an encrypted program

hello fellows, i got new project for my c class but i have failed to do it. i would be very appreciated if you help to do this program.her it comes:

Sample Run 1:
input 1: coggvogcvvgpwpfgtdtkfig
output 1: ameetmeattenunderbridge

Sample Run 2:
input 2: oushpcaptfcanshohashfcghohwcb
output 2: agetbombfromzetatmetrostation

Sample Run 3:
input 3: vorjhzincjonziyhzydx
output 3: atwomenshotsendmedic

i am sending you the original project sheet paper :
C PROJECT

Lieutenant, we are catching encrypted messages from enemy lines. Our mathematicians solved the
encryption pattern. A message like:
coggvogcvvgpwpfgtdtkfig
ameetmeattenunderbridge
This is a modified version of Caesar encryption. First character is used to define SHIFT: distance of
first character to letter ‘a’. If all other characters are shifted with the same amount, message appears
(here, shift value is -2 (from letter ‘c’ to letter ‘a’)). Shifting is also cyclic, meaning that if we shift ‘c’ for -
(‘c’->‘b’->’a’->’z’->’y’)
Right now, our mathematicians are decrypting messages by hand with all their efforts but we need a C
program from YOU to rapidly decrypt all the incoming messages.
• Input will only consist of the encrypted message, and enter (‘\n’) at the end.
• Your output MUST only consist of decrypted message (see sample runs below) and one enter
(‘\n’) at the end
• See sample runs ABOVE for more details

İ WOULD BE REALLY APPRECATED İF ANY OF YOU HELP TO DO THİS PROJECT

THANX

No one here is going to do your homework/project/assignment for you!
Show your effort.

Quote:

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Originally Posted by Bayint Naung

No one here is going to do your homework/project/assignment for you!
Show your effort.

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i know that sir, i just want some help from experinced c programmers.

First, I'd code up a function or two to handle the Caesar shift. If you don't have a clue about that, Wikipedia has a good write up on it. (and lots of other algorithms).

Get that working well, and then add in your functions for the reading of the input and make sure your output meets the requirements, as well.

Right now, we can't help you much, because you haven't studied the problem, and produced questions that show what has you stumped, at the moment.

When you have detailed questions, about specific code or logic that we can see, then we can be much more helpful.

Yes, better hurry your effort too... It sounds like there is a war at stake.

On a sidenote I remember that when I was in college we had to write a very similar program in MIPS assembly language which was far more challenging than doing it in C. That's just to encourage you a bit :)

Quote:

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Originally Posted by makonikor

i know that sir, i just want some help from experinced c programmers.

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Simply pasting in the assignment text is not "asking for help", it's asking for someone to do it for you.

If you were actually interested in learning any of this, you'd start to write some code and then paste the code you're stuck on or have problems with in here, asking for specific help. THAT'S how you ask for help. What you do is ask for a service. (And, of course, if you want to pay for it, someone's guaranteed to want to help you :-)

And use the code-tag when posting code, so that it's easier to read and more tidy allover.

I am also trying to learn C programming, but so far i have not been very successful. The main reason is that i find the assignments in many C programming books very boring, and therefore fail to solve them. The projekt posted in this thread seemed very interesting, and i could not resist to try solving it. The program is properly not very well written, but i am quite happy with it. I did not create a separate function for input as i could not figure out to return a array pointer to main. The message can maximum contain 100 characters. I had alot of fun with solving the shift cyclic.

decrypt_message.c

Code:

---

#include <stdio.h>

#define MESSAGE_MAX_SIZE 100

void caesar_shift(char encrypted_message[MESSAGE_MAX_SIZE], int character_counter)
{

        int result = 0;
        int result_counter = 0;

        int shift = encrypted_message[0] - 'a';

        for(result_counter = 0; result_counter < character_counter; result_counter++)
        {
                result = encrypted_message[result_counter] - shift;

                if(result < 'a')
                {
                        result = ('z' + 1) - ('a' - result);
                }

                printf("%c", result);
        }

        printf("\n");
}


main()
{

        char encrypted_message[MESSAGE_MAX_SIZE];

        int character = 0;
        int buffer_overflow_check = 0;
        int character_counter = 0;

        while(character != '\n' && buffer_overflow_check < MESSAGE_MAX_SIZE)
        {
                character = getchar();

                encrypted_message[buffer_overflow_check++] = character;
                character_counter++;
        }

        caesar_shift(encrypted_message, character_counter);
}

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The program do have a small flaw, as an ekstra character is appended to the message:

Sample run 1:

Code:

---

coggvogcvvgpwpfgtdtkfig
ameetmeattenunderbridge"

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Does anyone know why this is happening?

If anyone has links to C learning projekts like the one in this thread, please post them.

Good effort, but it seems that you have a loop for the message logic, and yet, in Caesar shift function, you use another loop. Shouldn't one loop be controlling the message, and the other function just do the "shifting" of ONE letter?

That's my mantra these days - always keep your logic as simple as you can. If you don't need two loops (and you don't), then don't have two loops.

Also, just being picky, the assignment wants the newline, appended onto the end of the char array, not just printed out.

I like the way you found the relationship between the letters in the shifting, though.

Quote:

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Originally Posted by Adak

Good effort, but it seems that you have a loop for the message logic, and yet, in Caesar shift function, you use another loop. Shouldn't one loop be controlling the message, and the other function just do the "shifting" of ONE letter?

That's my mantra these days - always keep your logic as simple as you can. If you don't need two loops (and you don't), then don't have two loops.

Also, just being picky, the assignment wants the newline, appended onto the end of the char array, not just printed out.

I like the way you found the relationship between the letters in the shifting, though.

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Thanks for the critique, this is always a great way to learn something new.

I do have one while loop in main that fill the encrypted_message array with characters from an input source, and checking for newline and overflow of the buffer. My first thought was to create a function called get_message() that would handle the input, but as arrays was involved i could not get this to work, so i handle this in main instead. The for loop in the function caesar_shift() do the shifting and output of the result. With my current knowledge in C, to reduce the two loops to one i could put the whole thing in main, but when using functions i would not know how to use only one loop?

Regarding the newline, i do not read it as they want it appended to the array:

Code:

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Your output MUST only consist of decrypted message (see sample runs below) and one enter (‘\n’) at the end

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If they really wanted the newline appended to the array, i guess i could create an if statement that checked for the last character in the array, and the inserted a newline after that, but this would be more complicated that just printing it at the end.

It's great that you solved the problem but you shouldn't have posted your entire code here for the OP to prey on.

Quote:

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Originally Posted by claudiu

It's great that you solved the problem but you shouldn't have posted your entire code here for the OP to prey on.

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Maybe not the best idea to post it in full, but for others to come with suggestions why the ekstra character is appended to the result, they should see the code.

Good work and great attitude, kasj!

kasj: Here's a very good article explaining how to return arrays from functions. Or rather, how to work around it, since just returning an array from a function (which is easy in f.ex. Java, PHP and "newer" high-level languages) isn't possible in C. http://www.eskimo.com/~scs/cclass/int/sx5.html

By the way: I wholehartedly agree with your comment about the typical training assignments in most programming books. Problem is, when you write code for a living, most of the problems aren't too interesting in themselves then either :-)

Great article. I tried the examples and i must admit that it is very simple to "return" arrays, simply making the array static will be enough in some cases. I have tried to change my program with the new knowledge, i have used method two in the article by passing an array as an argument to my new function.

I have also solved the problem with the ekstra character that was appended to the output, at the solution was very simple. When entering a message and pressing enter, the variable character_counter vil of course count the newline, and then when function caesar_shift() prints out the array, an ekstra character will be displayed. So now i just substract 1 from character_counter before returning. The main() function is really clean now

decrypt_message.c

Code:

---

#include <stdio.h>

#define MESSAGE_MAX_SIZE 100

int get_message(int encrypted_message[MESSAGE_MAX_SIZE])
{

        int character = 0;
        int buffer_overflow_check = 0;
        int character_counter = 0;

        while(character != '\n' && buffer_overflow_check < MESSAGE_MAX_SIZE)
        {
                character = getchar();
                encrypted_message[buffer_overflow_check++] = character;
                character_counter++;
        }

        return character_counter - 1;
}

void caesar_shift(int character_counter, int encrypted_message[MESSAGE_MAX_SIZE])
{

        int result = 0;
        int result_counter = 0;

        int shift = encrypted_message[0] - 'a';

        for(result_counter = 0; result_counter < character_counter; result_counter++)
        {
                result = encrypted_message[result_counter] - shift;

                if(result < 'a')
                {
                        result = ('z' + 1) - ('a' - result);
                }

                printf("%c", result);
        }

        printf("\n");
}


main()
{
        int encrypted_message[MESSAGE_MAX_SIZE];

        caesar_shift(get_message(encrypted_message), encrypted_message);
}

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Output:

Code:

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coggvogcvvgpwpfgtdtkfig
ameetmeattenunderbridge

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