Vigenere cipher in cs50 Segfau [closed] - c

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I am not experienced coder.Please dont judge for bad styling.I've run into an issue with the vingenere cipher
It keeps giving me segfau
#include <stdio.h>
#include <cs50.h>
#include <ctype.h>
#include <stdlib.h>
#include <string.h>**
int main(int argc,string argv[])
{
while (argc!=2)
{
printf("Incorrect input\n");
return 1;
}
printf("plaintext:");
string plaintext=get_string();
string num=argv[1];
int keylength=strlen(num);
for (int j=0, n=strlen(plaintext); j<n; j++)
{
char letter=plaintext[j];
if (isalpha(letter))
{
This is the reason of Segmantation Fault error.
If i don't check if num is upper or lower case,
code runs normally.
if (isupper(letter))
{
if (isupper(num))
{
printf ("%c",((((letter-65)+(num[j%keylength]-65))%26)+65));
}
else
{
printf ("%c",((((letter-65)+(num[j%keylength]-97))%26)+65));
}
}
else
{
if (isupper(num))
{
printf ("%c",((((letter-97)+(num[j%keylength]-65))%26)+97));
}
else
{
printf ("%c",((((letter-97)+(num[j%keylength]-97))%26)+97));
}
}
}
else
{
printf ("%c",letter);
}
}
printf("\n");
return 0;
}

Answer 1

Most likely, you meant
if (isupper(num[j % keylength))
Passing a string (really a char *) to isupper() is an undefined operation, and may produce weird effects depending on how isupper() is implemented. The GNU implementation ---
#define __ctype_lookup(__c) ((__ctype_ptr__+sizeof(""[__c]))[(int)(__c)])
#define isupper(__c) ((__ctype_lookup(__c)&(_U|_L))==_U)
--- uses a lookup table with all sorts of fun pointer math and casting under the hood, so it likely allows a char * to be passed without the compiler complaining, but it's definitely not going to produce good results. Change your if statement, and it should work better.