这个程序首先要求用户选择加/减/乘/除，如图片所示，输入d后，仍然显示要求输入a或s或m或d，相关的是char get_choice()函数，请看下问题出在哪里？

#include <stdio.h>
char get_choice(); /*读取加减乘除选项*/
char get_first(); /*只读取第一个字符*/
float get_num(); /*读取要计算的数字*/

int main()
{
    int choice;
    float num_1,num_2;
        
    while((choice=get_choice()) != 'q') /*期望输入字符a,s,m,d*/
    {
        printf("Enter first number:\n");
        num_1 = get_num();                /*输入第一个数字*/
        printf("Enter second number:\n");
        num_2 = get_num();                /*输入第二个数字*/
        switch(choice)
        {
            case 'a':    printf("%.2f + %.2f = %.2f.\n",num_1+num_2);/*如果输入字符a,做加法*/
                        break;
            case 's':    printf("%.2f - %.2f = %.2f.\n",num_1-num_2);/*如果输入字符s,做减法*/
                        break;
            case 'm':    printf("%.2f * %.2f = %.2f.\n",num_1*num_2);/*如果输入字符m,做乘法*/
                        break;
            case 'd':    if(num_2 == 0)                                /*做除法，如果分母等于0，要求重新输入*/
                            {
                                printf("Please enter a number other than 0:");
                                while((num_2 = get_num()) == 0)        /*一直输入，直到分母不等于0*/
                                    continue;
                                printf("%.2f / %.2f = %.2f.\n",num_1/num_2);
                                break;
                            }
                        else
                             printf("%.2f / %.2f = %.2f.\n",num_1/num_2);
                             break;
             default:    printf("Error!\n");
                         break;
        }
        printf("Bye!.\n");
    }
   
}

char get_choice()
{
    int ch;
   
    printf("Enter the operation of your choice:\n");
    printf("a. add            s. subtract\n");
    printf("m. multiply        d. divide\n");
    printf("q. quit\n");
    ch = get_first();
    while((ch != 'a' || ch != 's' || ch != 'm' || ch != 'd') && ch != 'q') /*如果输入的字符不等于a或s或m或d，并且不是q，那么要求重新输入*/
    {
        printf("Please respond with a,s,m,d or q.\n");
        ch = get_first();
    }
    return ch;
}

char get_first()
{
    int ch;
   
    ch = getchar();
    while(getchar() != '\n')  
        continue;            /*只读取第一个字符，跳过后续字符包括回车行*/
    return ch;
}

float get_num()
{
    float num;
    char ch;
   
    while((scanf("%f",&num)) != 1)    /*如果输入的不是数字*/
    {
        while((ch=getchar()) != '\n')
            putchar(ch);
        printf(" is not number.\n");
        printf("Please input a number:\n");
    }
    return num;
}