I get "/opt/sandbox50/bin/run.sh: line 31: 304..." when I try to include j-- in my loop

Question

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

int main (int argc, string argv[])
{
    //checks if argc is 2
    if (argc != 2)
    {
        printf("ERROR\n");
        return 1;
    }

    string k = argv[1];
    string p = GetString();

    for (int i = 0, p_strlen = strlen(p); i < p_strlen; )
    {
        for (int j = 0, k_strlen = strlen(k); j < p_strlen; j++, i++)
        {
            int num_k_array = j % k_strlen;
            int vigenere = k[num_k_array];   

            if (isalpha(p[i]), isupper(p[i]))
            {
                int vigenere_key = vigenere - 65;
                int letter = p[i] - 65;
                int result = ((letter + vigenere_key) % 26) + 65;
                printf("%c", result);
            }   

            else if (isalpha(p[i]), islower(p[i]))
            {
                int vigenere_key = vigenere - 97;
                int letter = p[i] - 97;
                int result = ((letter + vigenere_key) % 26) + 97;
                printf("%c", result);
            }
            else
            {
                printf("%c", p[i]);
                j--;
            }
        }        
    }
    printf("\n");
    return 0;
}

Answer 1

I compiled your code fine. I ran it like this:

~/workspace/help/pset2/vigenere/Cristian $ ./vigenere aaaa
abcd efgh ijklmnop
abcd efgh ijklmnopSegmentation fault

I ran it again with gdb to see what was happening every step of the way. It all goes smoothly until the inner loop makes i reach p_strlen. Why? Because at that point j is behind if there are any special characters.

In the example run above

p_strlen = 18

when

i = 18

j is not:

j = 15

Because you are comparing against j on line 20 the loop keeps going. i gets incremented to 19 and you try to access memory you should not access. Therefore, you get a Segmentation Fault.

I suggest you compare against i on line 20 instead of j.

Your approach is very interesting. I had prejudice against your code as soon as I saw a loop inside of a loop for this program. But when I saw that you don't increment in the outer loop, I started to like it more and more. It is always amusing to see how many unique solutions can a problem have.

Awesome code!