Java超出范围

Question

My program is nearly done except for one problem. I'm having an out of scope problem with the for loop. The goal of the program is to compound monthly interest for a user inputted amount & term.

An example of output at $5000 principal with 5% interest for 3 years would be:

Month:  Interest:   Principal:
1       $20.83      $5020.83 
2       $20.92      $5041.75
etc      etc         etc 

 Starting Balance        = $ 5000.00 // having problem outputting these w/ for-loop
 Final Account Balance   = $ 5807.36 // System.out.print keeps repeating multiple times
 Total Interest Paid     = $  807.36 // but i can't use variables outside of loop

My problem is that during my for loop, I keep outputting Starting Balance, Final Balance and Total Interest every time the program goes through the loop. but if I try to use the variables outside the loop it goes out of scope and if I try to declare variables outside of the loop I can't use them inside the loop because it's already been declared in the constructor.

Can anyone give me some hints or advice?

My code:

    public class Calculator
{

    public Calculator()
    {
        Scanner input = new Scanner(System.in);
        boolean error = false;

        while (!error){
        System.out.print("Please input the following: principal, interest rate, term >> ");
        double principal = input.nextDouble();
        double interest_rate = input.nextDouble(); 
        int term = input.nextInt();
        String Month = input.next();


         char dollar_sym = 36;

        if (interest_rate <= 0 || term <= 0 || principal <= 0) // input validation
           {
             System.out.println("The term, interest rate and principal must be greater 
             than zero");
             continue; 
           }
        if (!Month.equals("month")) // input validation
           {
             System.out.println("Please input month after term"); 
             continue; 
           }

        System.out.println("Month: " + "  Interest: " + "Principal:  "); 

        if (Month.equals("month"))
        {
           for (int month = 1; month <= term; month++)
            { 
              double interest = (principal * interest_rate / 100) / 12;
              principal = principal + interest; 

              System.out.printf("%4d      %c%5.2f    %c%5.2f\n", month, 
              dollar_sym, interest, dollar_sym, principal );

              double start_principal = principal - interest; // problem
              double final_principal = principal; // problem 
              double total_interest = interest * interest_rate; // problem

              System.out.println(" Starting balance = " + start_principal ); // problem
              System.out.println("Final account balance = " + final_principal ); // problem
              System.out.println("Total Interest Paid = " + total_interest); // problem
           } 
         }
       }
     }
   }

Answer 1

Declare them before the loop begins, so they will exist inside the loop and after it:

double start_principal = 0;
double final_principal = 0;
double total_interest = 0;

Scanner input = new Scanner(System.in);
boolean error = false;

while (!error) {
    // ...
}

// ...

Answer 2

In my answer, I am assuming that when you say goes out of scope , you mean that you get a compile time error. (note, it would make the answer easier to address if you provided the error message and the line that is causing the error message).

The scope of a variable refers to where the variable is accessible. For example, if you declare a variable inside of an if statement, the scope is that if statement. Some example code:

public void updateStatus(){
    Boolean shouldCheckStatus = true;//this variable is declared inside of the updateStatus method, so that is the scope of the variable
    if(shouldCheckStatus == true){
        Int hitCounter = 0;//this variable is declared inside of the if statement, so it is only accessible inside of the if statement
        //do some work
        if(hitCounter > 100){
            self.registerHits(hitCounter);//hitCounter is still accessible here, because it is still inside of the if statement
        }
        else{
            shouldCheckStatus = false;
        }
    }//close the if statement and so close the scope...
    //the hitCounter variable is no longer in scope, because we are no longer in the if statement
    //but shouldCheckStatus is still in scope, because we are still in the method
    if(shouldCheckStatus == true){
       self.callAnotherMethod();
    }
}

So in your problem, you need to declare your variable above where you want to use it, inside of the scope that you want to use it. And then not declare it again. So declare before the loop.