C编程：for循环和中断

Question

Can anybody tell me why the loop does not exit whenever I press letter X? How to make the program not get the value of backspace and enter into the array?

#include <stdio.h>
#include <stdlib.h>
#include<math.h>
#define N 2
#define M 4


int main()
{
int i,j,a[N][M];

for(i=0;i<N;i++)
{
    for(j=0;j<M;j++)
    {
        scanf("%c",&a[i][j]);
        if(a[i][j]=='X')
            break;
    }
        if(a[i][j]=='X')
            break;
}
    return 0;
}

Answer 1

Change scanf("%c",&a[i][j]); to scanf(" %c",&a[i][j]);

This allows for any spaces to be bypassed before scanning the character.

Answer 2

There are two problems in your code:

• The first one is already pointed out by Rishikesh Raje: You need to add a space to the scanf() command in order to eat up the scanned "\\n" characters.

• Then, you scan characters (%c) and try to store them in an int-array. Use

   char a[N][M]; 

  instead. My gcc gives a warning at your erroneous code. Other compilers may silently ignore this.

  Still, in an little-endian-environment (Like PC's) one could think: a char stored at the address of an int-variable should result in the same value. However, the char-value occupies only one byte, the remaining bytes (3 or more) keep uninitialized. If there were zero-bytes before, than a[i][j] will be 'X', otherwise, it will be some random number.

  This explains the behaviour, I think you observed: The program stopped randomly at some 'X' but not always.

Answer 3

Change the type of array a from int to char

int i,j,a[N][M];

change to

int i,j;
char a[N][M];

Usually gcc would warn you too for this:

so.c: In function ‘main’:
so.c:16:9: warning: format ‘%c’ expects argument of type ‘char *’, but argument 2 has type ‘int *’ [-Wformat=]
         scanf("%c",&a[i][j]);

Answer 4

Make it an char array instead of an int array

char a[N][M];

Instead of,

int a[N][M];