ISC COMPUTER SCIENCE PRACTICAL Solved ~ 2004

The computer department of the Agency of International Espionage is trying to decode intercepted messages.

The agency's spies have determined that the enemy encodes messages by first converting all characters to

their ASCII values and then reversing the string. For example, consider A_z ( the underscore is just to hightlight the space).

The ASCII values of A, <space>, z are 65,32,122 respectively.

Concatenate them to get 6532122, then reverse this to get 2212356 as the coded message.

Write a program which reads a coded message and decodes it.

It will contain only alphabets (A......Z, and a........z) and spaces.

ASCII values of A.......Z are 65........90 and those of a.....z are 97......122.

Test your program for the following data and some random data.

SAMPLE DATA:

INPUT:

Encoded Message: 2312179862310199501872379231018117927

OUTPUT:

The Decoded Message: Have a Nice Day

The idea behind the program is to store the Encoded message in a String.

Reverse the string and extract the individual characters and convert it 

into its corresponding integer values.

So, if input is 2212356 we will reverse it to get
6532122

Now the integer values 6,5,3,2,1,2,2 will be stored in some array;

To get the integer value of the character '6' (say) we need to subtract 48 from it.

since 54 is the ascii value of '6'. thus 54-48 = 6.

This has been achived by the following code

 public void reverseString() {

        int j = 0;

        for ( int i = Decryptmsg.length()-1; i >= 0; i--) {

              codeValues[j++] = Decryptmsg.charAt(i)-48; // for extracting the integer value at the i-th position from the String msg in

            }                                          // in reverse order

        }  

Now the idea is to extrac the ascii values from the array and print the 

corresponding character

For example, we need to extrac 65, 32 and 122 from the array.

For this note that the ascii values of characters ranges from 65 to 90

and 97 to 122 and one extra value of 32 for the space.

So we will take the first 3 values from the integer, 6 5 and 2 and convert

it into decimal number by 6*10*10+5*10+2=652

but this is outside our range, so we need to take only 6 and 5 and convert it

to get 65. then print the corresponding character. For this we have used the code

while ( i < Decryptmsg.length() ) {

              if (i < Decryptmsg.length() && i+1 < Decryptmsg.length() && i+2 < Decryptmsg.length())

                     tmp = codeValues[i]*10*10+codeValues[i+1]*10+codeValues[i+2];

              else

                     tmp = codeValues[i]*10+codeValues[i+1];

              if (tmp >= 97 && tmp <=122) {

                     System.out.print((char)tmp); // prints the character for the corresponding ascii

                     i +=3;

                     }

              else {

                     tmp = codeValues[i]*10+codeValues[i+1];

                     System.out.print((char)tmp); // prints the character for the corresponding ascii

                     if(tmp==32)

                           System.out.print("");

                     i +=2;

                     }

              }

Below is the link of solved ISC Computer Science Practical Paper year wise.

ISC COMPUTER SCIENCE PRACTICALS SOLVED YEAR WISE 

JAVA CODE

import java.util.*;

import java.io.*;

public class ISC2004Q1 {

    public static void main(String[] args) throws IOException

        {

        String msg;

        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));   

        System.out.println(" Enter the sentence (Encoded Message):");

        msg = br.readLine();   

        Decrypt ob = new Decrypt(msg);

        ob.reverseString();

        System.out.println("OUTPUT The Decoded Message: ");

        ob.decryptCode();       

        }

    }

class Decrypt {

    Decrypt(String msg) {

        Decryptmsg = msg;

        codeValues = new int[Decryptmsg.length()];

        }

    public void reverseString() {

        int j = 0;

        for ( int i = Decryptmsg.length()-1; i >= 0; i--) {

              codeValues[j++] = Decryptmsg.charAt(i)-48; // for extracting the integer value at the i-th position from the String msg in

            }                                          // in reverse order

        }   

    public void decryptCode() {

       int tmp,i=0;

        while ( i < Decryptmsg.length() ) {

              if (i < Decryptmsg.length() && i+1 < Decryptmsg.length() && i+2 < Decryptmsg.length())

                     tmp = codeValues[i]*10*10+codeValues[i+1]*10+codeValues[i+2];

              else

                     tmp = codeValues[i]*10+codeValues[i+1];

              if (tmp >= 97 && tmp <=122) {

                     System.out.print((char)tmp); // prints the character for the corresponding ascii

                     i +=3;

                     }

              else {

                     tmp = codeValues[i]*10+codeValues[i+1];

                     System.out.print((char)tmp); // prints the character for the corresponding ascii

                     if(tmp==32)

                           System.out.print("");

                     i +=2;

                     }

              }

       }   

    private String Decryptmsg;   

    private int[] codeValues;

    }