checking for invalid move

[trogne]

I don't understand what he's doing for an invalid tic-tac-toe move at the end.
It is not working. I get "Invalid move" but I cannot enter a new move.
Why "getch()" ?
What is he trying to do ?

#include <stdio.h>
#include <stdlib.h>
#include <conio.h>

char square[10] = { 'o', '1', '2', '3', '4', '5', '6', '7', '8', '9' };

int choice, player;

int checkForWin();
void displayBoard();
void markBoard(char mark);

int main()
{
  int gameStatus;

  char mark;

  player = 1;

  do {
    displayBoard();

    // change turns
    player = (player % 2) ? 1 : 2;

    // get input
    printf("Player %d, enter a number: ", player);
    scanf("%d", &choice);

    // set the correct character based on player turn
    mark = (player == 1) ? 'X' : 'O';

    // set board based on user choice or invalid choice
    markBoard(mark);

    gameStatus = checkForWin();

    player++;

  } while (gameStatus == -1);

  if (gameStatus == 1)
    printf("==>\aPlayer %d win ", --player);
  else
    printf("==>\aGame draw");

  return 0;
}

/*********************************************
FUNCTION TO RETURN GAME STATUS
1 FOR GAME IS OVER WITH RESULT
-1 FOR GAME IS IN PROGRESS
O GAME IS OVER AND NO RESULT
 **********************************************/
int checkForWin()
{
  int returnValue = 0;

  if (square[1] == square[2] && square[2] == square[3]) {
    returnValue = 1;
  } else if (square[4] == square[5] && square[5] == square[6])
    returnValue = 1;

  else if (square[7] == square[8] && square[8] == square[9])
    returnValue = 1;

  else if (square[1] == square[4] && square[4] == square[7])
    returnValue = 1;

  else if (square[2] == square[5] && square[5] == square[8])
    returnValue = 1;

  else if (square[3] == square[6] && square[6] == square[9])
    returnValue = 1;

  else if (square[1] == square[5] && square[5] == square[9])
    returnValue = 1;

  else if (square[3] == square[5] && square[5] == square[7])
    returnValue = 1;

  else if (square[1] != '1' && square[2] != '2' && square[3] != '3' &&
           square[4] != '4' && square[5] != '5' && square[6] != '6'
           && square[7] != '7' && square[8] != '8' && square[9] != '9')
    returnValue = 0;
  else
    returnValue = -1;

  return returnValue;
}

/*******************************************************************
FUNCTION TO DRAW BOARD OF TIC TAC TOE WITH PLAYERS MARK
 *******************************************************************/
void displayBoard()
{
  system("cls");                // clear screen, de stdlib.h

  printf("\n\n\tTic Tac Toe\n\n");

  printf("Player 1 (X)  -  Player 2 (O)\n\n\n");

  printf("     |     |     \n");
  printf("  %c  |  %c  |  %c \n", square[1], square[2], square[3]);

  printf("_____|_____|_____\n");
  printf("     |     |     \n");

  printf("  %c  |  %c  |  %c \n", square[4], square[5], square[6]);

  printf("_____|_____|_____\n");
  printf("     |     |     \n");

  printf("  %c  |  %c  |  %c \n", square[7], square[8], square[9]);

  printf("     |     |     \n\n");
}

/***************************************
set the board with the correct character,
x or o in the correct spot in the array
****************************************/
void markBoard(char mark)
{
  if (choice == 1 && square[1] == '1')
    square[1] = mark;

  else if (choice == 2 && square[2] == '2')
    square[2] = mark;

  else if (choice == 3 && square[3] == '3')
    square[3] = mark;

  else if (choice == 4 && square[4] == '4')
    square[4] = mark;

  else if (choice == 5 && square[5] == '5')
    square[5] = mark;

  else if (choice == 6 && square[6] == '6')
    square[6] = mark;

  else if (choice == 7 && square[7] == '7')
    square[7] = mark;

  else if (choice == 8 && square[8] == '8')
    square[8] = mark;

  else if (choice == 9 && square[9] == '9')
    square[9] = mark;
  else {
    printf("Invalid move ");

    player--;
    getch();                    // conio.h
  }
}

[trogne]

I see that my problem happens when I select a letter instead of a number.

When I do that, the next "scanf("%d", &choice);" does not prompts the user and gets the last value used.

Seems that scanf accepts the character from getch without prompting the user a second time ?

[laserlight]

I see that my problem happens when I select a letter instead of a number.

When I do that, the next "scanf("%d", &choice);" does not prompts the user and gets the last value used.

Seems that scanf accepts the character from getch without prompting the user a second time ?

What happens is that the standard input stream enters a failed state. You need to clear the state and remove the bad input.

Instead of this, another option when dealing with such line-based input to read the entire line into a string then parse the string. This way, if the input is invalid, you just discard the string and move on to read the next line entered.

[trogne]

thank you !