Pset 2 Vigenere - User input validation (using for loop inside of 'else if' )?

Question

I am stuck on validating the user input for the key phrase in vigenere (there aren't a lot of questions on here about that, so I am guessing I am missing something rather obvious...).

I was trying to use !isalpha() and after learning that I cannot pass a string into the function, I tried using a for loop to iterate through the char's in the key phrase. However, my program won't compile, giving me the error statement:

"expected expression"

(with the little green ^ pointing at the beginning of the for loop).

My suspicion is that I've got the syntax wrong, but I have not been able to find any resource on using for loops as a condition inside an if statement (maybe this just does't work?).

Looking at my code, can anyone give me a hint as to why I am seeing this error or point to toward a more elegant solution for user input validation?

Many thanks!

vigenere.c

int main (int argc, string argv[])
{
    // check user input
    if (argc !=2)
    {
        printf("please follow command ./vigenere with a key word (letters only)\n");
        return 1;
    }

    else if (
        for (int h = 0, n = strlen(argv[1]); h < n; h++)
        {
            if ( !isalpha(argv[1][h]) )
        }
    )
    {
        printf("please follow command ./vigenere with a key word (letters only)\n");
        return 1;
    }

Answer 1

You haven't structured the statements correctly. an IF statement must have the following structure:

if( <test condition> )  <single statement or code block in curly braces> ;

You have an opening parenthesis followed by a for statement instead of a test condition and a closing parenthesis. Then you have another if statement with a test condition, but no closing semicolon or paired curly braces.

You would benefit greatly from reviewing the class lectures and shorts on constructing if statements, for loops, and the use of parentheses and curly braces.

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