I get Segmentation fault (core dumped) in Caesar

Question

I'm trying to solve the Caesar pset, and I think I'm mostly on the right way, but I just started getting the error "Segmentation fault (core dumped)". I've attached the code, hopefully someone can point out, what I did wrong.

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

char rotate(char c, int n);

int main(int argc, string argv[])
{
    // SAVE CLA AS VARIABLES
    string plaintext = get_string("plaintext: ");
    int key = atoi(argv[1]);
    string ciphertext = "";
    int length  = strlen(plaintext);

    for (int i = 0; i < length; i++)
    {
        ciphertext[i] = rotate(plaintext[i], key);
    }

    printf("%s\n", ciphertext);
}

char rotate(char c, int n)
{
    //test if c = key is in right range
    c = c + n;

    while (c > 122)
    {
        c = c - 122 + 64;
    }


    return c;
}

Answer 1

Your code shouldn't do:

string ciphertext = "";
int length  = strlen(plaintext);

Instead try:

int length = strlen(plaintext);
char ciphertext[length]; // you here create an array of size length

Remember that strings are arrays of chars, so its better practice to use chars than strings. The error appears because strings are literally references to arrays, so by creating a reference to nothing (""), you cannot then easily change the content writing in memory you aren't allowed to write.