这个程序调试时出错，实在不知道为什么，请高手指点，Thank you!!
#include <stdio.h>

int main(void)
{
    char ch, a[50];
    int i = 0, j;

    printf ( "Please input strings:" );
    while ( ( ch = getchar () ) != '#' )
    {
        if ( ch != '.' )
            putchar(ch);
        else
        {
            ch = '!';
            putchar(ch);
        }
        while ( 1 )
        {
            a[i] = ch;
            i++;
        }
    }
    j = i;
    for ( i = 0; i <= j; i++ )
    {
        if ( a[i] == '!' )
        {
            a[i] = '!!';
            putchar(a[i]);
        }
        else
            putchar(a[i]);
    }
    getch();

    return 0;
}