Thread: Can't stop while loop

I'm trying to create a connect four game, but I've ran into a problem while trying to read valid user input.

Code:

// 4 in a row game

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

#define NUM_ROWS 6
#define NUM_COLS 7

#define NO_PLAYER 0
#define PLAYER_1 1
#define PLAYER_2 2

#define TRUE 1
#define FALSE 0

void printBoard(int board[NUM_COLS][NUM_ROWS]);
void prompt(int player);
int input(int board[NUM_COLS][NUM_ROWS], int player);

int main(int argc, char **argv) {
    int board[NUM_COLS][NUM_ROWS] = {{NO_PLAYER}}; //(a,b) = (row,col)
    int round = 1;
    int player;
    
    printBoard(board);
    
    player = (round-1)%2 + 1;
    prompt(player);
    while (input(board, player) == FALSE) {
        printf("(!) Invalid input\n\n");
        prompt(player);
    }

    return EXIT_SUCCESS;
}


void printBoard(int board[NUM_COLS][NUM_ROWS]) {
    int i,j;
    
    printf("\n");
    for (i=0; i<NUM_ROWS; i++) {
        printf("\t\t");
        for (j=0; j<NUM_COLS; j++) {
            if (board[i][j] == NO_PLAYER)
                printf(".");
            else if (board[i][j] == PLAYER_1)
                printf("X");
            else if (board[i][j] == PLAYER_2)
                printf("O");
            else
                printf("E");
            printf("  ");       
        }    
        printf("\n");
    }
    printf("\n\t\t");
    for (i=0; i<NUM_COLS; i++) {
        printf("%d  ", i+1);
    }
    printf("\n\n");

    return;
}

void prompt(int player) {
    printf("Player %d, enter position 1-%d: ", player, NUM_COLS);
    return;
}

int input(int board[NUM_COLS][NUM_ROWS], int player) {
    int position;
    int numScan;
    int i;
    
    numScan = scanf("%d", &position);
    if (numScan != 1 || position < 1 || position > NUM_COLS
        || board[position][0] != NO_PLAYER) 
        return FALSE;
    
    for (i=NUM_ROWS-1; i>=0; i--) {
        if (board[position][i] == NO_PLAYER)
            board[position][i] == player;
    }
    
    return TRUE;
}

If the user enters an invalid number, then it works fine, but if a character is entered, the while loop in main never breaks. How can I fix this?

When a character is entered, "scanf()" fails and leaves that character in the input buffer. Then it loops, tries to read the same input, fails, etc. That is the cause of your infinite loop in that situation.

One option would be to flush the input buffer if "scanf()" fails, so you start the next iteration of the loop with a clean slate.

Also, check line 83 - you have a mistake there.

Code:

main.c||In function 'input':|
main.c|83|warning: statement with no effect|
||=== Build finished: 0 errors, 1 warnings ===|

Originally Posted by Matticus

When a character is entered, "scanf()" fails and leaves that character in the input buffer. Then it loops, tries to read the same input, fails, etc. That is the cause of your infinite loop in that situation.

One option would be to flush the input buffer if "scanf()" fails, so you start the next iteration of the loop with a clean slate.

I see. Sadly, I don't know how to do that.

Originally Posted by Matticus

Also, check line 83 - you have a mistake there.

Code:

main.c||In function 'input':|
main.c|83|warning: statement with no effect|
||=== Build finished: 0 errors, 1 warnings ===|

Thanks, I noticed it too, and I would've spotted it even earlier if I used the -Wall flag

This might suit your needs:

Code:

    numScan = scanf("%d", &position);
    if(numScan != 1)                // if "scanf" failed,
        while(getchar() != '\n')    //   eat up all characters in the
            ;                       //   input buffer up to the newline

Review that carefully and make sure you understand how it works. If you're not certain, just ask for clarification.

This is not the most elegant approach, and will not cover all circumstances (e.g. if the user just presses enter without typing anything), but should do you well for now.

In the future, you might consider making a separate function just for reading a number from stdin. This can use the "fgets()" and "sscanf()" combination to read the input without having to worry about the issues usually encountered with "scanf()" alone.

Originally Posted by Matticus

This might suit your needs:

Code:

    numScan = scanf("%d", &position);
    if(numScan != 1)                // if "scanf" failed,
        while(getchar() != '\n')    //   eat up all characters in the
            ;                       //   input buffer up to the newline

Review that carefully and make sure you understand how it works. If you're not certain, just ask for clarification.

This is not the most elegant approach, and will not cover all circumstances (e.g. if the user just presses enter without typing anything), but should do you well for now.

I think I see what it's doing. It's going to be consuming each character in the input buffer including the new line character, but breaks from the while loop when that happens so it stops waiting for more chars by that point.

Originally Posted by Matticus

In the future, you might consider making a separate function just for reading a number from stdin. This can use the "fgets()" and "sscanf()" combination to read the input without having to worry about the issues usually encountered with "scanf()" alone.

Could I get some help with this please? I'm willing to try and do it, but I'm rusty with each of those functions.

Originally Posted by hk_mp5kpdw

Looks like you need to increment round in the loop and also reset player so that it switches back and forth.

I wasn't quite done at that point. I now have

Code:

// 4 in a row game

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

#define NUM_ROWS 6
#define NUM_COLS 7

#define NO_PLAYER 0
#define PLAYER_1 1
#define PLAYER_2 2

#define TRUE 1
#define FALSE 0

void printBoard(int board[NUM_ROWS][NUM_COLS]);
void prompt(int player);
int input(int board[NUM_ROWS][NUM_COLS], int player, int *play);
int gameOver(int board[NUM_ROWS][NUM_COLS], int play);
int win(int board[NUM_ROWS][NUM_COLS], int play);

int main(int argc, char **argv) {
    int board[NUM_ROWS][NUM_COLS] = {{NO_PLAYER}}; //(a,b) = (row,col)
    int round = 1;
    int player;
    int play = 0;
    int *playP = &play;
    
    printBoard(board);
    
    while (gameOver(board, play) == FALSE) {
        player = (round-1)%2 + 1;
        prompt(player);
        while (input(board, player, playP) == FALSE) {
            printf("(!) Invalid input\n\n");
            prompt(player);
        }
        printBoard(board);
        round++;
    }

    return EXIT_SUCCESS;
}


void printBoard(int board[NUM_ROWS][NUM_COLS]) {
    int i,j;
    
    printf("\n");
    for (i=0; i<NUM_ROWS; i++) {
        printf("\t\t");
        for (j=0; j<NUM_COLS; j++) {
            if (board[i][j] == NO_PLAYER)
                printf(".");
            else if (board[i][j] == PLAYER_1)
                printf("X");
            else if (board[i][j] == PLAYER_2)
                printf("O");
            else
                printf("E");
            printf("  ");       
        }    
        printf("\n");
    }
    printf("\n\t\t");
    for (i=0; i<NUM_COLS; i++) {
        printf("%d  ", i+1);
    }
    printf("\n\n");

    return;
}

void prompt(int player) {
    printf("\t\tPlayer %d: ", player);
    return;
}

int input(int board[NUM_ROWS][NUM_COLS], int player, int *play) {
    int position;
    int numScan;
    int i;
    
    numScan = scanf("%d", &position);
    if (numScan != 1 || position < 1 || position > NUM_COLS
        || board[0][position] != NO_PLAYER) 
        return FALSE;
        
    *play = position;
    
    for (i=NUM_ROWS-1; i>=0; i--) {
        if (board[i][position-1] == NO_PLAYER) {
            board[i][position-1] = player;
            break;
        }
    }
    
    return TRUE;
}

int gameOver(int board[NUM_ROWS][NUM_COLS], int play) {
    int i;
    
    for (i=0; i<NUM_COLS; i++)
        if (board[0][i] == NO_PLAYER) break;
    if (i == NUM_COLS && board[0][i-1] != NO_PLAYER) return TRUE;
    
    if (win(board, play) == TRUE) return TRUE;
    
    return FALSE;
}

int win(int board[NUM_ROWS][NUM_COLS], int play) {

    return FALSE;
}

You should look up documentation on fgets and sscanf, but you could do something like this.

Code:

int getInt(){    
    const int STR_SIZE = 20;
    int integer;
    char stringInt[STR_SIZE];
    fgets(stringInt, STR_SIZE, stdin);
    sscanf(stringInt, "%d", &integer);
    return integer;
}

Originally Posted by vart

if scanf fails you will return value of uninitialized variable integer

I don't think he's gonna have that problem for whats he's doing.

Not sure if it's a bad thing. It's quite common throughout history. A great example is this:

https://www.youtube.com/watch?v=36tsp2rTACo