package com.github.HarryHook.coding2017.stack.expr;
import java.util.List;
import java.util.Stack;
public class PostfixExpr {
String expr = null;
public PostfixExpr(String expr) {
this.expr = expr;
}
public float evaluate() {
TokenParser parser = new TokenParser();
List<Token> tokens = parser.parse(this.expr);
Stack<Float> numStack = new Stack<>();
for (Token token : tokens) {
if (token.isNumber()) {
numStack.push(new Float(token.getIntValue()));
} else {
Float f2 = numStack.pop();
Float f1 = numStack.pop();
numStack.push(calculate(token.toString(), f1, f2));
}
}
return numStack.pop().floatValue();
}
// 注意,此时计算的顺序和前序的次序相反
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) {
PostfixExpr expr = new PostfixExpr("9 3 1-3*+ 10 2/+");
System.out.println("The result of the expression: " + expr.evaluate());
}
}