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thanks for the answer. I tried to use a for loop, like this,

 for (int i = 0; i <= argc; i++)
       {
           if (isalpha(*argv[1]))
           {
               printf ("Usage: ./caesar key\n");
               return 0;
           }
       }

but it didn't help. I think the main problem was what to put in the square brackets here (*argv[1]). Any ideas? Thanks in advance!

0

Why is argc even involved in this? argc is the number of arguments (each is a string) used to execute the program. argv[0] is the program name. argv[1] is the first string that follows, and so on. You need to understand that argv is an array of strings, not a single string (i.e., an array of chars.) So, if a program is executed with 4 parameters, then argc would be 5 and argv would be an array of 5 strings.

You need to use a for loop to step through the string argv[1] to check each char. To do that, you need to realize that argv is a 2 dimensional array and you're checking all the chars in one row. Think in terms of argv[1][i]. The for loop needs to run from 0 to strlen(argv[1]).

Hopefully, this explains it in enough detail that you can figure out how to code it.

Happy programming.

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