Vigenere segmentation fault!

Question

My code compiles but when I run it I get the error: Segmentation fault (core dumped). I looked at similar questions to this, but I think I'm using isalpha correctly. Any insight will be appreciated. (As an aside, I find my code highly unreadable. If anyone has any tips for me regarding readability and simplicity that would be great!) Here's my code:

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


int cipher(int x);
string k;
string s;

int main(int argc, string argv[]){

    if (argc != 2){
        printf("I can't do that.  argc must be 2.\n");
        return 1;
    }

    string k = argv[1];

    for (int i = 0, n = strlen(k); i < n; i++){
        if (!isalpha(k[i])){
            printf("Vigenere key has non-alphabetical characters.\n");
            return 1;
        }

    }

    string s = GetString();

    for (int i = 0, n = strlen(s); i < n; i++){
        printf("%c", cipher(s[i]));
    }

    printf("\n");

}


int pos = 0;

int cipherlow(int s);
int cipherupp(int s);


int cipher(int s){
    int j = strlen(k);

    if(isalpha(s)){
        //get pos in keyword
        pos = pos + 1;
        pos = pos % j;

        if(islower(s)){
        //print: pos in keyword * shift + s[i]
            printf("%c", cipherlow(s));
        }
        else{
            printf("%c", cipherupp(s));
        }
    }
    else{
        //just print s[i]
        printf("%c", s);
    }


    return 0;
}

int cipherlow(int s){
    int p = k[pos] + s - 97;
    p = p % 26 + 97;
    return p;
}

int cipherupp(int s){
    int p = k[pos] + s - 65;
    p = p % 26 + 65;
    return p;
}  

Answer 1

This is what happens when you redeclare a variable, and a good example of why global variables can cause problems.

Here's what's happening. You first declare string k; as a global var outside of main and all of the other functions. Next, you declare it again inside of main with string k = argv[1];. Later, you call cipher() and the first line inside of cipher is int j = strlen(k);. Unfortunately, since you are no longer inside of main(), the k that points to argv[1] is out of scope and cannot be accessed here. Instead, cipher() is using the global k, which is currently = null. So, when you try to get strlen(k), you get a seg fault.

There is no need to use global variables in this pset. You should try to recode your work to eliminate them.

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