Below are the code. To fix the error, I simply rewrote the code in getFraction() method to den = Integer.parseInt(fracValue.substring(fracValue.indexOf("/") 1, fracValue.length())) , by adding 1. I have never seen or learn this during my courses and I just encounter this while doing project. I want to understand what the code did in num = Integer.parseInt(fracValue.substring(0, fracValue.indexOf("/"))) and den = Integer.parseInt(fracValue.substring(fracValue.indexOf("/"), fracValue.length())), we are converting the numerator to int and the num is all the values before the / and the den is all the values after the /. Am I right ? My second question is that why do we need to add 1 after the indexOf("/") ? Is it so we are taking the values after the /?

import java.util.Scanner;

public class FractionCalculator {
    public static Scanner input = new Scanner(System.in);

    public static void main(String[] args) {
        intro();

        while (true) {
            String operation = getOperation();
            Fraction frac1 = getFraction();
            Fraction frac2 = getFraction();
            Fraction result = new Fraction(1,1);
            String result2 = "";
            if (operation.equals("=")) {
                System.out.println(frac1 " " operation " " frac2 " is " frac1.equals(frac2));
            } else {
                if (operation.equals(" ")) {
                    result=frac1.add(frac2);
                } else if (operation.equals("-")) {
                    result=frac1.subtract(frac2);
                } else if (operation.equals("/")) {
                    if(frac2.getNumerator()==0) {
                        result2="Undefined";
                    } else {
                        result=frac1.divide(frac2);
                    }
                } else if (operation.equals("*")) {
                    if(frac2.getNumerator()==0) {
                        result2 = "Undefined";
                    } else {
                    result=frac1.multiply(frac2);
                    }
                }

            //print results
        }   if (result2!="") {// division and multiplication by zero is undefined
                System.out.println(frac1 " " operation " " "0" " = " result2);
            } else if (result.getNumerator()%result.getDenominator() == 0) {
                System.out.println(frac1 " " operation " " frac2 " = " (result.getNumerator()/ result.getDenominator()));
            } else {
                System.out.println(frac1 " " operation " " frac2 " = " result.toString());
            }
        }
    }

    public static void intro() {
        System.out.println("\nThis program is a fraction calculator");
        System.out.println("It will add, subtract, multiply and divide fractions until you type Q to quit.");
        System.out.println("Please enter your fraction in the form a/b, where a and b are integers.");
        for (int i=0; i<80; i  ) {
            System.out.print("-");
        }
    }

    public static String getOperation() {
        System.out.println("\nPlease enter an operation ( , -, /, *, = or \"Q\" to quit): ");
        Scanner input = new Scanner(System.in);
        String operation = input.nextLine();

        int x = 0;

        while (x == 0) {
            if (operation.equals(" ") || operation.equals("-") || operation.equals("/") || operation.equals("*") || operation.equals("=")) {
                x  ;
            } else if (operation.equalsIgnoreCase("q")) {
                System.exit(0);
            } else {
                System.out.println("Invalid input, enter valid operation ( , -, /, *, = or \"Q\" to quit)");
                operation = input.nextLine();
            }
        }
        return operation;
    }

    public static boolean validFraction(String input) {
        boolean valid;

        if (input.startsWith("-")) {
            input = input.replaceFirst("-",""); // or use 'input.substring("1", input.length())';
        }
        if (input.contains("-") || input.charAt(input.indexOf("/") 1)==('0') || input.contains(" "))  {
            valid = false;
        } else if (input.contains("/")) {
            input = input.replace("/", "");
        }
        if (input.matches("^[0-9] $") && input.length() > 0) {
                valid = true;
             } else {
                valid = false;
             }
        return valid;
    }

    public static Fraction getFraction() {
        System.out.println("Please enter a fraction (a/b) or integer (a): ");
        String fracValue = input.nextLine();

        //validate input
        while (!validFraction(fracValue)) {
            System.out.println("Please enter a fraction (a/b) or integer (a): ");
            fracValue = input.nextLine();
        }

        //convert to numerator, denominator
        int num = 0;
        int den = 0;
        if (fracValue.contains("/")) {
            num = Integer.parseInt(fracValue.substring(0, fracValue.indexOf("/")));
            den = Integer.parseInt(fracValue.substring(fracValue.indexOf("/"), fracValue.length()));
        } else {
            num = Integer.parseInt(fracValue);
            den = 1;
        }

        // return fraction
        Fraction fracConv = new Fraction(num, den);
        return fracConv;
    }
}

CodePudding user response：

substring(int start, int end) includes the character at start and excludes the character at end. Since the integer cannot be parsed with a / in it, you need to do fracValue.indexOf("/") 1 to get just the numerical part of the denominator.

CodePudding user response：

Firstly you have to understand your input if your input is String a = "6 7", it means your string length is 3, and alloted indexes are 0,1 and 2 where 0 index is '6', 1 index is ' ' and 2 index is '7' So, when you use a.substring(0, a.indexOf(" ")) it means you are saying

a.indexOf(" ") = 1 index

you should get a string including '0' index but not 1 index, because substring works with inclusive first param and exclusive second param.

In case of this a.substring(a.indexOf(" ") 1, a.length())

you don't want to include ' ' in your denominator, So you should not include 1 index, that's why you are adding ( 1) with indexof. So, by adding 1 you are saying only pick value from a.indexOf(" ") 1, i.e. 2 index, and length of string is 3, which means you will get string inclusive of 2 index, i.e 7

if your input is "6 7" in that case you should use 'trim()' before using Integer.parseInt.