How Do I Rotate A Char Alphabetically By Integer X?

Question

I've been trying different solutions but am not sure where to look for the solution.

I prompt the user for "plaintext" and whatever their input is, the chars in their input need to be rotated alphabetically by a number (aka the key) which they provide.

For example: plaintext: HELLO would spit out ciphertext: IFMMP if the key were 1.

Assuming the key will always be a number, here is what my code looks like which attempts to rotate each char by Key: 1. I'm a real noob so please break it down is possible.

{
     string s = get_string("plaintext: ");                                    
     printf("ciphertext: %s\n", s + 1);
}   

The remaining code (which includes identifying and filtering out the key is:

#include <cs50.h>
#include <stdio.h>
#include <string.h>
#include <ctype.h>
#include <stdlib.h>

int main(int argc, string argv[])
{                
    int counter = 0;    
    if (argc == 2)
    {
        for(int k = 0; k <strlen(argv[1]); k++)
        {
            if (isdigit(argv[1][k]))
            {
                counter++;         
            }
        }   
        if (strlen(argv[1]) == counter)
        {
            string s = get_string("plaintext: ");
            if(s)
            {
                
                printf("ciphertext: %s\n", s + 1);
                free(s);
            }
        }   
        else
        {
            printf("Usage: ./caesar key\n");
        }
    }
    else
    {
        printf("Usage: ./caesar key\n");
    }                     

}

Any assistance would be appreciated.

Answer 1

You can achieve this with typecasting:

char input = 'f';
char output = (char)((int)input + 1);

You should also handle edge cases, i.e. when the character value is near the end of the alphabet.

Answer 2

I do not have cs50, so cannot test everything in your code, but addressing your primary question: How Do I Rotate A Char Alphabetically By Integer X, the code section you have identified as where the rotation occurs:

 if (strlen(argv[1]) == counter)
 {
      string s = get_string("plaintext: ");                                    
      printf("ciphertext: %s\n", s + 1);
 } 

But this section doesn't really rotate the text. It rather attempts to print the string obtained from the call to get_string after adding 1 to it. First, this is not modifying s at all. Second, s + 1 is not a legal operation. To rotate s, additional code is needed between those two lines.

If I understand get_string(), it is essentially a combination of printf(), fgets(,,stdin) and strdup(). So after the call to that function you are left with s which will be populated with whatever the user typed into stdin. For illustration, let say s contains "what the user typed". How that string is represented in memory, including the NULL terminator, can be depicted as follows:

 |w|h|a|t| |t|h|e| |u|s|e|r| |t|y|p|e|d|0|

To rotate this by X, as you have stated, each character needs to be modified by adding the value X, in this case 1

Showing ASCII equivalent values:

 |119|104|97|116|32|116|104|101|32|117|115|101|114|32|116|121|112|101|100|0|

The rotated string then would be each value + 1:
For the general case, assume rotation value is read in from command line argument and placed into int n = 0;

earlier in code:

if(isdigit(argv[2][0]))
{
    n = atoi(argv[2])   
}

The rotation: (the following is complete. It may need a little debugging around the corner case [where values wrap around the end of ASCII values when added to n.] Lines are commented indicating where I used ANSI C instead of CS50. [I do not have CS50])

int main(int argc, char *argv[])//note: command line requires a numeric
                                // argument be used:  prog.exe 12 (for n == 12)
{
    //char s[80];//used to test (I do not have cs50)
    char *rotated = NULL;
    int i;

    if(argc != 2) 
    {
        printf("Usage: prog.exe <positive int value.>\nExiting\n");
        return 0;
    }
    if(isdigit(argv[1][0]) == 0)
    {
        printf("Usage: prog.exe <positive int value.>\nExiting\n"); 
        return 0;
    }

    int n = atoi(argv[1]);
    string s = get_string("plaintext: ");
 //   printf("Enter text to be rotated\n");//used to test (I do not have cs50)
 //   scanf("%[^\n]", s);

   // string rotated = strdup(s); //preserve s by manipulating identical string
    rotated = StrDup(s);
    if(rotated)
    {   //advance value by n
        int origVal = 0;
        int len = strlen(rotated);
        for(i=0;i<len;i++)
        {
            origVal = rotated[i];
            if((rotated[i] + n) <= 127) 
            {
                rotated[i] += n;
            }
            else
            {
                rotated[i] = abs(127 - (origVal + n));
                if(rotated[i] == 0) rotated[i] += 23; //skip to printable characters (http://facweb.cs.depaul.edu/sjost/it212/documents/ascii-pr.htm)             
                //if(rotated[i] == 0) rotated[i] += 1; //skip only ASCII NULL value
            }       
        } 
        rotated[i] = 0; //adding NULL to end of string 
        printf("ciphertext: %s\n", rotated);
        free(rotated);
    }
    return 0;   
}