Why am I getting segmentation error in a Caesar cipher program?

Question

I'm trying to write a program that uses Caesar's algorithm to cipher a string input. I'm a beginner to C but I can understand basic codes. So to cipher the text I wrote this code, but when I enter the input, I get an error that says

Segmentation fault (core dumped)

I tried to do some debugging by removing the else condition at the end and the program kind of worked for short inputs of 2-3 letters

Can someone please help me with this issue?

I'm using the CS50's header only to get the string in the first place.

#include <stdio.h>
#include <cs50.h>
#include <string.h>
#include <ctype.h>
#include <stdlib.h>
int main(int argc, char * argv[])
{

    char name[] = "";
    strcat(name, argv[1]);
    int key = atoi(name);
    string plaintext = get_string("plaintext: ");
    int length = strlen(plaintext);
    char ciphertext[] = "";
    for(int i = 0; i < length; i++)
    {
        int skipCount = 0;
        if(isalpha(plaintext[i]))
        {
            while(skipCount < key)
            {
                char tmp = (char) ((int) plaintext[i] + 1);
                if(isalpha(tmp))
                {
                    ciphertext[i] = tmp;
                    skipCount++;
                }
                else
                {
                    if (isupper(plaintext[i]))
                    {
                        tmp = 'A';
                        skipCount++;
                    }
                    if (islower(plaintext[i]))
                    {
                        tmp = 'a';
                        skipCount++;
                    }
                }
            }
        }
        else ciphertext[i] = plaintext[i];
    }
    printf("%s\n", ciphertext);
}

Answer 1

What you need to understand about C, is that it does not automatically allocate memory.

You have to it your self!

This line:

char name[] = "";

creates an array of size 1, which holds a single character - the "null" character = '\0';

It signifies an empty string.
You can not copy any larger string in to it, because all strings in C must have a null character at the end, so there isn't enough room for even a single readable character.

As a beginner, you would need to decide what the maximum length of the string you want will be, and declare the array to be of proper size:

char name[255];

This is one example that can hold up to 254 characters, plus the terminating null character.