appendix", "liver", "gizzard"}; Dictionary dict = new Dictionary(10); for(int i = 0; i < words.length; i++) { dict.insert(words[i], i*2.5); } System.out.println(dict); } }

Infix expression evaluttor (modify this one):

import java.util.Stack;

public class EvaluateExpression {

public static float solveExpression(String expression) {

char[] tokens = expression.toCharArray(); //Number Stack Stack valueStack = new Stack(); //Operator Stack Stack opsStack = new Stack();

for (int i = 0; i < tokens.length; i++) { if (tokens[i] >= '0' && tokens[i] <= '9') { StringBuffer sbuf = new StringBuffer();

while (i < tokens.length && ((tokens[i] >= '0' && tokens[i] <= '9') || (tokens[i] == '.'))) { sbuf.append(tokens[i++]); }

valueStack.push(Float.parseFloat(sbuf.toString()));

} else if (tokens[i] == '(') { opsStack.push(tokens[i]); } else if (tokens[i] == ')') {

while (opsStack.peek() != '(') { valueStack.push(performOperation(opsStack.pop(), valueStack.pop(), valueStack.pop())); } opsStack.pop(); } else if (tokens[i] == '+' || tokens[i] == '-' || tokens[i] == '*' || tokens[i] == '/') {

while (!opsStack.empty() && isPrecedence(tokens[i], opsStack.peek())) { valueStack.push(performOperation(opsStack.pop(), valueStack.pop(), valueStack.pop())); } opsStack.push(tokens[i]); } }

while (!opsStack.empty()) { valueStack.push(performOperation(opsStack.pop(), valueStack.pop(), valueStack.pop())); } return valueStack.pop();

}

public static boolean isPrecedence(char op1, char op2) { if (op2 == '(' || op2 == ')') { return false; } if ((op1 == '/' || op1 == '*') && (op2 == '-' || op2 == '+')) { return false; } else { return true; } } public static float performOperation(char op, float a, float b) {

switch (op) { case '+': return b + a;

case '-': return b - a;

case '*': return b * a;

case '/': if (a == 0) { throw new UnsupportedOperationException("Cannot divide by zero"); } return b / a; } return 0.0f; }

public static void main(String[] args) {

System.out.println("10.0 + 3 * 45.0 = " + EvaluateExpression.solveExpression("10.0 + 3 * 45.0")); System.out.println("100 * ( 2 + 12 ) = " + EvaluateExpression.solveExpression("100 * ( 2 + 12 )")); System.out.println("100 * ( 2 + 12 ) / 14 = " + EvaluateExpression.solveExpression("100 * ( 2 + 12 ) / 14"));

}

}