Compare operators

[Torvald Helmer]

I am trying to compare two operators.
The operators are: + - * / ^
They have all different priority
^ = 3
* = 2
/ = 2
+ = 1
- = 1

How can I compare a character '*' with
a character '+' of which one has the
highest priority?

[Jesper de Jong]

I saw your question in the forums at Sun's website too.

First of all, you need to get exactly clear what you want. You have two strings containing two operators, for example "*" and "+". Now what output exactly do you want?

Do you just want to know which of the two has the highest priority, for example?

You have to write a piece of code that contains a data structure to keep track of the priorities of the different operators, and then you need to lookup the priorities of the two you want to compare. For the data structure, try a HashMap. For example (uses Java 5.0 features):

Map<String, Integer> priorities = new HashMap<String, Integer>();
 
priorities.put("^", 3);
priorities.put("*", 2);
priorities.put("*", 2);
priorities.put("+", 1);
priorities.put("-", 1);
 
// Now you can lookup the priority of an operator using get(), for example:
String op = "+";
int prio = priorities.get(op);
System.out.printf("The priority of %s is %d", op, prio);

[Stan James]

I'd probably try a map or a lookup of some kind so I could say:

Integer priority1 = priorityMap.get( operator1 );
Integer priority2 = priorityMap.get( operator2 );
return priority1.compareTo( priority2 );

Does that look like it would work? This is a problem solved in every CS program in the world, so somebody has probably found a slicker way to do it by now.

[Torvald Helmer]

Jesper de Jong!!!

I thought this really would work, but
surely I have missed something.

This is my code:

import java.util.*;
import javax.swing.*;
 
public class InfixToPostfix {
 
public static void main(String[] args) {

Object slutt       = "#";

String infixStr = JOptionPane.showInputDialog("Tast inn infix-uttrykket: ");
infixStr    += slutt;
String postfixStr= "";

System.out.println("Infix: "+infixStr);

Stack stack = new Stack();
stack.add(slutt);
System.out.println ("Stack is empty: "+stack.empty ()+ ", Stack: " + stack);


Map <String, Integer> priorities = new HashMap<String, Integer>();
priorities.put("^", 3);
priorities.put("*", 2);
priorities.put("*", 2);
priorities.put("+", 1);
priorities.put("-", 1);
priorities.put("#", 0);

//String op = "+";

Object c;
String stackOp;
String infixOp;
int infixPri;
int stackPri;

for (int i=0; i<infixStr.length(); i++) {
c = infixStr.charAt(i);
infixOp= c.toString();
infixPri = priorities.get(infixOp);

System.out.println("A: "+infixPri+infixOp);

if (infixOp == "+" || infixOp == "-" || infixOp == "*" || infixOp == "/" || infixOp == "^") {
if (stack.empty()) {
stack.push(String.valueOf(c));
}
else {
while (!stack.empty()) {
stackOp = (String) stack.pop();
stackPri = priorities.get(stackOp);
if (stackPri >= infixPri) {
postfixStr += stackOp;
}
else stack.push(String.valueOf(c));
}
}
}
else if (c == "#") {
System.out.println("Postfix: "+ postfixStr);
System.exit(0);
}
else {
postfixStr += c;
}
}
}
}

At these lines: "stack.push(String.valueOf(c));"
I get this message -->

Type safety: The method push(Object) belongs to the raw type Stack. References to generic type
Stack<E> should be parameterized

[Joel McNary]

You're getting that because you haven't declared the type of object that your stack holds. Note that that should just be a warning, not an error.

To fix it, you would declare your stack:

Stack<String> stack = new Stack<String>();

[Torvald Helmer]

thanks Joel McNary!!

Now that problem is out of the way.

But there's still something that messes things up.

When I cmopile the code I get this:
______________
Exception in thread "main" java.lang.NullPointerException
at InfixToPostfix.main(InfixToPostfix.java:40)
______________

line 40 is: infixPri = priorities.get(infixOp);

[Ernest Friedman-Hill]

Your "for" loop loops over every character in "infixStr", but the HashMap only contains entries for a few characters. When the loop hits a character not in the Map, you'll see this exception. So you need to check whether a given character is an operator or not, first. You could just split this line into two and not use auto-unboxing:

Integer mapValue = priorities.get(infixOp);
if (mapValue == null)
    continue; // not an operator
infixPri = mapValue;