How Do I Rotate A Char Alphabetically By Integer X? - c

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

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;
}

Answer 2

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.