Programming Forums - error trappring and looping

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error trappring and looping

the code below is a simple menu driven program to calculate loan stuffs. i have to beable to trap bad user input and tie that with looping of the program. also the main goal of this assignmnet is to use methods. there are my problems

1. in displayMenu(),as it is now, the user must pick a letter from the menu but what if the use picks a bad letter? i have to trap bad input, tell the use that it was a bad input and then get good input. i know this involves some sorta loop and conditional. i dont know how to test input if it is not P,I,T or Q and loop based on that.

import java.util.Scanner;      //console I/O
import java.util.Currency;     //dollar format
import java.util.Locale;       //get defual location info (US)
import java.text.NumberFormat; //numerical formating
 
 
public class Assignment1b
{
    //intialize new scanner object as global so other methods can use it
    //is there a better way to do this? should ask Hoagland.
    static Scanner scanner = new Scanner(System.in);
 
    public static void main(String[] args)
    {  
        //displays the menu and returns a valid user selection
        char choice = displayMenu();//hold menu choice   
        //print the cost by calling other functions
        loanCaculator(choice);        
 
    }
 
    //foobar. need to error check for bad inputs in some sorta loop i dont know how.
    public static char displayMenu()
    {
        System.out.println("     Loan Caluclator");
        System.out.println("**************************");
        System.out.println("Calulate monthly (P)ayment");
        System.out.println("Calulate  total  (I)ntrest");
        System.out.println("Calulate  total  (L)oan");
        System.out.println("to exit program  (Q)uit\n");
        System.out.print("Select P,I,L or Q: ");
 
        String stringInput = scanner.next();
        char charInput = Character.toUpperCase(stringInput.charAt(0));
 
 
        switch(charInput)
        {
            /*Fall thru on purpose. these are blank b/c if the user enters such
            values they are valid and will be returened out side the switch
            statement. other wise  all non P,I,T,Q values will be caught
            as bad values and not be returned.*/            
            case 'P':
            case 'I':
            case 'T':
            break;
 
            //return Q to main so loop the whole program in main
            case 'Q':
                System.out.println("Exiting program....\n");
                System.exit(0);
            break;
 
            default:
                System.out.println("No vaild entry.Run again using only P, I, T or Q\n");
            break;
        }
        return charInput;       
    }
 
    public static void loanCaculator(char choice)
    {
        //intialize new US currency format
        NumberFormat nf = NumberFormat.getCurrencyInstance(Locale.US);             
 
        System.out.print("Enter the principle of the loan: ");
        double amount = scanner.nextDouble();
 
        double rate = 0.0;//scope
        do
        {
            System.out.print("Enter the intrest rate between 1% and 100%: ");
            rate = scanner.nextDouble();
        }
        while(rate > 100.0 || rate < 1.0);
 
        System.out.print("Enter the term of loan: ");
        double term =scanner.nextDouble();
 
        System.out.print("The total cost is ");
 
        switch(choice)
        {
            case 'P':
                System.out.println(nf.format(calcPayment(amount, rate, term)));
            break;
 
            case 'I':
                System.out.println(nf.format(clacIntrest(amount, rate, term)));
            break;
 
            case 'T':
                System.out.println(nf.format(calcTotalLoan(amount, rate, term)));
            break;
        }                
    }
 
    public static double calcPayment(double amount, double rate, double term)
    {
        return (amount * rate) / (1.0 - Math.pow((1.0 + rate), (-term)));
    }
 
    public static double clacIntrest(double amount, double rate, double term)
    {
        return term * calcPayment(amount, rate, term) - amount; 
    }
 
    public static double calcTotalLoan(double amount, double rate, double term)
    {
        return amount + clacIntrest(amount, rate, term);
    }
 
}

you need to throw the menu into a loop and not break out of it if it hits the default switch statement.

Quote:

Originally Posted by Serinth (Post 122928)

you need to throw the menu into a loop and not break out of it if it hits the default switch statement.

some thing like this?

import java.util.Scanner;
 
public class Test3
{  
    public static void main(String[] args)
    { 
        //intialize new scanner object  
        Scanner scanner = new Scanner(System.in);
 
        boolean repete = false;
        char charInput;
        String stringInput = "";
 
        do 
        {
            System.out.println("     Loan Caluclator");
            System.out.println("**************************");
            System.out.println("Calulate monthly (P)ayment");
            System.out.println("Calulate  total  (I)ntrest");
            System.out.println("Calulate  total  (L)oan");
            System.out.println("to exit program  (Q)uit\n");
            System.out.print("Select P,I,L or Q: ");
 
            stringInput = scanner.next();
            charInput = Character.toUpperCase(stringInput.charAt(0));
 
            switch(charInput)
            {         
                case 'P':
                case 'I':
                case 'L':
                    repete = false;
                break;
 
                //return Q to main so loop the whole program in main
                case 'Q':
                    System.out.println("Exiting program....\n");
                    System.exit(0);
                break;
 
                default:
                    System.out.println("No vaild entry.Run again using only P, I, T or Q\n");
                    repete = true;
            }
        }
        while(repete);
 
        System.out.println(charInput);        
    }
}

it works but my teacher says not to use flags. :confused:

This is not so much a language issue as it is a design issue. User input devices rarely, if ever, give you directly useful information. They provide binary patterns that indicate that this or that key was pressed, or this or that point on the screen was touched, or whatever. Your machine is not smart. It is merely a reactive device constrained by its hardware implementation.

Your machine has little, if any, utility, out of the box. Free (seemingly) utility comes from those who have decided, for whatever reason, to make your job easier.

When you press a key (if you have keys) on your device, it doesn't matter if that key is labeled "ENTER", or "BOINK MY WIFE". That key produces a binary output. Someone gets to determine what it means.

In the most common systems available to novices today (desktop/laptop machines), this is a two-step process, or more. The input device produces a code. The OS converts this code to some other code or sequence of codes. The language (which has to be defined, implementation-wise) for that OS converts this other code to some other code, or sequence of codes. The user of that language has to decide whether this latter code represents, say, text, or a number, or some control mechanism. Fortunately, for most users, someone else has done that. Your responsibility is to read what that someone else has said about how that code, so thoughtfully provided, works. Two people working for you may have different ideas.

User input is particularly invidious. If you're reading a file, you might have failures because of your own errors, or a corrupt file, or a broken mechanism. With human users, one has the added burden of dealing with inattention, stupidity, maliciousness, or a number of other things. You must know what you're writing, or what your utilities think of certain kinds of input, and how they deal with it.

Failing to contend with this situation is, at best, ignorance. At worst, it's negligence or overt malice. Such actions may subject you to no penalties whatsoever. They could conceivably subject you to rather harsh penalties meted out in a court of civil or criminal justice.

Budding professionals need to think about and deal with these things. Read your documentation with an eye for your concerns.

I took some time to review a few things and came up with this. I would like your comments and suggestions for making the code more effient and "better". thanks for the help so far.

/*
Author: *mrynit*
Date made: 1-20-07
last update: 1-22-07
Purpose: provide a menue based program to calculate various loan related values.
the main method loops untill user selects the exit command. there is error trapping
for bad menue opptions and rate that is not between 1% and 100%. all other inputs
are not tested for invalid input.
*/
 
import java.util.Scanner;      //console I/O
import java.util.Currency;     //dollar format
import java.util.Locale;       //get defual location info (US)
import java.text.NumberFormat; //numerical formating
 
 
public class Assignment1d//4 reversions latter...
{
    public static void main(String[] args)
    {
        displayMenu();//put the menu on screen       
        /*choice out here so its scope can be used in conditional of while
        the vaild user choice is passed to the local main var choice. this block
        is here to make the program loop*/
        char choice = getMenuChoice();
        while(choice != 'Q')
        {
            /*when loanCaculator() is finished the menu is put back on screen 
            and the user can run the program again or pick Q to exit*/
            loanCaculator(choice);
            displayMenu();
            choice = getMenuChoice();
        }
        System.out.println("\nGood-bye\n");
    }
 
    //displays the menu and prompt for menu choice
    //the method is void and accepts no parramaters
    public static void displayMenu()
    {
        System.out.println("     Loan Caluclator");
        System.out.println("**************************");
        System.out.println("P. Calulate monthly Payment");
        System.out.println("I. Calulate total Intrest");
        System.out.println("T. Calulate  toatl Loan");
        System.out.println("Q. Exit program\n");
        System.out.print("Select P,I,T or Q: ");
        //no new line b/c sacnner input creats a newline.
        //do this so input is on the same line as the prompt
    }
 
    /*this method gets the user menu choice thru scanner object.
    the method returns a vaild char of P,I,T or Q to the main method in the form
    of the char variable choice.*/
    public static char getMenuChoice()
    {
        //intialize new scanner object  
        Scanner scanner = new Scanner(System.in);        
 
 
        String stringInput = scanner.next();//get input as string
        //conver the above string into a single upper case char
        char charInput = Character.toUpperCase(stringInput.charAt(0));
 
        /*error trapping
        if charInput == Q then the while condition is false and the char 'Q' is
        returned to main. Else then it is looped until a true statment is found.
        when the ifs are true the char is returned to main. when none of the ifs
        are true then the user has to renter a menu choice. this choice is then
        tested running thru the while loop again untill a valid choice is given.
        This way traps only good input and ingore all other input. is there a 
        better to hanndle this?
        */
        while(charInput != 'Q')
        {
            if (charInput == 'P')
                return charInput;
            else if (charInput == 'I')
                return charInput;
            else if(charInput == 'T')
                return charInput;
            else
            {
                System.out.print("Select P,I,T or Q: ");
                stringInput = scanner.next();
                charInput = Character.toUpperCase(stringInput.charAt(0));
            }//else               
        }//while loop
        return charInput;//return user input as upper case char
   }//end getMenuChoice   
 
 
    /*This method accepts the user's VALID menue choice and asks for input for 
    amount, rate and term. note that all these vars are double b/c they under go
    floating point opps. the method then uses a swtich statment to pick the right
    method to call for calculation. the call statmetns in the switch print out
    the value of the returned methods*/
    public static void loanCaculator(char choice)
    {
        //intialize new US currency format
        NumberFormat nf = NumberFormat.getCurrencyInstance(Locale.US);
 
        //intialize new scanner object in here 
        Scanner scanner = new Scanner(System.in);           
 
        System.out.print("Enter the principle of the loan: ");
        double amount = scanner.nextDouble();
 
        //scope: rate must be out here to be used in while condition
        //do this to get rate inclusively between 1% to 100%
        double rate = 0.0;
        do
        {
            System.out.print("Enter the intrest rate between 1% and 100%: ");
            rate = scanner.nextDouble();
        }
        while(rate > 100.0 || rate < 1.0);
 
        System.out.print("Enter the term of loan: ");
        double term =scanner.nextDouble();
 
        System.out.print("The total cost is ");
 
        //call the right method and return its value in local currancey format
        //new lines are added for output formating
        switch(choice)
        {
            case 'P':
                System.out.println(nf.format(calcPayment(amount, rate, term)) + "\n\n");
            break;
 
            case 'I':
                System.out.println(nf.format(clacIntrest(amount, rate, term))+ "\n\n");
            break;
 
            case 'T':
                System.out.println(nf.format(calcTotalLoan(amount, rate, term))+ "\n\n");
            break;
        }                
    }
 
    //these calc methods are not accurate to the real thing... takl to teacher...  
    public static double calcPayment(double amount, double rate, double term)
    {
        return (amount * rate) / (1.0 - Math.pow((1.0 + rate), (-term)));
    }
 
    public static double clacIntrest(double amount, double rate, double term)
    {
        return term * calcPayment(amount, rate, term) - amount; 
    }
 
    public static double calcTotalLoan(double amount, double rate, double term)
    {
        return amount + clacIntrest(amount, rate, term);
    }
 
}

One key is to look at your logic carefully. You're hacking a path through the jungle over and over when your destination is always the same. Why not just,

public static char getMenuChoice()

{

    Scanner scanner = new Scanner(System.in);

    String stringInput;

    char cIn;    

    do

    {

        stringInput = scanner.next();

        cIn = Character.toUpperCase(stringInput.charAt(0));

        if ((cIn == 'P') || ((cIn == 'I') || ((cIn == 'L') || ((cIn == 'Q')) return cIn;

        System.out.print("Select P,I,T or Q: ");

    } while (true)

}