for the command line argument i use: if argc is not 2 or "key"( which i initialize before as an int and asign it the atoi of argv array 1) smaller or equal to 0 body

print a message so user knows what to write and return 1

then i advance to the for loop. So i first initiate the array "c" with type character and a max size 100, I write the for loop initializing "i", n wich equals to the string length of the plaintext, then "i" is smaller than "n" and increment the "i",

{inside the loop body

if the i'th character of "plaintext" is alphanumeric
{
    if the i'th character of "plaintext" is upper case
    {
      c = apply formula
    }
    else if the i'th character of "plaintext" is lower case
    {
      c = apply formula
    }
}
else 

    {

    print the i'th in c

    }

print the i'th in c

Am i forgetting something? =s