How do I check for symbols using isalpha?

Question

I have an issue with checking for non alpha symbols

if(check n for alpha)
 {
    if check for lower symbols

        //statement

    else check for upper symbol
        //statement
   print (a)
 }
   else (n is non alpha)
      should print (n)

a is my encrypted text and n is the text that user write.
I get output like this

jharvard@appliance (~/Dropbox/pset2): ./vigenere baz
worldsayhello
xoqmdrbygflkp
jharvard@appliance (~/Dropbox/pset2): ./vigenere baz
world say
xoqmd tas

Answer 1

Per the manual page for isalpha()

isalpha()
checks for an alphabetic character; in the standard "C" locale, it is equivalent to (isupper(c) || islower(c)).

In other words, isalpha() basically returns true if the passed in char is an alphabetical character. Otherwise it returns false. Meanwhile,

if (isalpha(c))
{
    // c is alphabetical
    if (isupper(c))
    {
        // c is uppercase
    }
    else
    {
        // c is lowercase
    }
}
else
{
    // c is non-alphabetical
}

Also, how can I check again and again the length of my word and the text? If the text is longer than the word, I get an error.

Calculating the index of the current char in the keyword using the remainder operator is one way of achieving that. Given the expression a % b where a and b are integers, mathematically, the result would be >= 0 && < b.

Since we're supposed to increase the index of the current char in the keyword by one as we encipher the current plain character in the plain text, if the length of the number of plain characters in the plain text is greater than the length of the keyword, eventually, the index of the current char in the keyword will become equal to (and exceed, if we continue) the length of the keyword and we'd wind up with invalid index(es).

Taking the remainder of dividing the index of the current char in the keyword by the length of the keyword will wrap the index back to 0 once it's equal to the length of the keyword.

---

Edit #0: after looking at the code, I could find the following problems

1. In general, it's not a good practice to have something like

   if (condition)
   {
       // do nothing
   }
   else
   {
       // do something
   }
   

   you could directly have something like that instead

   if (!condition)
   {
       // do something
   }
   

2. The cases that should be covered are

   a. if the current character in the plain text is an uppercase char

   • if the current character in the keyword is an uppercase char

   • if the current character in the keyword is a lowercase char

   b. if the current character in the plain text is a lowercase char

   • if the current character in the keyword is an uppercase char

   • if the current character in the keyword is a lowercase char

   what you're covering though is

   a. if the current character in the plain text is an uppercase char

   • if the current character in the keyword is an uppercase char

   b. if the current character in the plain text is a lowercase char

   • if the current character in the keyword is a lowercase char

   c. if the current character in the plain text is a lowercase char OR (the current character in the keyword is an uppercase char OR a lowercase char)

   Now, you claim that your cases work well, but they're really difficult to read and understand.

3. You shouldn't calculate the index of the current char in the keyword in terms of i because it always increases whether the current char in the plain text is an alphabetical char or not. Rather, you should calculate it in terms of j.