Check that program was run with one command-line argument. Iterate over the provided argument to make sure all characters are digits. Convert that command-line argument from a string to an int.
code :
int main (int argc , string argv[] ) { if ( argc != 2) { printf("Usage: ./caesar key"); return 1; } else if ( isalpha(argv1)) { printf("Usage: ./caesar key"); return 1; } else{ int key = atoi( argv1); return 0; } }