I have a segmentation fault in Caesar in pset 2

Question

Okay, first time writing here and I am a complete noobie. The program compiles and I have run the debugger, and it seems that I have a segmentation fault which I have marked. Also, I have used a lot of comments to explain my logic, so please forgive me if it looks messy. Please help!!

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

int k; // Global variable

int main(int argc, string argv[]) // We need a command line argument, and argc counts the number of 
                                  // words in input and string argv stores them
{   
   if (argc != 2) // we need two command line arguments so anything else should prompt the user to 
                  // enter in the correct format,ie, ./caesar key, where key is the second 
                  // commandline argument 
   {
      printf("Usage: ./caesar key\n"); 
      return 1; // Terminates the program prematurely.
   }
    if (argc == 2) //when the user inputs correctly
    {
       if (isdigit(argv[1]) == 0) // the second commandline argument that is the key, should be 
                                  // purely numeric. this checks for that. returns a boolean value, 
                                  // true / false or 1 / 0
       {
          printf("Usage: ./caesar key\n");// if it is false, the program is terminated with the 
                                          // message again
          return 1; // same as before
       }

segmentation fault starts here

       if (isdigit(argv[1]) == 1) // the condition if key is numeric, which is what we want
       {
           k = (int) argv[1]; //stores the key
           string plaintext = get_string("Plaintext: "); // Gets the plaintext from user as a string 
           int strlength = strlen(plaintext); // stores the length of the string as an integer
           char ciphertext[strlength]; // declaring the cipher text
           for (int i = 0; i <= strlength; i++) // for loop for encoding with key
           {
              char a = plaintext[i]; // stores the value of the i th index of plaintext, such that 
                                     // plaintext itself does not get modified as it is an array
              if (isalpha(a) == 1) // sees wheter the coresponding index , now stored in a is a part 
                                   // of the alphabet, returns true if yes
              {
                 if (isupper(a) == 1) //sees whether the index is upper case character, returns true 
                                      // if yes  
                 {
                    char b = a; // storing a in b, such that a does not change
                    b -= 65; //converting ASCII value of block letters to Alphabetical index, ie , 0- 
                             //25, so that they can get encrypted
                    b = (b + k) % 26; //encryption with key k
                    b += 65; //converting back to ASCII
                    ciphertext[i] = b; //storing encrypted index in ciphertext
                 }

                 if (islower(a) == 1) //same story as above, checking and encrypting for lower case
                 {
                     char c = a;
                     c -= 97;
                     c = (c + k) % 26;
                     c += 97;
                     ciphertext[i] = c;
                 }
              }
              if (isalpha(a) == 0) // we do not want to encrypt space or special characters like .,;@ 
                                 //etc or numbers, so we us isalpha to stay clear of them
              {
                 ciphertext[i] = a; // storing the special characters as it is.
              }
          }   
        printf("\nCiphertext: %s\n", ciphertext); //printing out ciphertext
        return 0; // Command to terminate the program at the right moment
    }
 }
}

Answer 1

A segmentation fault occurs when you try to access memory that "doesn't belong to you". In your case, in the following for loop:

for (int i = 0; i <= strlength; i++) // for loop for encoding with key

because you set the loop to run until i = strlength, you'll end up accessing ciphertext[strlength], but that value does not exist.

To elaborate with an example, let's use strlength = 3.

Your program creates an array of size 3 when you use char ciphertext[strlength];.

Thus, you can only access ciphertext[0], ciphertext[1] and ciphertext[2] (3 items, starting from index 0). However when you try to access ciphertext[i] where i = strlength, then you'll be attempting to access ciphertext[3], which is why the segmentation fault occurs.

Hope this helps and do mark this as an answer if it does!

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Side note: You might want to look up the documentation for isdigit(), otherwise your current program might run into another problem after fixing the segmentation fault!