Pset2 Vigenere Segmentation fault, have read prev answers but still stuck

Question

I keep getting a seg fault when I run vigenere with a keyword, I think I may be using the isalpha() function incorrectly, please can someone take a look and give me some pointers pls? Below are a couple of lines of code to help get to the root of the problem:

int key = atoi(argv[1]);
char letr[key];

for (int c = 0, p = strlen(plaintext); c < p; c++)
{
for(int j = 0, k = strlen(argv[1]); j < k; j++)
    {
    if (isupper(letr[c]))
    {
    printf("%c", (plaintext[key] - 65 + key) % 26 + 65);

Pls also let me know if you spot any potential pitfalls with my formula.

Many thanks

Answer 1

The reason my seg fault still came up after making changes following @DinoCoderSaurus' advice regarding the atoi function was that I hadn't converted the string input to char when using isalpha to ensure the keyword entered only contained alphabetical characters.

To correct this, I used a for loop to iterate over and check each character. It was a Duh! moment

Thank you for your help @DinoCoderSaurus'

Answer 2

Actually, it's the atoi function here int key = atoi(argv[1]);. Since vigenere takes a "word" as the argument, it cannot convert it to an "integer".

From man atoi:

.... atoi() does not detect errors.

So atoi(argv[1]) is probably returning 0, so char letr[key]; is creating a "0 length" array. It's downhill from there.

You are on the right track. Think about using "ascii arithmetic" to transform the characters in argv[1] into what you want.

Before you get too far into this, you will (almost) always run into problems if you attempt vigenere with nested loops. Remember from the spec "Your program must only apply Vigenère’s cipher to a character in p if that character is a letter. ". So there is "skipping" involved. Basically you need to process each letter in the plaintext exactly once. If you have an inner loop that processes strlen(key) times, it will process each letter in the plaintext strlen(key) times.