Tic-Tac-Toe user input looping endlessly when alphabetical character entered - c

Question

So I am making a tic tac toe game in C and I am having a problem where I am asking the user to undo their move by pressing 2. Each time it is their turn, they can either undo a move or continue to select there next turn by selecting another key.
It works how it should but I have a problem where if the user enters an alphabetical character, the program just repeatedly loops through saying that this is not a number when it checks my GetHumanMove function. Does anyone know how to make it stop doing this and just say this is not a number once without it looping the program?
Here's my code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <conio.h>
#include <windows.h>
//Tic-Tac-Toe game in C
//20/03/2019
//5x5 Board with 4 in a row to win. Also has playable AI that randomly blocks human moves.
//Also has replay function using a doubly linked list
/*
int board[49] = {
;,;, ;, ;, ;, ;, ;,
;,1, 2, 3, 4, 5 ,;,
;,6, 7, 8, 9, 10,;,
;,11,12,13,14,15,;,
;,16,17,18,19,20,;,
;,21,22,23,24,25,;,
;,;, ;, ;, ;, ;, ;,
}
*/
struct node
{
int side;
int move;
int place;
struct node * prev;
struct node * next;
};
void append(struct node **, int, int, int);
void display(struct node *);
void delete(struct node **, int);
//Constants of pieces, border and empty places
enum { NOUGHTS, CROSSES, BORDER, EMPTY };
//Constants for human/computer win or draw
enum { HUMANWIN, COMPWIN, DRAW };
const int directions[4] = { 1, 7, 6, 8 };
int moveCount = 0;
//Values of original places to play
const int ConvertTo49[25] = {
8, 9, 10,11,12,
15,16,17,18,19,
22,23,24,25,26,
29,30,31,32,33,
36,37,38,39,40
};
void append(struct node ** list, int side, int move, int place)
{
struct node *temp , *current = *list;
if(*list == NULL)
{
*list = (struct node *) malloc(sizeof(struct node));
(*list) -> prev = NULL;
(*list) -> side = side;
(*list) -> move = move;
(*list) -> place = place;
(*list) -> next = NULL;
}
else
{
while(current -> next != NULL)
current = current -> next;
temp = (struct node *) malloc(sizeof(struct node));
temp -> side = side;
temp -> move = move;
temp -> place = place;
temp -> next = NULL;
temp -> prev = current;
current -> next = temp;
}
}
void delete(struct node ** list, int num)
{
struct node *temp = *list;
while(temp != NULL)
{
if(temp -> move == num)
{
if(temp == *list)
{
*list = (*list) -> next;
(*list) -> prev = NULL;
}
else
{
if(temp -> next == NULL)
temp -> prev -> next = NULL;
else
{
temp -> prev -> next = temp -> next;
temp -> next -> prev = temp -> prev;
}
free(temp);
}
return ;
}
temp = temp -> next;
}
printf("Element %d not found in the supplied list \n\n", num);
}
//Loop through a direction until a border square is hit
int GetNumForDir(int startSq, const int dir, const int *board, const int us)
{
int found = 0;
while(board[startSq] != BORDER)
{
if(board[startSq] != us)
{
break;
}
found++;
startSq += dir;
}
return found;
}
//Loop through directions and find four in a row
int FindFourInARow(const int *board, const int ourIndex, const int us)
{
int dirIndex = 0;
int dir = 0;
int countFour = 1;
for(dirIndex = 0; dirIndex < 4; ++dirIndex)
{
dir = directions[dirIndex];
countFour += GetNumForDir(ourIndex + dir, dir, board, us);
countFour += GetNumForDir(ourIndex + dir * -1, dir * -1, board, us);
if(countFour == 4)
{
break;
}
countFour = 1;
}
return countFour;
}
//Set up board structure
void InitialiseBoard(int *board)
{
//index variable
int index;
//set whole board to border squares initially
for(index = 0; index < 49; ++index)
{
board[index] = BORDER;
}
//Set up empty squares for placing pieces
for(index = 0; index < 25; ++index)
{
board[ConvertTo49[index]] = EMPTY;
}
}
//Print the board
void PrintBoard(const int *board)
{
//index variable
int index;
//Set pieces and places
char pceChars[] = "OX| ";
printf("\n\n\n");
//Loop through and only draw the actual playable squares, leaving out the border squares
for(index = 0; index < 25; ++index)
{
//Put lines inbetween places
if(index==1 || index==2 || index==3 || index==4 ||
index==6 || index==7 || index==8 || index==9 ||
index==11 || index==12 || index==13 || index==14 ||
index==16 || index==17 || index==18 || index==19 ||
index==21 || index==22 || index==23 || index==24)
{
printf(" |");
}
//Put space at start to space out board from edge of screen
if(index==0)
{
printf(" ");
}
//Print new line every 5 places
if(index!= 0 && index==5 || index==10 || index==15 || index==20)
{
printf("\n");
printf(" -------------------");
printf("\n");
printf(" ");
}
//Print each playable piece
printf(" %c", pceChars[board[ConvertTo49[index]]]);
}
printf("\n\n");
}
//Check if board has empty spaces
int HasEmpty(const int *board)
{
int index;
for(index = 0; index < 25; ++index)
{
if(board[ConvertTo49[index]] == EMPTY) return 1;
}
return 0;
}
//Function for making move of current player
void MakeMove(int *board, const int sq, const side)
{
//set board place that either player decides
board[sq] = side;
}
void UndoMove(int *board, int sq, int side)
{
//set board place that either player decides
board[sq] = side;
}
//Help the computer find a winning move
int GetWinningMove(int *board, const int side)
{
int ourMove = -1;
int winFound = 0;
int index = 0;
for(index = 0; index < 25; ++index)
{
if(board[ConvertTo49[index]] == EMPTY)
{
ourMove = ConvertTo49[index];
board[ourMove] = side;
if(FindFourInARow(board, ourMove, side) == 4)
{
winFound = 1;
}
board[ourMove] = EMPTY;
if(winFound == 1)
{
return ourMove;
}
ourMove = -1;
};
}
return ourMove;
}
//Get computer player move
int GetComputerMove(int *board, const int side)
{
int index;
int numFree = 0;
int availableMoves[25];
int randMove = 0;
//Set random number to randomly run a function
int randFunction = 0;
randFunction = (rand() % 2);
//Go for the winning move
randMove = GetWinningMove(board, side);
if(randMove != -1)
{
return randMove;
}
//If random function is 1, stop any winning move from the human
if(randFunction == 1)
{
randMove = GetWinningMove(board, side ^ 1);
if(randMove != -1)
{
return randMove;
}
}
randMove = 0;
//Loop through all squares and put piece in random place
for(index = 0; index < 25; ++index)
{
if(board[ConvertTo49[index]] == EMPTY)
{
availableMoves[numFree++] = ConvertTo49[index];
};
}
randMove = (rand() % numFree);
return availableMoves[randMove];
}
//Get human player move
int GetHumanMove(const int *board)
{
//Array for user input
int userInput;
char term;
//Start moveOK at 0
int moveOK = 0;
int move = -1;
//Loop through until move being made is valid
while(moveOK == 0)
{
printf("\n");
//Ask user for input of 1-9
printf("Please enter a place on the board to make your move, from 1 to 25: ");
//Make sure that user doesnt enter long string with number on the end that eventaully passes tests in the while loop
scanf("%d%c", &userInput, &term);
if(term != '\n')
{
move = -1;
printf("This is not a number\n");
continue;
}
//Check if input is in proper range
if(userInput < 1 || userInput > 25)
{
move = -1;
printf("Invalid Range\n");
continue;
}
//decrement move to get the array location of the playable places
userInput--;
//Check if place selected is already taken
if(board[ConvertTo49[userInput]] != EMPTY)
{
move = -1;
printf("Square not available\n");
continue;
}
//Set move ok to 1 after passing all the tests
moveOK = 1;
}
//Print move being made and return it
printf("\nHuman making Move at square: %d\n",(userInput+1));
return ConvertTo49[userInput];
}
//Run Game function
void RunGame()
{
struct node *list;
list = NULL;
//Display
printf("\n\n Noughts And Crosses Game:");
printf("\n =========================");
//game over variable
int gameOver = 0;
//Start on noughts side
int side = NOUGHTS;
//Record last move
int lastMoveMade = 0;
//Create new board
int board[49];
//Count of first move
int initialMove = 0;
//Initialise the Board
InitialiseBoard(&board[0]);
//Print the board to screen
PrintBoard(&board[0]);
//While game isn't over
while(!gameOver)
{
initialMove++;
//Human
if(side==NOUGHTS)
{
//Ask to undo move after first move played
if(initialMove > 1)
{
while(list -> prev != NULL)
list = list -> prev;
int undo;
printf("\n\nWould you like to undo your move? \nEnter 2 to undo or any other key/s to continue: ");
scanf("%d", &undo);
if( undo == 2)
{
while(list -> next != NULL)
list = list -> next;
char pceChars[] = "OX| ";
//printf(" %c", pceChars[board[ConvertTo49[index]]]);
int count = 2;
while(count != 0)
{
UndoMove(&board[0], list -> place, EMPTY);
list = list -> prev;
delete(&list, list -> next -> move);
moveCount--;
count--;
}
PrintBoard(&board[0]);
}
}
//Make new move for human
lastMoveMade = GetHumanMove(&board[0]);
MakeMove(&board[0], lastMoveMade, side);
moveCount++;
int humanMove;
//Find the position the computer is making on the 5x5 board
for(humanMove = 0; humanMove < 25; ++humanMove)
{
if(ConvertTo49[humanMove] == lastMoveMade)
{
break;
}
}
append(&list, 0, moveCount, lastMoveMade);
side = CROSSES;
}
//Computer
else
{
//Make move for computer
lastMoveMade = GetComputerMove(&board[0], side);
MakeMove(&board[0], lastMoveMade, side);
int compMove;
//Find the position the computer is making on the 5x5 board
for(compMove = 0; compMove < 25; ++compMove)
{
if(ConvertTo49[compMove] == lastMoveMade)
{
break;
}
}
printf("\nComputer making move at square: %d", compMove+1);
moveCount++;
append(&list, 1, moveCount, lastMoveMade);
side = NOUGHTS;
PrintBoard(&board[0]);
}
//If three in a row exists, game over
if(FindFourInARow(board, lastMoveMade, side ^ 1) == 4)
{
gameOver = 1;
if(side == NOUGHTS)
{
printf("\nGame Over!\n");
printf("Computer Wins! :(\n");
}
else
{
PrintBoard(&board[0]);
printf("\nGame Over!\n");
printf("Human Wins! :D\n");
}
}
//If no more moves, game is a draw
if(!HasEmpty(board))
{
PrintBoard(&board[0]);
printf("\n\nGame Over!\n");
gameOver = 1;
printf("It's a draw! :/\n");
}
}
/*
*
* Match Replay
*
*/
int answer;
printf("\n\nWould you like to replay the match? \nEnter 1 to replay or any other key/s to quit: ");
scanf("%d", &answer);
if(answer == 1)
{
InitialiseBoard(&board[0]);
while ( list != NULL )
{
MakeMove(&board[0], list -> place, list -> side);
PrintBoard(&board[0]);
char who[7];
if(list -> side == 0)
{
strcpy(who, "NOUGHTS");
//who = "NOUGHTS";
}
else
{
strcpy(who, "CROSSES");
//who = "CROSSES";
}
int moveMade;
//Find the position the computer is making on the 5x5 board
for(moveMade = 0; moveMade < 25; ++moveMade)
{
if(ConvertTo49[moveMade] == list -> place)
{
break;
}
}
printf ("\nMove:%2d Side:%8s Place: %d\n", list -> move, who, moveMade+1);
Sleep(2000);
list = list -> next ;
}
}
else
{
printf("\nGood Game! See ya!\n");
getch();
}
}
int main()
{
//Create random number
srand(time(NULL));
//Call Run Game to play
RunGame();
return 0;
}

Answer 1

Input that doesn't match your format doesn't get consumed by scanf. It remains in the input stream. So when you type a letter it fails to match the number you wanted %d and stays in the input stream.
scanf("%d%c", &userInput, &term);
Instead of scanf, it would be simpler to use getline to consume the entire line of input. And then parse the result with sscanf (the string-based version of the function) or another method.
char *line = NULL;
size_t len = 0;
while ((read = getline(&line, &len, stdin)) != -1) {
// Do something with line
int scanned = sscanf(line, "%d%c", &userInput);
if (scanned == EOF || scanned != 2) {
printf("Line failed to match\n");
}
}

Answer 2

Just read the manual page:
The format string consists of a sequence of directives which describe
how to process the sequence of input characters. If processing of a
directive fails, no further input is read, and scanf() returns. A
"failure" can be either of the following: input failure, meaning that
input characters were unavailable, or matching failure, meaning that
the input was inappropriate (see below).
You do not check the return value of scanf(). So you try to scan the same non-digit as digit all the time.