Bool true/false for error checking

[gubhaus]

Hi, I am working on a lab assignment and I am having trouble with this one section. I need to write something that will print an error message and not run the program if the user enters anything besides lowercase x's and o's. This is what I have so far

 // Check board contains only x's and o's
   bool valid = true;
   //TBA

   if (!valid)
   {
      cout << "Sorry, you can only enter x's and o's\n";
      exit(1);
   }

Any help is appreciated. Thanks!

[Chervil]

The comment here,

// Check board contains only x's and o's 

suggests that you have some sort of array or structure where the board data is stored, and that you want to check the entire board?

Or do you really just want to check the last character entered by the user?

As a matter of user-friendly design, it might be a good idea to allow the user to enter either upper or lower case characters, and convert the input to lower case just after it has been input, before the validation.

http://www.cplusplus.com/reference/cctype/tolower/

[gubhaus]

It's an array for a tic tac toe game. Here's the entire code.

#include <cstdlib>
#include <iostream>
using namespace std;

int main()
{
   // Declare 2D array
   const int SIZE = 3;
   char board[SIZE][SIZE];

   // Read x's and o's
   cout << "Enter x's and o's on board (L-R, T-B): ";
   for (int r = 0; r < SIZE; r++)
      for (int c = 0; c < SIZE; c++)
         cin >> board[r][c];

   // Print 2D array
   cout << "\n+---+---+---+\n";
   for (int r = 0; r < SIZE; r++)
   {
      cout << "| ";
      for (int c = 0; c < SIZE; c++)
         cout << board[r][c] << " | ";
      cout << "\n+---+---+---+\n";
   }

   // Check board contains only x's and o's
   bool valid = true;
   if (!valid)
   {
      cout << "Sorry, you can only enter x's and o's\n";
      exit(1);
   }

   // Check first diagonal to see who wins
   char winner = ' ';
   if ((board[0][0] == board[1][1]) &&
       (board[1][1] == board[2][2]))
      winner = board[0][0];

   // Check second diagonal to see who wins

   if ((board[0][2] == board[1][1]) &&
       (board[1][1] == board[2][0]))
      winner = board[0][0];


   // Check rows to see who wins
   for (int r = 0; r < SIZE; r++)
      if ((board[r][0] == board[r][1]) &&
          (board[r][1] == board[r][2]))
         winner = board[r][0];

   // Check columns to see who wins
   for (int c = 0; c < SIZE; c++)
      if ((board[c][0] == board[c][1]) &&
          (board[c][1] == board[c][2]))
         winner = board[c][0];


   // Print winner
   if (winner != ' ')
      cout << "Congratulations " << winner << " is the winner\n";
   else
      cout << "Sorry, no one wins\n";
   return 0 ;
}

I also need to fix the column section of the code but it's not my priority right now. On normal circumstances I would use the upper to lower idea but because this is just a lab and I'm not concerned with it being perfect.

[Chervil]

Well, if you wanted to check the entire array, you'd need a nested for loop (similar to the existing loop) and a check something like this:

    if (board[r][c] != 'o' && board[r][c] != 'x')
    {
        valid = false;
    }

That if statement is quite tricky because it is checking for two possible values, and a common mistake is to put a logical or || in the middle. But because you are checking for not-equal, the logical and && is needed.

Another way to write it is like this, with an extra set of parentheses:

    if ( !(board[r][c] == 'o' || board[r][c] == 'x') )
    {
        valid = false;
    }

[gunnerfunner]

Chervil - this was a great point, I was doing exactly(!) the same and v nicely put

That if statement is quite tricky because it is checking for two possible values, and a common mistake is to put a logical or || in the middle. But because you are checking for not-equal, the logical and && is needed.