求助：如何用两个字符替换一个字符？

#define NUM 500
int main(void)
{
    int i, j, n;

    char ch[NUM*2];
    i=j=0;
   
    while ((ch[i]=getchar())!='#')   //#结束
    {
        if (ch[i] == '!')       //用两个感叹号代替一个感叹号
        {
            ch[i] = '!';
            for(n=NUM;n>=i;n--)
                ch[n+1] =ch[n];
            ch[i+1]= '!';
            putchar(ch[i]);
            putchar(ch[i+1]);
            j++;            
        }
        else if (ch[i] == '.')    //用一个感叹号代替句号
        {
            ch[i] = '!';
            putchar(ch[i]);
            j++;
        }
        else
            putchar(ch[i]);        
    }   
    printf("\n%5d\n", j);          //替换了几次
    system("pause");
    return 0;
}