package com.github.HarryHook.coding2017.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(this.expr);
Stack<Token> exprStack = new Stack<>();
Stack<Float> numStack = new Stack<>();
for (Token token : tokens) {
exprStack.push(token);
}
System.out.println(tokens);
while (!exprStack.isEmpty()) {
Token t = exprStack.pop();
if (t.isNumber()) {
numStack.push(new Float(t.getIntValue()));
} else if(t.isOperator()){
Float f1 = numStack.pop();
Float f2 = numStack.pop();
numStack.push(calculate(t.toString(), f1, f2));
}
}
return numStack.pop().floatValue();
}
private Float calculate(String op, Float f1, Float f2) {
if (op.equals("+")) {
return f1 + f2;
}
if (op.equals("-")) {
return f1 - f2;
}
if (op.equals("*")) {
return f1 * f2;
}
if (op.equals("/")) {
return f1 / f2;
}
throw new RuntimeException(op + " is not supported");
}
public static void main(String[] args) {
PrefixExpr expr = new PrefixExpr("-++6/*2 9 3 * 4 2 8");
System.out.println("The result of the expression: " + expr.evaluate());
}
}