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Thread: Datatype comparisons in do while loops

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    Default Datatype comparisons in do while loops

    Hi, I'm having an awkward time dealing with the comparison of user input:

    package randomExperiments;
     
    import java.util.Scanner;
     
    public class RandomExperiments {
     
    	public static void main(String[] args){
     
    		String password = "Megaman X";
    		Scanner str = new Scanner(System.in);
    		do{
    			System.out.println("We're still trapped in the do. Guess the password to get out!");
    			//String password = "Megaman X";
    			//Scanner str = new Scanner(System.in);
    			String strGuess = str.next();
    		}while(!strGuess.equals(password));
    		str.close();
     
    		int magicNumber = 40;
    		Scanner num = new Scanner(System.in);
    		do{
    			System.out.println("We're still trapped in the do. Guess the magic number to get out!");
    			int intGuess = num.nextInt();
    		}while(intGuess != magicNumber);
    		num.close();
    	}
    }

    Line 22 says, "strGuess cannot be resolved".

    Line 30 says, "intGuess cannot be resolved to a variable".

    What's wrong?
    Last edited by SamJava_the_Hut; December 6th, 2018 at 02:43 AM.

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    Default Re: Datatype comparisons in do while loops

    What's wrong?
    The variable is not defined in the scope where it is being referenced. Define it outside of the loop.
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Datatype comparisons in do while loops

    Why do you continue to 1) use multiple Scanner instances of System.in and 2) close them? And here's a freebie. You're going to have problems with your password. I suggest you use a print statement to print out the user's input to see what happens.

    Regards,
    Jim

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    Default Re: Datatype comparisons in do while loops

    Quote Originally Posted by jim829 View Post
    Why do you continue to 1) use multiple Scanner instances of System.in and 2) close them?
    Sorry Jim. I'm having a hard time wrapping my head around only being able to use one scanner in an entire class. It puzzles me that Java would have such a crippling limitation. In my inexperienced understanding, it seems like it would be common necessity to get user input more than once (and consequently, for more than one variable).

    Quote Originally Posted by Norm View Post
    The variable is not defined in the scope where it is being referenced. Define it outside of the loop.
    I suspected it was a scope problem, and your reply confirms that. But, at the same time, wouldn't defining strGuess outside of the loop defeat the whole purpose not letting the user out of the do while loop until he/she enters a string that matches the password?

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    Default Re: Datatype comparisons in do while loops

    defining strGuess outside of the loop
    You can define a variable in one place with an initial value and assign it other values in other places.
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Datatype comparisons in do while loops

    Quote Originally Posted by SamJava_the_Hut View Post
    Sorry Jim. I'm having a hard time wrapping my head around only being able to use one scanner in an entire class. It puzzles me that Java would have such a crippling limitation. In my inexperienced understanding, it seems like it would be common necessity to get user input more than once (and consequently, for more than one variable).
    You are missing the point. It is not a crippling limitation. First you may do it if you want. It is just inefficient and unnecessary. You can take as many inputs from a single scanner as you need to. Here is an example:
    import java.util.Scanner;
     
    public class ScannerDemo {
     
       public static void main(String[] args) {
    	  // get rid of annoying (and inappropriate) resource not closed message from compiler
    	  @SuppressWarnings("resource")
     
    	  Scanner input = new Scanner(System.in);
    	  System.out.print("Please enter your first name: ");
    	  String name = input.next();
     
              System.out.print("Please enter your age:");
    	  int age = input.nextInt();
     
    	  input.nextLine(); // get rid of lingering new line from previous input
     
    	  System.out.print("Please list three strings or ints on one line:");
    	  String line = input.nextLine();
     
    	  System.out.println("Your name is " + name);
    	  System.out.println("Your age is " + age + " years.");
    	  System.out.println("You chose to list\n" + line);
       }
    }

    As you learn more about Scanner you will find that in some situations there is a lingering new line in the buffer. So you need to read it and discard before carrying on. I recommend you carefully read the Scanner API documentation. It is a commonly used class and will be of great benefit to you.

    Regards,
    Jim

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    Default Re: Datatype comparisons in do while loops

    Quote Originally Posted by jim829 View Post
    You can take as many inputs from a single scanner as you need to.
    OK, now I think I understand. However, in the context of what I'm trying to do, I'm still having variable scope issues:

    package randomExperiments;
     
    import java.util.Scanner;
     
    public class RandomExperiments {
     
    	public static void main(String[] args){		
     
     
    		String password = "Megaman X";
    		Scanner input = new Scanner(System.in);		
    		do{
    			System.out.println("We're still trapped in the do. Guess the password to get out!");
    			String pw = input.nextLine();
    		}while(!pw.equals(password));//while(!input.equals(password));
     
    		int magicNumber = 40;		
    		do{
    			System.out.println("We're still trapped in the do. Guess the magic number to get out!");
    			int intGuess = input.nextInt();
    		}while(!input.equals(magicNumber));
     
    		String test = "password";
    		while(!input.equals(test)){
    			System.out.println("Your guess still doesn't match. Try again.");
    			String guess = input.nextLine();
    		}
    	}
    }

    On line 16:

    If I use
    while(!pw.equals(password))
    , pw cannot be resolved.

    If I use
    while(!input.equals(password))
    while(!input.equals(password)), pw from line 15, and input from line 12 are seen as never used.

    intGuess on line 21 and guess on line 27 are also looked at by the program as unused.

    Is there any way around this?

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    Default Re: Datatype comparisons in do while loops

    pw cannot be resolved.
    Define pw before the loop. Assign it a value inside the loop.
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Datatype comparisons in do while loops

    Just as a heads up, I have a slightly different program on a new hard drive.

    I now have a problem with line 29. Here's the code:
    package sam;
    import java.text.DecimalFormat;
    import java.text.NumberFormat;
    import java.util.Scanner;
     
     
    public class Test {
     
    	public static void main(String[] args){
     
    		String password = "Megaman X";
    		String pw = "";
    		Scanner input = new Scanner(System.in);		
    		do{
    			System.out.println("We're still trapped in the do. Guess the password to get out!");
    			pw = input.nextLine();
    			if(pw.equals(password)){
    				System.out.println("The password has been guessed. Exiting this do while loop.");
    			}
    		}while(!pw.equals(password));//while(!input.equals(password));
    		System.out.println("We're out of the do while loop for password " + password + ".");
     
     
    		int magicNumber = 40;
    		int intGuess;
    		do{
    			System.out.println("We're still trapped in the do. Guess the magic number to get out!");
    			intGuess = input.nextInt();
    		}while(!input.equals(magicNumber));//(input != magicNumber);
     
    		String test = "password";
    		while(!input.equals(test)){
    			System.out.println("Your guess still doesn't match. Try again.");
    			String guess = input.nextLine();
    		}		
     
    	}//end of main
    }//end of class Test

    If I use
    while(!input.equals(magicNumber));
    The error is "The method equals(Object) in the type Object is not applicable for the arguments (int)".

    If I use
    while(input != magicNumber);
    The error is "Incompatible operand types Scanner and int".

    How do I fix this while statement?
    Last edited by SamJava_the_Hut; December 31st, 2018 at 02:40 AM. Reason: I got the string comparison do while loop working. I now need help on the int comparison do while loop instead.

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    Default Re: Datatype comparisons in do while loops

    How do I fix this while statement?
    I assume you want to compare the value entered by the user with the value of magicNumber.
    That value is in the variable: intGuess. Use that in the while statement

    The input variable refers to an instance of the Scanner class. Use it with Scanner class methods to get user input. For example:
    	pw = input.nextLine();
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Datatype comparisons in do while loops

    Based on everything that I've learned from this thread, i have come to a working solution:

    package randomExperiments;
     
    import java.util.Scanner;
    import java.util.Random;
     
    public class RandomExperiments {
     
    	public static void main(String[] args){		
     
    		String strGuess = "";
    		int intGuess;
     
    		String password = "Megaman X";
    		Scanner input = new Scanner(System.in);		
    		do{
    			System.out.println("We're still trapped in the do. Guess the password to get out!");
    			strGuess = input.nextLine();
    		}while(!strGuess.equals(password));		
    		System.out.println("We have guessed correctly for strGuess.");
     
    		System.out.println("We will now start guessing for intGuess in the next do while loop.");
    		int magicNumber = 40;		
    		do{
    			System.out.println("We're still trapped in the do. Guess the magic number to get out!");
    			intGuess = input.nextInt();
    		}while(intGuess != magicNumber);
    		System.out.println("We have guessed correctly for intGuess.");
     
    	}
    }

    I want to thank jim and norm to start incrementing my thanks counter, but I don't know how. So, thanks for your help in this thread

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