Postfix Evaluation using a Stack, Help?

[dalangley]

Alright so my task is to design a driver that accepts postfix expressions in the form of:

BC+
ACE^
A/B
etc.

First off, I must read input from the user in that form and get the integer value of the letters where:

public static int intValue(char letter)
{
int value =0;
if ( letter >= 'A' || letter <= 'Y')
value = (int) letter - 64;
else if (letter == 'Z')
value = 0;
return value;
}

Basically 'A' = 1, 'B' = 2, etc with 'Z' = 0.

I have gotten this far, but to no avail. I type in an expression such as BC+ and it does nothing but give me these errors:

Too few operands or too many operators.
Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 2
at java.lang.String.charAt(String.java:687)
at StackClient.main(StackClient.java:52)


import java.util.Scanner;

public class StackClient {

/**
* @param args the command line arguments
*/
public static void main(String[] args)
{
char operator = 0;
String expression;
int result = 0, loc = 0;

System.out.println("\t\tProgram #2 Stacks\n\nThis program will read expressions from user input into\n" +
"a stack where the given expressions will be evaluated with different operators.\n" +
"It will also employ the use of push and pop to mutate and access the data in" +
"the stack.\n");
Stack theStack = new Stack(10);
Scanner scan = new Scanner(System.in);

System.out.println("Please enter an expression in postfix form using letters (A,B,C, etc.):");
expression = scan.next();

while(true)
{


if (theStack.isEmpty())
System.out.println("Too few operands or too many operators.");
else
theStack.pop();

if (theStack.isNotEmpty())
System.out.println("Too many operands.");
theStack.flush();

for (int i = loc; i <= expression.length(); i++)
theStack.push(theStack.intValue((char) expression.charAt(i)));
theStack.push(expression.charAt(loc));

if (expression.charAt(loc) == operator)
theStack.push(expression.charAt(loc));

switch (operator)
{
case '+': result = theStack.pop() + theStack.pop();

case '-': result = theStack.pop() - theStack.pop();

case '/': result = theStack.pop() / theStack.pop();

case '%': result = theStack.pop() % theStack.pop();

case '^': result = theStack.pop() * theStack.pop();

}

Where am I messing up? I need the program to take in those expressions and evaluate in postfix form. Do I have the loops wrong? Am I not searching the expression right? Any ideas?

[MK12]

EDIT: I had an idea but was wrong when I tested. Sorry.
-MK12

[dalangley]

The method intValue accepts a char type paramater and typecasts it in the evalutation to return the ascii value minus 64. Says so in the method. I am positive that works. Ive tested that much. I need to evaluate the postfix expression using a Stack.