Raks-coder
diff --git a/‎FAT-Practice/.idea/workspace.xml
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diff --git a/‎FAT-Practice/out/production/FAT-Practice/ISBN.class
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diff --git a/‎FAT-Practice/out/production/FAT-Practice/InvalidInputException.class
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diff --git a/‎FAT-Practice/out/production/FAT-Practice/META-INF/FAT-Practice.kotlin_module
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diff --git a/‎FAT-Practice/out/production/FAT-Practice/Package1/NumberManipulation.class
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diff --git a/‎FAT-Practice/src/ISBN.java
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diff --git a/‎FAT-Practice/src/Package1/NumberManipulation.java
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diff --git a/‎README.md
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@@ -0,0 +1,31 @@
+import Package1.NumberManipulation;
+
+import java.util.Scanner;
+
+class InvalidInputException extends Exception{
+    InvalidInputException(String msg){
+        super(msg);
+    }
+}
+
+public class ISBN {
+    public static void main(String[] args) {
+        Scanner sc = new Scanner(System.in);
+        String is = sc.nextLine();
+        try{
+            if(is.length() != 9){
+                throw new InvalidInputException("ISBN must be exactly 9 digits");
+            }
+            else{
+                NumberManipulation n = new NumberManipulation();
+                Integer i = Integer.parseInt(is);
+                n.extractDigits(i);
+                System.out.println("Last digit = " + n.findLastDigit());
+                System.out.println("New ISBN = " + is + n.findLastDigit());
+            }
+        }
+        catch(Exception e){
+            System.out.println(e);
+        }
+    }
+}
@@ -0,0 +1,25 @@
+package Package1;
+
+public class NumberManipulation {
+    int i;
+    int arr[] = new int[9];
+
+    public void extractDigits(int num){
+
+        for(i = 8; i >= 0; i--){
+            arr[i] = num%10;
+            num = num/10;
+        }
+    }
+
+    public int findLastDigit(){
+        int n = 1, sum = 0;
+
+        for(i = 0; i < 9; i++) {
+            sum += arr[i]*n;
+            n++;
+        }
+
+        return sum;
+    }
+}
@@ -206,7 +206,10 @@ previous numbers, i.e., `T(n)=T(n-1)+T(n-2)+T(n-3), T(1)=T(2)=1, and T(3)=2`). [
 
 17. Student volunteers from three batches of B.Tech are selected to ensure the smooth conduct of a Technical event. Assume that only 4 volunteers from batch1, 3 volunteers from batch2, 1 volunteer from batch3 are selected. Create a 2-D ragged array of strings to store their register numbers batch wise. The first row stores register numbers of the volunteers in batch1, second row – batch2 and so forth. Use for-each loop to traverse the array elements.[Ans](https://github.com/jacobjohn2016/Java-Programming/blob/master/FAT-Practice/src/volunteers.java)
 
-18. 
+18. An ISBN (International Standard Book Number) consists of 10 digits d1d2d3d4d5d6d7d8d9d10. The last digit d10 is a checksum, which is calculated from the other nine digits using the following formula: `(d1 * 1 + d2 * 2 + d3 * 3 + d4 * 4 + d5 * 5 + d6 * 6 + d7 * 7 + d8 * 8 + d9 * 9)% 11`.
+    * If the checksum is 10, the last digit is denoted X according to the ISBN convention. For the input 013601267, the program should display 0136012671.
+    * Define a package ‘Package1’ with the class ‘NumberManipulation’ which contains an integer array (size 9) as its instance variable and two methods `extractDigits( )` and `findLastDigit( )`. The `extractDigits( )` method must take an integer number as argument and extract the individual digits of the number and store them in the 9-element integer array. The method `findLastDigit( )` should find out the 10th digit using the above formula and return it.
+    * Define a main class outside of the package created above and read the 9-digit number as a String. Throw a user defined exception "InvalidInputException" if the length of the input String is not exactly 9 digits and prompt the user to enter the input again. If valid, convert the input into integer and pass it to the method `extractDigits( )` and also invoke `findLastDigit( )` method which would return the last digit. If the last digit is 10, append ‘X’ to the original string else append the digit itself’ and display the ISBN number.
 
 ## Contributions
 * Initial Author - [jacobjohn2016](github.com/jacobjohn2016)