(Vigenere)How to make sure the user input alpha?

Question

https://pastebin.com/giThivuC

The above is part of my work.I would like to know how to ensure the user input the alpha after ./vigenere ? Coz k = atoi(argv[1]) seems doesn't work. Besides, what is segmentation fault?

Answer 1

Unlike Caesar, the key is now a string, not an integer. Why is the code trying to convert argv[1] to an int with a call to atoi()?

A segmentation fault is a runtime error where the program is attempting to access an unauthorized area of memory. In this case, it's a common mistake by new programmers. The isalpha() function takes a single char and checks whether it is a letter or not. Now, remember that argv[ ] is an array of strings (actually an array of char arrays). The code above, isalpha(argv[1]) tries to shove an entire string down it's throat and it chokes, throwing a seg fault error. The code has to look at each individual character in the string, one at a time, using isalpha. Good place to use a for loop to check argv[1][x].

Finally a hint. Chars can be treated as unsigned, one byte integers. You don't have to convert them using atoi or any other function. You might want to do some basic math on them, depending on your logic, and that is allowed.

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Answer 2

Maybe you have to check if (isalpha(argv[1])) before of use atoi function,

You have at line 15:

k = atoi(argv[1]);
if (isalpha(k))
{
   ...
}

Try this way:

if (isalpha(argv[1]))
{
   k = atoi(argv[1]);
   ...
}

Segmentation fault indicates that you're trying to read/write on something that you shouldn't.