package dataStructure_7_PreAndPost;
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> operatorStack = new Stack();
Stack<Float> operandStack = new Stack();
for(int i = tokens.size()-1; i >= 0; i--){
if(tokens.get(i).isNumber()){
operandStack.push(new Float(tokens.get(i).getIntValue()));
}else if(tokens.get(i).isOperator()){
checkOperandStack(operandStack.size());
Float operand = operandStack.pop();
Float operanded = operandStack.pop();
Float result = calculate(tokens.get(i).toString(),operand, operanded);
operandStack.push(result);
}
}
return operandStack.pop();
}
private boolean checkOperandStack(int size) {
if(size < 2){
throw new RuntimeException("������ջ�е�Ԫ��̫��");
}
return true;
}
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");
}
}