package com.coding.basic.stack.expr;
import java.util.List;
import java.util.Stack;
public class PrefixExpr {
String expr = null;
public PrefixExpr(String expr) {
this.expr = expr;
}
public float evaluate() {
TokenParser parser = new TokenParser();
List<Token> tokens = parser.parse(expr);
Stack<Token> opStack = new Stack();
Stack<Float> numStack = new Stack();
Stack<Token> exprs = new Stack<Token>();
for(Token token : tokens) {
exprs.push(token);
}
while(!exprs.isEmpty()) {
Token token = exprs.pop();
if (token.isDigit()) {
numStack.push(new Float(token.getIntValue()));
} else {
Float f1 = numStack.pop();
Float f2 = numStack.pop();
numStack.push(calculate(token.toString(), f1, f2));
}
}
return numStack.pop().floatValue();
}
private Float calculate(String op, Float f1, Float f2) {
System.out.println(f1 + "=====" +f2);
switch (op) {
case "*":
System.out.println(" * ");
return f1 * f2;
case "/":
System.out.println("/");
return f1 / f2;
case "+":
System.out.println("+");
return f1 + f2;
case "-":
System.out.println("-");
return f1 - f2;
}
throw new RuntimeException(op + " is not supported");
}
}