Reading character with scanf()

问题

This code is for game of craps.

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

int roll_dice(void);
bool play_game(void);

int main()
{
   int i, ch,win = 0,lose = 0;
   bool flag;
   srand((unsigned)time(NULL));

   do
   {
       flag = play_game();
       if(flag)
       {
           printf("You win!");
           win++;
       }
       else
       {
           printf("You lose!");
           lose++;
       }

       printf("\n\nPlay again(Y/N)? ");
       scanf("%c", &ch);
       ch = getchar();
       printf("\n");
   }while(ch == 'Y' || ch == 'y');

   printf("\nWins: %d   Losses: %d",win,lose);
   return 0;
}

int roll_dice(void)
{
    return rand()%6 + rand()%6 + 2;
}

bool play_game(void)
{
   int sum = roll_dice();

   printf("You rolled: %d\n", sum);
   if(sum == 7 || sum == 11)
       return 1;
   else if(sum == 2 || sum == 3 || sum == 12)
       return 0;    
   else
   {
       int point = sum;
       printf("Your point is: %d\n", point);
       do
       {
           sum = roll_dice();
           printf("You rolled: %d\n", sum);
           if(sum == 7)
               return 0;            
       }while(point != sum);
       return 1;
   }                    
}

I have problem only with code snippet

 printf("\n\nPlay again(Y/N)? ");
 scanf("%c", &ch);
 ch = getchar();
 printf("\n");

I have used, because it terminates after one iteration whatever user input Y or N. I thought I am doing wrong by placing ch = getchar() to eat up \n, I removed it and placed a space before conversion specifier and replaced it by " %c" which also did't work.When I replaced the conversion specifier by %d it works fine.
Is anything going wrong with this?
I visited this post and it is saying same thing I did.

回答1:

The posted code has undefined behaviour because ch is of type int and the format specifier %c must match a char.

When I replaced the conversion specifier %d it works fine.

When you switch to %d the scanf() fails, because Y or y is not an int, so no input is consumed (apart from leading whitespace which discards the new line character on subsequent iterations of the loop) and the subsequent ch = getchar() actually reads the user entered character, and the code works by fluke. Always check the return value of scanf(), which returns the number of assignments made.

回答2:

You convert the character with scanf(), and then overwrite it with getchar() immediately afterwards. I wouldn't expect it to work, unless you type "yy" before typing ENTER, but then your second confirmation would fail.

BTW, use the space in " %c".

回答3:

scanf("%c", &ch);
ch = getchar();

And that's how you lost the previous char stored in ch. How about

ch = fgetc(stdin);
while (fgetc(stdin) != '\n')
    ;

instead?

回答4:

printf("Play again? ");
scanf(" %c", &char);

this code works for me. The project is from K.N.King's "C programming : A modern approach" book. I met with this problem before and had the same problem. On page 224 there is a guess.c example project which includes exactly the same command "ask" ("play again"). And author used scanf(" %c", &command); (he used command instead of ch) and it did work. I remember I used it during the "game of craps" project but it did not work. Probably I missed something. Overall, the expression above 100% does work.

来源：https://stackoverflow.com/questions/17497111/reading-character-with-scanf

标签

char

scanf

specifier