Reject multiple column letters in get_input

[EricccTaiwan]

This update adds validation to reject inputs with multiple column letters, such as "aa", which were previously treated as out of bounds instead of being explicitly rejected.

Additionally, a comment was updated for clarity, changing "row number" to "row numbers" for consistency. #38

Before

$ ./ttt
 1 |             
 2 |             
 3 |             
 4 |             
---+-------------
      A  B  C  D
X> a5
Invalid operation: index exceeds board size
X> aa
Invalid operation: index exceeds board size
X> aa5
Invalid operation: index exceeds board size
X> d5
Invalid operation: index exceeds board size
X> dd
Invalid operation: index exceeds board size
X> ^C
$

After

$ ./ttt
 1 |             
 2 |             
 3 |             
 4 |             
---+-------------
      A  B  C  D
X> a5
Invalid operation: index exceeds board size
X> aa
Invalid operation: multiple column alphabets detected
X> aa5
Invalid operation: multiple column alphabets detected
X> d5
Invalid operation: index exceeds board size
X> dd
Invalid operation: multiple column alphabets detected
X> ^C
$

[jserv]

Thank @EricccTaiwan for contributing!

[visitorckw]

I'm quite late to respond. But the isalpha(line[i]) check seems to be checked twice in the same path, so it looks like a redundant condition to me?

[EricccTaiwan]

I'm quite late to respond. But the isalpha(line[i]) check seems to be checked twice in the same path, so it looks like a redundant condition to me?

Inputs like aa previously showed "index exceeds board size," which wasn’t accurate since the issue was multiple column letters. Also, isalpha(line[i]) is checked twice, which might be redundant, there could be a better way to handle this. I'll look into it!