Thread: Can someone please tell me why my do while loop isn't working?

Hi, everyone, can someone please take a look at my code and let me know why my do while loop does not work? The program runs properly until I add the do while loop. I referenced my book and have no idea how to do this differently.

Ashley

import java.util.Scanner;
 
public class Project2
{
	public static void main(String[] args)
	{
 
		//variables
		double future = 0;
		double rate = 0;
		double numYears = 0;
		char runAgain;
		String input;
 
		System.out.println("This will find present value.");
		presentValue(future, rate, numYears);
 
 
	}
 
	public static double presentValue(double future, double rate, double numYears)
	{
 
		//creat a scanner object to read input.
		Scanner keyboard = new Scanner(System.in);
 
		//Get input from user
 
			do
			{
 
			System.out.println("What do you want the future value to be? ");
			future = keyboard.nextDouble();
 
			System.out.println("What is the annual percentage rate? ");
			rate = keyboard.nextDouble();
 
			System.out.println("How long will you keep your money in the account? (in years)");
			numYears = keyboard.nextDouble();
 
			//present = (future)/(1+rate)^numYears
 
			//variables
			double present = (future)/Math.pow(1+rate,numYears);
 
			System.out.println("The present value is " + present + " .");
 
			System.out.println("Would you like to run the program again? (Enter 'y' for yes " +
									" or 'n' for no");
			input = keyboard.nextLine();
			repeat = input.charAt(0);
 
			} while (runAgain == 'y' || runAgain == 'Y');
 
			return present;
 
 
		}
}

I also tried adding the "while" part after the return statement and that doesn't change anything. Thank you in advance.

Ashley

Yes I know it won't compile. It did fine before I tried adding the loop. Here are the errors:

Project2.java:50: error: cannot find symbol
			input = keyboard.nextLine();
			^
  symbol:   variable input
  location: class Project2
Project2.java:51: error: cannot find symbol
			runAgain = input.charAt(0);
			^
  symbol:   variable repeat
  location: class Project2
Project2.java:51: error: cannot find symbol
			runAgain = input.charAt(0);
			         ^
  symbol:   variable input
  location: class Project2
Project2.java:53: error: cannot find symbol
			} while (runAgain == 'y' || runAgain == 'Y');
			         ^
  symbol:   variable runAgain
  location: class Project2
Project2.java:53: error: cannot find symbol
			} while (runAgain == 'y' || runAgain == 'Y');
			                            ^
  symbol:   variable runAgain
  location: class Project2
Project2.java:55: error: cannot find symbol
			return present;
			       ^
  symbol:   variable present
  location: class Project2
6 errors

In your code, the variable runAgain is not assigned, so you can not continue to run your code repeatedly.

just modify the line: repeat = input.charAt(0) to runAgain = input.charAt(0)

Yes I noticed that. I fixed that and still no fix.

Sorry, I'll make sure to include that next time. They are declared in the main where the other variables are declared. The other inputs work just fine. Again, if the while loop is taken out then the program executes fine and the output is correct. I just have to add the do while loop to give the user as many repeat attempts as they wish, as long as they enter "y" or "Y".

Here are the variables, and they are declared in the main:

//variables
		double future = 0;
		double rate = 0;
		double numYears = 0;
		char runAgain;
		String input;

Ashley

char runAgain;
String input;

should be declared in presentValue

present should be declared before while loop

It is declared before the while. I moved it up to declare it before the 'do' and and it runs, but then gets an error, incorrect output of "0.0", and then it closes before it gives the option of running again. I will post my new code and the new errors as well.

import java.util.Scanner;
 
public class Project2
{
	public static void main(String[] args)
	{
 
		//variables
		double future = 0;
		double rate = 0;
		double numYears = 0;
 
 
		System.out.println("This will find present value.");
		presentValue(future, rate, numYears);
 
 
 
 
	}
 
	public static double presentValue(double future, double rate, double numYears)
	{
 
		//creat a scanner object to read input.
		Scanner keyboard = new Scanner(System.in);
 
		char runAgain;
		String input;
 
		//Get input from user
 
		double present = (future)/Math.pow(1+rate,numYears);
 
			do
			{
 
			System.out.println("What do you want the future value to be? ");
			future = keyboard.nextDouble();
 
			System.out.println("What is the annual percentage rate? ");
			rate = keyboard.nextDouble();
 
			System.out.println("How long will you keep your money in the account? (in years)");
			numYears = keyboard.nextDouble();
 
			//present = (future)/(1+rate)^numYears
 
			//variables
 
 
			System.out.println("The present value is " + present + " .");
 
 
			System.out.println("Would you like to run the program again? (Enter 'y' for yes " +
									" or 'n' for no");
			input = keyboard.nextLine();
			runAgain = input.charAt(0);
 
			} while (runAgain == 'y' || runAgain == 'Y');
 
			return present;
 
 
 
		}
}

ERRORS:

 ----jGRASP exec: java Project2
 
This will find present value.
What do you want the future value to be? 
10000.00
What is the annual percentage rate? 
.0575
How long will you keep your money in the account? (in years)
10
The present value is 0.0 .
Would you like to run the program again? (Enter 'y' for yes  or 'n' for no
Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 0
	at java.lang.String.charAt(String.java:695)
	at Project2.presentValue(Project2.java:58)
	at Project2.main(Project2.java:15)
 
 ----jGRASP wedge2: exit code for process is 1.
 ----jGRASP: operation complete.

replace keyboard.nextLine() to keyboard.next()

It compiles now, but the output is 0.0. I'm pretty sure that's because I have the equation before it actually receives the input. But if I put the equation after the input it gives me that same error again with the "return present;" statement.

It seems working fine on my PC:

 
What do you want the future value to be? 
2
What is the annual percentage rate? 
3
How long will you keep your money in the account? (in years)
4
The present value is 0.0078125 .
Would you like to run the program again? (Enter 'y' for yes  or 'n' for no
y
What do you want the future value to be? 
10000.00
 
What is the annual percentage rate? 
.0575
How long will you keep your money in the account? (in years)
10
The present value is 5717.369223381666 .
Would you like to run the program again? (Enter 'y' for yes  or 'n' for no

Weird. No idea why mine won't work then. Thank you for your help.