Adding order of operations in Java

[ratyo8]

Hi everyone,

I am trying to write a java program that can be used as a calculator. I want it to be able to evaluate an expression of any length with any number of operands or operators (as long as they're in one line). So far I have made a program that can evaluate expressions correctly, however it can only evaluate an expression with two operands and one operator. The other problem I have is that I want it to be able to use the order of operators correctly (so I need it to be able to evaluate negatives first, followed by * and / and then + and -). So far the program can evaluate expressions with negatives, but it will not evaluate the other operators in order and cannot evaluate more than one. Any help I can get would be appreciated! Here is the code I have so far (it is in two files entitled "LabCalculator.java" and "LabExpression.java"):


Part one
public class LabCalculator
{
String expstring;
int cursor;

public LabCalculator()
{
expstring = null;
}

public LabCalculator(String instring)
{
expstring = instring;
cursor = 0;
}

public void setExpression(String estring)
{
expstring = estring;
cursor = 0;
}

public int getResult()
{
if (expstring == null)
{
System.err.println("There is no expression to evaluate.");
System.err.println("Call setExpression() before calling getResult().");
return 0;
}
int result = 0;
cursor = 0;
int op1 = getOperand();
char operator = getOperator();
int op2 = getOperand();
switch(operator)
{
case '+':
result = op1 + op2;
break;
case '-':
result = op1 - op2;
break;
case '*':
result = op1 * op2;
break;
case '/':
if (op2 == 0)
{
System.err.println("Can not divide by 0.");
return 0;
}
else
{
result = op1 / op2;
break;
}
}
return result;
}

private int getOperand()
{
char nextchar = expstring.charAt(cursor);
int value = 0;
int neg=0;
if (nextchar == '-')
{
neg=1;
cursor++;
nextchar = expstring.charAt(cursor);
}

while (cursor < expstring.length() && Character.isDigit(nextchar))
{
value = (value * 10 + Character.digit(nextchar, 10));
cursor++;
if (cursor < expstring.length())
nextchar = expstring.charAt(cursor);

}

if (neg==1)
return -value;
else
return value;
}

private char getOperator()
{
char operator = expstring.charAt(cursor);
cursor++;
return operator;
}
}

Part Two
import java.util.*;

class LabExpression
{
Scanner keyreader;
LabCalculator calc;

public LabExpression()
{
keyreader = new Scanner(System.in);
calc = new LabCalculator();
}

public void run()
{
String expression;
showMenu();
expression = getNextExpression();
while(!expression.equals("quit"))
{
calc.setExpression(expression);
System.out.print(expression + " = ");
System.out.println(calc.getResult());
expression = getNextExpression();
}
}

private String getNextExpression()
{
System.out.print("? ");
return keyreader.next();
}

private void showMenu()
{
System.out.println("Enter a simple arithmetic expression");
System.out.println("Or enter \"quit\" to terminate the program");
}

public static void main(String[] args)
{
LabExpression dialog = new LabExpression();
dialog.run();
}
}

[Norm]

There are well defined algorithms for parsing expressions. You need to find one and implement it in java.
I wouldn't try to patch one together. The logic has been worked out.